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First element on the periodic table, hydrogen is truly in a class by itself. It does not belong to any family of elements, and though it is a nonmetal, it appears on the left side of the periodic table with the metals. The other elements with it in Group 1 form the alkali metal family, but obviously, hydrogen does not belong with them. Indeed, if there is any element similar to hydrogen in simplicity and abundance, it is the only other one on the first row, or period, of the periodic table: helium. Together, these two elements make up 99.9% of all known matter in the entire universe, because hydrogen atoms in stars fuse to create helium. Yet whereas helium is a noble gas, and therefore chemically unreactive, hydrogen bonds with all sorts of other elements. In one such variety of bond, with carbon, hydrogen forms the backbone for a vast collection of organic molecules, known as hydrocarbons and their derivatives. Bonded with oxygen, hydrogen forms the single most important compound on Earth, and the most important complex substance other than air: water. Yet when it bonds with sulfur, it creates toxic hydrogen sulfide; and on its own, hydrogen is extremely flammable. The only element whose isotopes have names, hydrogen has long been considered as a potential source of power and transportation: once upon a time for airships, later as a component in nuclear reactionsand, perhaps in the future, as a source of abundant clean energy.


The Essentials

The atomic number of hydrogen is 1, meaning that it has a single proton in its nucleus. With its single electron, hydrogen is the simplest element of all. Because it is such a basic elemental building block, figures for the mass of other elements were once based on hydrogen, but the standard today is set by 12C or carbon-12, the most common isotope of carbon.

Hydrogen has two stable isotopesforms of the element that differ in mass. The first of these, protium, is simply hydrogen in its most common form, with no neutrons in its nucleus. Protium (the name is only used to distinguish it from the other isotopes) accounts for 99.985% of all the hydrogen that appears in nature. The second stable isotope, deuterium, has one neutron, and makes up 0.015% of all hydrogen atoms. Tritium, hydrogen's one radioactive isotope, will be discussed below.

The fact that hydrogen's isotopes have separate names, whereas all other isotopes are designated merely by element name and mass number (for example, "carbon-12") says something about the prominence of hydrogen as an element. Not only is its atomic number 1, but in many ways, it is like the number 1 itselfthe essential piece from which all others are ultimately constructed. Indeed, nuclear fusion of hydrogen in the stars is the ultimate source for the 90-odd elements that occur in nature.

The mass of this number-one element is not, however, 1: it is 1.008 amu, reflecting the small quantities of deuterium, or "heavy hydrogen," present in a typical sample. A gas at ordinary temperatures, hydrogen turns to a liquid at 423.2°F (252.9°C), and to a solid at 434.°F (259.3°C). These figures are its boiling point and melting point respectively; only the figures for helium are lower. As noted earlier, these two elements make up all but 0.01% of the known elemental mass of the universe, and are the principal materials from which stars are formed.

Normally hydrogen is diatomic, meaning that its molecules are formed by two atoms. At the interior of a star, however, where the temperature is many millions of degrees, H2 molecules are separated into atoms, and these atoms become ionized. In other words, the electron separates from the proton, resulting in an ion with a positive charge, along with a free electron. The positive ions experience fusionthat is, their nuclei bond, releasing enormous amounts of energy as they form new elements.

Because the principal isotopic form of helium has two protons in the nucleus, it is natural that helium is the element usually formed; yet it is nonetheless trueamazing as it may seemthat all the elements found on Earth were once formed in stars. On Earth, however, hydrogen ranks ninth in its percentage of the planet's known elemental mass: just 0.87%. In the human body, on the other hand, it is third, after oxygen and carbon, making up 10% of human elemental body mass.

Hydrogen and Bonding

Having just one electron, hydrogen can bond to other atoms in one of two ways. The first option is to combine its electron with one from the atom of a nonmetallic element to make a covalent bond, in which the two electrons are shared. Hydrogen is unusual in this regard, because most atoms conform to the octet rule, ending up with eight valence electrons. The bonding behavior of hydrogen follows the duet rule, resulting in just two electrons for bonding.

Examples of this first type of bond include water (H2O), hydrogen sulfide (H2S), and ammonia (NH3), as well as the many organic compounds formed on a hydrogen-carbon backbone. But hydrogen can form a second type of bond, in which it gains an extra electron to become the negative ion H, or hydride. It is then able to combine with a metallic positive ion to form an ionic bond. Ionic hydrides are convenient sources of hydrogen gas: for instance, calcium hydride, or CaH2, is sold commercially, and provides a very convenient means of hydrogen generation. The hydrogen gas produced by the reaction of calcium hydride with water can be used to inflate life rafts.

The presence of hydrogen in certain types of molecules can also be a factor in intermolecular bonding. Intermolecular bonding is the attraction between molecules, as opposed to the bonding within molecules, which is usually what chemists mean when they talk about "bonding."

Hydrogen's Early History

Because it bonds so readily with other elements, hydrogen almost never appears in pure elemental form on Earth. Yet by the late fifteenth century, chemists recognized that by adding a metal to an acid, hydrogen was produced. Only in 1766, however, did English chemist and physicist Henry Cavendish (1731-1810) recognize hydrogen as a substance distinct from all other "airs," as gases then were called.

Seventeen years later, in 1783, French chemist Antoine Lavoisier (1743-1794) named the substance after two Greek words: hydro (water) and genes (born or formed). It was another two decades before English chemist John Dalton (1766-1844) formed his atomic theory of matter, and despite the great strides he made for science, Dalton remained convinced that hydrogen and oxygen in water formed "water atoms." Around the same time, however, Italian physicist Amedeo Avogadro (1776-1856) clarified the distinction between atoms and molecules, though this theory would not be generally accepted until the 1850s.

Contemporary to Dalton and Avogadro was Swedish chemist Jons Berzelius (1779-1848), who developed a system of comparing the mass of various atoms in relation to hydrogen. This method remained in use for more than a century, until the discovery of neutrons, protons, and isotopes pointed the way toward a means of making more accurate determinations of atomic mass. In 1931, American chemist and physicist Harold Urey (1893-1981) made the first separation of an isotope: deuterium, from ordinary water.


Deuterium and Tritium

Designated as 2H, deuterium is a stable isotope, whereas tritium3His unstable, or radioactive. Not only do these two have names; they even have chemical symbols (D and T, respectively), as though they were elements on the periodic table. Just as hydrogen represents the most basic proton-electron combination against which other atoms are compared, these two are respectively the most basic isotope containing a single neutron, and the most basic radioisotope, or radioactive isotope.

Deuterium is sometimes called "heavy hydrogen," and its nucleus is called a deuteron. In separating deuteriuman achievement for which he won the 1934 Nobel PrizeUrey collected a relatively large sample of liquid hydrogen: 4.2 qt (4 l). He then allowed the liquid to evaporate very slowly, predicting that the more abundant protium would evaporate more quickly than the heavier isotope. When all but 0.034 oz (1 ml) of the sample had evaporated, he submitted the remainder to a form of analysis called spectroscopy, adding a burst of energy to the atoms and then analyzing the light spectrum they emitted for evidence of differing varieties of atoms.

With an atomic mass of 2.014102 amu, deuterium is almost exactly twice as heavy as protium, which has an atomic mass of 1.007825. Its melting points and boiling points, respectively 426°F (254°C) and 417°F (249°C), are higher than for protium. Often, deuterium is applied as a tracer, an atom or group of atoms whose participation in a chemical, physical, or biological reaction can be easily observed.


In nuclear power plants, deuterium is combined with oxygen to form "heavy water" (D2O), which likewise has higher boiling and melting points than ordinary water. Heavy water is often used in nuclear fission reactors to slow down the fission process, or the splitting of atoms. Deuterium is also present in nuclear fusion, both on the Sun and in laboratories.

During the period shortly after World War II, physicists developed a means of duplicating the thermonuclear fusion process. The result was the hydrogen bombmore properly called a fusion bombwhose detonating device was a compound of lithium and deuterium called lithium deuteride. Vastly more powerful than the "atomic" (that is, fission) bombs dropped by the United States over Japan (Nagaski and Hiroshima) in 1945, the hydrogen bomb greatly increased the threat of worldwide nuclear annihilation in the postwar years.

Yet the power that could destroy the world also has the potential to provide safe, abundant fusion energy from power plantsa dream as yet unrealized. Physicists studying nuclear fusion are attempting several approaches, including a process involving the fusion of two deuterons. This fusion would result in a triton, the nucleus of tritium, along with a single proton. Theoretically, the triton and deuteron would then be fused to create a helium nucleus, resulting in the production of vast amounts of energy.


Whereas deuterium has a single neutron, tritiumas its mass number of 3 indicateshas two. And just as deuterium has approximately twice the mass of protium, tritium has about three times the mass, or 3.016 amu. Its melting and boiling points are higher still than those of deuterium: thus tritium heavy water (T2O) melts at 40°F (4.5°C), as compared with 32°F (0°C) for H2O.

Tritium has a half-life (the length of time it takes for half the radioisotopes in a sample to become stable) of 12.26 years. As it decays, its nucleus emits a low-energy beta particle, which is either an electron or a subatomic particle called a positron, resulting in the creation of the helium-3 isotope. Due to the low energy levels involved, the radioactive decay of tritium poses little danger to humans. In fact, there is always a small quantity of tritium in the atmosphere, and this quantity is constantly being replenished by cosmic rays.

Like deuterium, tritium is applied in nuclear fusion, but due to its scarcity, it is usually combined with deuterium. Sometimes it is released in small quantities into the groundwater as a means of monitoring subterranean water flow. It is also used as a tracer in biochemical processes, and as an ingredient in luminous paints.

Hydrogen and Oxygen


Water, of course, is the most well-known compound involving hydrogen. Nonetheless, it is worthwhile to consider the interaction between hydrogen and oxygen, the two ingredients in water, which provides an interesting illustration of chemistry in action.

Chemically bonded as water, hydrogen and oxygen can put out any type of fire except an oil or electrical fire; as separate substances, however, hydrogen and oxygen are highly flammable. In an oxyhydrogen torch, the potentially explosive reaction between the two gases is controlled by a gradual feeding process, which produces combustion instead of the more violent explosion that sometimes occurs when hydrogen and oxygen come into contact.


Aside from water, another commonly used hydrogen-oxygen compound is hydrogen peroxide, or H2O2. A colorless liquid, hydrogen peroxide is chemically unstable (not "unstable" in the way that a radioisotope is), and decomposes slowly to form water and oxygen gas. In high concentrations, it can be used as rocket fuel.

By contrast, the hydrogen peroxide used in homes as a disinfectant and bleaching agent is only a 3% solution. The formation of oxygen gas molecules causes hydrogen peroxide to bubble, and this bubbling is quite rapid when the peroxide is placed on cuts, because the enzymes in blood act as a catalyst to speed up the reaction.

Hydrogen Chloride

Another significant compound involving hydrogen is hydrogen chloride, or HClin other words, one hydrogen atom bonded to chlorine, a member of the halogens family. Dissolved in water, it produces hydrochloric acid, used in laboratories for analyses involving other acids. Normally, hydrogen chloride is produced by the reaction of salt with sulfuric acid, though it can also be created by direct bonding of hydrogen and chlorine at temperatures above 428°F (250°C).

Hydrogen chloride and hydrochloric acid have numerous applications in metallurgy, as well as in the manufacture of pharmaceuticals, dyes, and synthetic rubber. They are used, for instance, in making pharmaceutical hydrochlorides, water-soluble drugs that dissolve when ingested. Other applications include the production of fertilizers, synthetic silk, paint pigments, soap, and numerous other products.

Not all hydrochloric acid is produced by industry, or by chemists in laboratories. Active volcanoes, as well as waters from volcanic mountain sources, contain traces of the acid. So, too, does the human body, which generates it during digestion. However, too much hydrochloric acid in the digestive system can cause the formation of gastric ulcers.

Hydrogen Sulfide

It may not be a pleasant subject, but hydrogenin the form of hydrogen sulfideis also present in intestinal gas. The fact that hydrogen sulfide is an extremely malodorous substance once again illustrates the strange things that happen when elements bond: neither hydrogen nor sulfur has any smell on its own, yet together they form an extremely noxiousand toxicsubstance.

Pockets of hydrogen sulfide occur in nature. If a person were to breathe the vapors for very long, it could be fatal, but usually, the foul odor keeps people away. The May 2001 National Geographic included two stories relating to such natural hydrogen-sulfide deposits, on opposite sides of the Earth, and in both cases the presence of these toxic fumes created interesting results.

In southern Mexico is a system of caves known as Villa Luz, through which run some 20 underground springs, many of them carrying large quantities of hydrogen sulfide. The National Geographic Society's team had to enter the caves wearing gas masks, yet the area teems with strange varieties of life. Among these are fish that are red from high concentrations of hemoglobin, or red blood cells. The creatures need this extra dose of hemoglobin, necessary to move oxygen through the body, in order to survive on the scant oxygen supplies. The waters of the cave are further populated by microorganisms that oxidize the hydrogen sulfide and turn it into sulfuric acid, which dissolves the rock walls and continually enlarges the cave.

Thousands of miles away, in the Black Sea, explorers supported by a grant from the National Geographic Society examined evidence suggesting that there indeed had been a great ancient flood in the area, much like the one depicted in the Bible. In their efforts, they had an unlikely ally: hydrogen sulfide, which had formed at the bottom of the sea, and was covered by dense layers of salt water. Because the Black Sea lacks the temperature differences that cause water to circulate from the bottom upward, the hydrogen sulfide stayed at the bottom.

Under normal circumstances, the wreck of a 1,500-year-old wooden ship would not have been preserved; but because oxygen could not reach the bottom of the Black Seaand thus wood-boring worms could not live in the toxic environmentthe ship was left undisturbed. Thanks to the presence of hydrogen sulfide, explorers were able to study the ship, the first fully intact ancient shipwreck to be discovered.


Together with carbon, hydrogen forms a huge array of organic materials known as hydrocarbonschemical compounds whose molecules are made up of nothing but carbon and hydrogen atoms. Theoretically, there is no limit to the number of possible hydrocarbons. Not only does carbon form itself into seemingly limitless molecular shapes, but hydrogen is a particularly good partner. Because it has the smallest atom of any element on the periodic table, it can bond to one of carbon's valence electrons without getting in the way of the others.

Hydrocarbons may either be saturated or unsaturated. A saturated hydrocarbon is one in which the carbon atom is already bonded to four other atoms, and thus cannot bond to any others. In an unsaturated hydrocarbon, however, not all the valence electrons of the carbon atom are bonded to other atoms.

Hydrogenation is a term describing any chemical reaction in which hydrogen atoms are added to carbon multiple bonds. There are many applications of hydrogenation, but one that is particularly relevant to daily life involves its use in turning unsaturated hydrocarbons into saturated ones. When treated with hydrogen gas, unsaturated fats (fats are complex substances that involve hydrocarbons bonded to other molecules) become saturated fats, which are softer and more stable, and stand up better to the heat of frying. Many foods contain hydrogenated vegetable oil; however, saturated fats have been linked with a rise in blood cholesterol levelsand with an increased risk of heart disease.


One important variety of hydrocarbons is described under the collective heading of petrochemicalsthat is, derivatives of petroleum. These include natural gas; petroleum ether, a solvent; naphtha, a solvent (for example, paint thinner); gasoline; kerosene; fuel for heating and diesel fuel; lubricating oils; petroleum jelly; paraffin wax; and pitch, or tar. A host of other organic chemicals, including various drugs, plastics, paints, adhesives, fibers, detergents, synthetic rubber, and agricultural chemicals, owe their existence to petrochemicals.

Then there are the many hydrocarbon derivatives formed by the bonding of hydrocarbons to various functional groupsbroad arrays of molecule types involving other elements. Among these are alcoholsboth ethanol (the alcohol in beer and other drinks) and methanol, used in adhesives, fibers, and plastics, and as a fuel. Other functional groups include aldehydes, ketones, carboxylic acids, and esters. Products of these functional groups range from aspirin to butyric acid, which is in part responsible for the smell both of rancid butter and human sweat. Hydrocarbons also form the basis for polymer plastics such as Nylon and Teflon.

Hydrogen for Transportation and Power

We have already seen that hydrogen is a component of petroleum, and that hydrogen is used in creating nuclear powerboth deadly and peaceful varieties. But hydrogen has been applied in many other ways in the transportation and power industries.

There are only three gases practical for lifting a balloon: hydrogen, helium, and hot air. Each is much less dense than ordinary air, and this gives them their buoyancy. Because hydrogen is the lightest known gas and is relatively cheap to produce, it initially seemed the ideal choice, particularly for airships, which made their debut near the end of the nineteenth century.

For a few decades in the early twentieth century, airships were widely used, first in warfare and later as the equivalent of luxury liners in the skies. One of the greatest such craft was Germany's Hindenburg, which used hydrogen to provide buoyancy. Then, on May 6, 1937, the Hindenburg caught fire while mooring at Lakehurst, New Jersey, and 36 people were killeda tragic and dramatic event that effectively ended the use of hydrogen in airships.

Adding to the pathos of the Hindenburg crash was the voice of radio announcer Herb Morrison, whose audio report has become a classic of radio history. Morrison had come to Lakehurst to report on the landing of the famous airship, but ended up with the biggestand most horrifyingstory of his career. As the ship burst into flames, Morrison's voice broke, and he uttered words that have become famous:"Oh, the humanity!"

Half a century later, a hydrogen-related disaster destroyed a craft much more sophisticated than the Hindenburg, and this time, the medium of television provided an entire nation with a view of the ensuing horror. The event was the explosion of the space shuttle Challenger on January 28, 1986, and the cause was the failure of a rubber seal in the shuttle's fuel tanks. As a result, hydrogen gas flooded out of the craft and straight into the jet of flame behind the rocket. All seven astronauts aboard were killed.


Despite the misfortunes that have occurred as a result of hydrogen's high flammability, the element nonetheless holds out the promise of cheap, safe power. Just as it made possible the fusion, or hydrogen, bombwhich fortunately has never been dropped in wartime, but is estimated to be many hundreds of times more lethal than the fission bombs dropped on Japanhydrogen may be the key to the harnessing of nuclear fusion, which could make possible almost unlimited power.

A number of individuals and agencies advocate another form of hydrogen power, created by the controlled burning of hydrogen in air. Not only is hydrogen an incredibly clean fuel, producing no by-products other than water vapor, it is available in vast quantities from water. In order to separate it from the oxygen atoms, electrolysis would have to be appliedand this is one of the challenges that must be addressed before hydrogen fuel can become a reality.

Electrolysis requires enormous amounts of electricity, which would have to be produced before the benefits of hydrogen fuel could be realized. Furthermore, though the burning of hydrogen could be controlled, there are the dangers associated with transporting it across country in pipelines. Nonetheless, a number of advocacy groupssome of whose Web sites are listed belowcontinue to promote efforts toward realizing the dream of nonpolluting, virtually limitless, fuel.


American Hydrogen Association (Web site). <> (June 1, 2001).

Blashfield, Jean F. Hydrogen. Austin, TX: Raintree Steck-Vaughn, 1999.

Farndon, John. Hydrogen. New York: Benchmark Books, 2001.

"Hydrogen" (Web site). <> (June 1, 2001).

Hydrogen Energy Center (Web site). <> (June 1, 2001).

Hydrogen Information Network (Web site). <> (June 1, 2001).

Knapp, Brian J. Carbon Chemistry. Illustrated by David Woodroffe. Danbury, CT: Grolier Educational, 1998.

Knapp, Brian J. Elements. Illustrated by David Woodroffe and David Hardy. Danbury, CT: Grolier Educational, 1996.

National Hydrogen Association (Web site). <> (June 1, 2001).

Uehling, Mark. The Story of Hydrogen. New York: Franklin Watts, 1995.



A type of chemical bonding in which two atoms share valence electrons.


A term describing an element that exists as molecules composed of two atoms.


A term describing the distribution of valence electrons when hydrogen atomswhich end up with only two valence electronsexperience chemical bonding with other atoms. Most other elements follow the octet rule.


The use of an electric current to cause a chemical reaction.


A nuclear reaction involving the splitting of atoms.


A nuclear reaction that involves the joining of atomic nuclei.


Any chemical compound whose molecules are made up of nothing but carbon and hydrogen atoms.


A chemical reaction in which hydrogen atoms are added to carbon multiple bonds, as in a hydrocarbon.


An atom or group of atoms that has lost or gained one or more electrons, and thus has a net electrical charge.


A form of chemical bonding that results from attractions between ions with opposite electric charges. The bonding of a metal to a nonmetal such as hydrogen is ionic.


Atoms that have an equal number of protons, and hence are of the same element, but differ in their number of neutrons. This results in a difference of mass. An isotope may either be stable or radioactive.


The center of an atom, a region where protons and neutrons are located, and around which electrons spin. The plural of "nucleus" is nuclei.


A term describing the distribution of valence electrons that takes place in chemical bonding for most elements, which end up with eight valence electrons. Hydrogen is an exception, and follows the duet rule.


At one time, chemists used the term "organic" only in reference to living things. Now the word is applied to most compounds containing carbon, with the exception of calcium carbonate (lime-stone) and oxides such as carbon dioxide.


An isotope subject to the decay associated with radioactivity. A radioisotope is thus an unstable isotope.


A term describing a hydrocarbon in which each carbon is already bound to four other atoms.


An atom or group of atoms whose participation in a chemical, physical, or biological reaction can be easilyobserved. Radioisotopes are often used astracers.


A term describing a hydrocarbon, in which the carbons involved in a multiple bond are free to bond with other atoms.


Electrons that occupy the highest principal energy level in an atom. These are the electrons involved in chemical bonding.

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Hydrogen (revised)


Note: This article, originally published in 1998, was updated in 2006 for the eBook edition.


Hydrogen is the most abundant element in the universe. Nearly nine out of every ten atoms in the universe are hydrogen atoms. Hydrogen is also common on the Earth. It is the third most abundant element after oxygen and silicon. About 15 percent of all the atoms found on the Earth are hydrogen atoms.

Hydrogen is also the simplest of all elements. Its atoms consist (usually) of one proton and one electron.

Hydrogen was first discovered in 1766 by English chemist and physicist Henry Cavendish (1731-1810). Cavendish was also the first person to prove that water is a compound of hydrogen and oxygen.

Some experts believe that hydrogen forms more compounds than any other element. These compounds include water, sucrose (table sugar), alcohols, vinegar (acetic acid), household lye (sodium hydroxide), drugs, fibers, dyes, plastics, and fuels.




Group 1 (IA)


Discovery and naming

Hydrogen was probably "discovered" many times. Many early chemists reported finding a "flammable gas" in some of their experiments. In 1671, for example, English chemist Robert Boyle (1627-91) described experiments in which he added iron to hydrochloric acid (HCl) and sulfuric acid (H2SO4). In both cases, a gas that burned easily with a pale blue flame was produced.

The problem with these early discoveries was that chemists did not understand the nature of gases very well. They had not learned that there are many kinds of gases. They thought that all the "gases" they saw were some form of air with impurities in it.

Cavendish discovered hydrogen in experiments like those that Boyle performed. He added iron metal to different acids and found that a flammable gas was produced. But Cavendish thought the flammable gas came from the iron and not from the acid. Chemists later showed that iron is an element and does not contain hydrogen or anything else. Therefore, the hydrogen in Cavendish's experiment came from the acid:

Hydrogen was named by French chemist Antoine-Laurent Lavoisier (1743-94). Lavoisier is sometimes called the father of modern chemistry because of his many contributions to the science. Lavoisier suggested the name hydrogen after the Greek word for "water former" (that which forms water). (See sidebar on Lavoisier in the oxygen entry in volume 2.)

Physical properties

Hydrogen is a colorless, odorless, tasteless gas. Its density is the lowest of any chemical element, 0.08999 grams per liter. By comparison, a liter of air weighs 1.29 grams, 14 times as much as a liter of hydrogen.

Hydrogen changes from a gas to a liquid at a temperature of -252.77°C (-422.99°F) and from a liquid to a solid at a temperature of -259.2°C (-434.6°F). It is slightly soluble in water, alcohol, and a few other common liquids.

Chemical properties

Hydrogen burns in air or oxygen to produce water:

It also combines readily with other non-metals, such as sulfur, phosphorus, and the halogens. The halogens are the elements that make up Group 17 (VIIA) of the periodic table. They include fluorine, chlorine, bromine, iodine, and astatine. As an example:

Occurrence in nature

Hydrogen occurs throughout the universe in two forms. First, it occurs in stars. Stars use hydrogen as a fuel with which to produce energy. The process by which stars use hydrogen is known as fusion. Fusion is the process by which two or more small atoms are pushed together to make one large atom. In most stars, the primary fusion reaction that occurs is:

This equation shows that four hydrogen atoms are squeezed together (fused) to make one helium atom. In this process, enormous amounts of energy are released in the form of heat and light.

Hydrogen also occurs in the "empty" spaces between stars. At one time, scientists thought that this space was really empty, that it contained no atoms of any kind. But, in fact, this interstellar space (space between stars) contains a small number of atoms, most of which are hydrogen atoms. A cubic mile of interstellar space usually contains no more than a handful of hydrogen and other atoms.

Hydrogen occurs on the Earth primarily in the form of water. Every molecule of water (H2O) contains two hydrogen atoms and one oxygen atom. Hydrogen is also found in many rocks and minerals. Its abundance is estimated to be about 1,500 parts per million. That makes hydrogen the tenth most abundant element in the Earth's crust.

Hydrogen also occurs to a very small extent in the Earth's atmosphere. Its abundance there is estimated to be about0.000055 percent. Hydrogen is not abundant in the atmosphere because it has such a low density. The Earth's gravity is not able to hold on to hydrogen atoms very well. They float away into outer space very easily. Most of the hydrogen that was once in the atmosphere has now escaped into outer space.


There are three isotopes of hydrogen, hydrogen-1, hydrogen-2, and hydrogen-3. Isotopes are two or more forms of an element. Isotopes differ from each other according to their mass number. The number written to the right of the element's name is the mass number. The mass number represents the number of protons plus neutrons in the nucleus of an atom of the element. The number of protons determines the element, but the number of neutrons in the atom of any one element can vary. Each variation is an isotope.

The three isotopes of hydrogen have special names. Hydrogen-1 is sometimes called protium. It is the simplest and most common form of hydrogen. Protium atoms all contain one proton and one electron. About 99.9844 percent of the hydrogen in nature is protium.

The man who gave hydrogen its name, Antoine-Laurent Lavoisier, is sometimes called the father of modern chemistry.

Hydrogen-2 is known as deuterium. A deuterium atom contains one proton, one electron, and one neutron. About 0.0156 percent of the hydrogen in nature is deuterium.

The third isotope of hydrogen, hydrogen-3, is tritium. An atom of tritium contains one proton, one electron, and two neutrons. There are only very small traces of tritium in nature.

Tritium is a radioactive isotope. A radioactive isotope is one that breaks apart and gives off some form of radiation. Some radioactive isotopes (such as tritium) occur in nature. They can also be produced in the laboratory. Very small particles are fired at atoms. These particles stick in the atoms and make them radioactive. Tritium is a widely used isotope and is now made in large amounts in the laboratory.

Tritium is widely used as a tracer in both industry and research. A tracer is a radioactive isotope whose presence in a system can easily be detected. The isotope is injected into the system at some point. Inside the system, the isotope gives off radiation. That radiation can be followed by means of detectors placed around the system.

Tritium is popular as a tracer because hydrogen occurs in so many different compounds. For example, suppose a scientist wants to trace the movement of water through soil. The scientist can make up a sample of water made with tritium instead of protium. As that water moves through the soil, its path can be followed by means of the radioactivity the tritium gives off.

Tritium is also used in the manufacture of fusion bombs. A fusion bomb is also known as a hydrogen bomb. In a fusion bomb, small atoms are squeezed together (fused) to make a larger atom. In the process, enormous amounts of energy are given off. For example, the first fusion bomb tested by the United States in 1952 had the explosive power of 15 million tons of TNT. A type of fusion bomb fuses tritium with deuterium to make helium atoms:

Stars use hydrogen as a fuel with which to produce energy.


The obvious source for hydrogen is water. The Earth has enough water to supply people's need for hydrogen. The problem is that it takes a lot of energy to split a water molecule:

In fact, it simply costs too much to make hydrogen by this method. The cost of electricity is too high. So it is not economical to make hydrogen by splitting water.

A number of other methods can be used to produce hydrogen, however. For example, steam can be passed over hot charcoal (nearly pure carbon):

The same reaction can be used with steam and other carbon compounds. For example, using methane, or natural gas (CH4), the reaction is:

Hydrogen can also be made by the reaction between carbon monoxide (CO) and steam:

Because hydrogen is such an important element, many other methods for producing it have been invented. However, the preceding methods are the least expensive.


The most important single use of hydrogen is in the manufacture of ammonia (NH3). Ammonia is made by combining hydrogen and nitrogen at high pressure and temperature in the presence of a catalyst. A catalyst is a substance used to speed up or slow down a chemical reaction. The catalyst does not undergo any change during the reaction:

Ammonia is a very important compound. It is used in making many products, the most important of which is fertilizer.

Hydrogen is also used for a number of similar reactions. For example, it can be combined with carbon monoxide to make methanolmethyl alcohol, or wood alcohol (CH3OH):

Tritium (hydrogen-3, the third isotope of hydrogen), is used in the manufacture of fusion bombs.

Like ammonia, methanol has a great many practical uses in a variety of industries. The most important use of methanol is in the manufacture of other chemicals, such as those from which plastics are made. Small amounts are used as additives to gasoline to reduce the amount of pollution released to the environment. Methanol is also used widely as a solvent (to dissolve other materials) in industry.

Another important use of hydrogen is in the production of pure metals. Hydrogen gas is passed over a hot metal oxide to produce the pure metal. For example, molybdenum can be prepared by passing hydrogen over hot molybdenum oxide:

The Hindenburg explosion

T he Hindenburg was Germany's largest passenger airship. It was built in 1936 as a luxury liner, and made the trip to the United States faster than an ocean liner.

The Hindenburg was designed to be filled with helium, a safer gas than the highly flammable hydrogen. But in those post-World War II days, the United States suspected that Germany's new leader, Adolf Hitler (1889-1945), had military plans for helium-filled ships. So the United States refused to sell helium to the Zeppelin air-ship company. Seven million cubic feet of hydrogen was used instead. This made the crew very nervous about the potential for fire. Passengers were even checked for matches as they boarded!

On May 3, 1937, the Hindenburg left Frankfurt, Germany, for Lakehurst, New Jersey. It travelled over the Netherlands, down the English Channel, through Canada, and into the United States. Bad weather forced the ship to slow down several times, lengthening the trip. But it finally approached the field in Lakehurst around 7:00 P.M. on May 6.

After several minutes of maneuvers due to rain and wind, crewmen dropped ropes to the ground at 7:21. The ship was 200 feet above ground. Four minutes later, a small flame emerged on the skin of the ship, and crewmen heard a pop and felt a shudder. Seconds later, the Hindenburg exploded. Flaming hydrogen blasted out of the top. Within 32 seconds, the entire airship had burned, the framework had collapsed, and the entire ship lay smoldering on the ground. Thirty-six people died. Amazingly, 62 survived.

Although claims of sabotage have always surrounded the Hindenburg tragedy, American and German investigators both agreed it was an accident. Both sides concluded that the airship's hydrogen was ignited probably by some type of atmospheric electric discharge. Witnesses had noticed some of the skin of the ship flapping; they also observed the nose of the ship rise suddenly. Both indicate the likelihood that free hydrogen had escaped. The Hindenburg disaster ended lighter-than-air air-ship travel for many decades.

Hydrogenation is an important procedure to the food industry. In hydrogenation, hydrogen is chemically added to another substance. The reaction between carbon monoxide and hydrogen is an example of hydrogenation. Liquid oils are often hydrogenated. Hydrogenation changes the liquid oil to a solid fat. Most kitchens contain foods with hydrogenated or partially hydrogenated oils. Vegetable shortening, such as Crisco, is a good example. Hydrogenation makes it easier to pack and transport oils.

Hydrogen is also used in oxyhydrogen ("oxygen + hydrogen") and atomic hydrogen torches. These torches produce temperatures of a few thousand degrees. At these temperatures, it is possible to cut through steel and most other metals. These torches can also be used to weld (join together with heat) two metals.

Another use for hydrogen is in Lighter-than-air balloons. Hydrogen is the least dense of all gases. So a balloon filled with hydrogen can lift very large loads. Such balloons are not used to carry people. The danger of fire or explosion is too great. On May 6, 1937, a hydrogen fire destroyed the German airship Hindenburg, as it was landing in Lakehurst, New Jersey; 36 people died. Today, hydrogen balloons are used for lifting weather instruments into the upper atmosphere.

One of the best known uses of hydrogen is as a rocket fuel. Many rockets obtain the power they need for lift-off by burning oxygen and hydrogen in a closed tank. The energy produced by this reaction provides thrust to the rocket.

Solving the world's energy problems

M ost people don't worry about filling their cars with gas. They seem to believe that there will always be enough coal, oil, and natural gas to keep civilization running. Those three fuelsthe "fossil fuels"are what keep people on the move today. They fuel cars and trucks, heat homes and offices, and keep factories operating.

But fossil fuels will not last forever. At some point, all the coal, oil, and natural gas will be gone. What source of energy will humans turn to?

Some people believe that hydrogen is the answer. They talk about the day when the age of fossil fuels will be replaced by a hydrogen economy.

"Hydrogen economy" refers to a world in which the burning of hydrogen will be the main source of energy and power. Hydrogen seems to be a good choice for future energy needs. When it burns, it produces only water:

A lot of energy is produced in this reaction. That energy can be used to operate cars, trucks, trains, boats, and airplanes. It can be used as a source of heat for keeping people warm and running chemical reactions.

Why doesn't a hydrogen economy exist today? The answer is easy. It is still too expensive to make hydrogen gas. No one has found a way to remove hydrogen from water or some other source at a low cost. It is still cheaper to mine for coal or drill for oil than to make hydrogen.

But that may not always be true. Some day, someone will find a way to make hydrogen cheaply. When that happens, the day of the hydrogen economy will have arrived.


Millions of hydrogen compounds are known. One of the most important groups of hydrogen compounds is the acids. An acid is any compound that contains hydrogen as its positive part. Common acids include: hydrochloric acid (HCl), sulfuric acid (H2SO4), nitric acid (HNO3), acetic acid (HC2H3O2), phosphoric acid (H3PO4), and hydrofluoric acid (HF).

Acids are present in thousands of natural substances and artificial products. The following list gives a few examples: vinegar, or acetic acid (HC2H3O2); sour milk, or lactic acid (C3H6O3); lemons and other citrus fruits, or citric acid (C6H8O7); soda water, or carbonic acid (H2CO3); battery acid, or sulfuric acid H2SO4); and boric acid (H3BO3).

Health effects

Hydrogen is essential to every plant and animal. Nearly every compound in a living cell contains hydrogen. It is harmless to humans unless taken in very large amounts. In this case, it is dangerous only because it cuts off the supply of oxygen humans need to breathe.

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hydrogen (hī´drəjən) [Gr.,=water forming], gaseous chemical element; symbol H; at. no. 1; interval in which at. wt. ranges 1.00784–1.00811; m.p. -259.14°C; b.p. -252.87°C; density 0.08988 grams per liter at STP; valence usually +1.

The Isotopes and Forms

Atmospheric hydrogen is a mixture of three isotopes. The most common is called protium (mass no. 1, atomic mass 1.007822); the protium nucleus (protium ion) is a proton. A second isotope of hydrogen is deuterium (mass no. 2, atomic mass 2.0140), the so-called heavy hydrogen, often represented in chemical formulas by the symbol D. The deuterium nucleus, or ion, is called the deuteron; it consists of a proton plus a neutron. The two isotopes are found in atmospheric hydrogen in the proportion of about 1 atom of deuterium to every 6,700 atoms of protium. Protium and deuterium differ slightly in their chemical and physical properties; for example, the boiling point of deuterium is about 3°C lower than protium. The properties of compounds they form differ depending on the ratio of the two isotopes present.

Deuterium oxide (D2O), the so-called heavy water, is present in ordinary water; the concentration of deuterium oxide is increased by electrolysis of the water. The melting point (3.79°C), boiling point (101.4°C), and specific gravity (1.107 at 25°C) of deuterium oxide are higher than those of ordinary water. Deuterium oxide is used as a moderator in nuclear reactors. Deuterium is also of importance because of the wide use it has found in scientific research; for example, chemical reaction mechanisms have been studied by the use of deuterium atoms as tracers (i.e., deuterium is substituted for atoms of ordinary hydrogen in compounds), making it possible to follow the course of individual molecules in a reaction.

Tritium (mass no. 3, atomic mass 3.016), a third hydrogen isotope, is a radioactive gas with a half-life of about 121/4 years; it is often represented in chemical formulas by the symbol T. It is produced in nuclear reactors and occurs to a very limited extent in atmospheric hydrogen. It is used in the hydrogen bomb, in luminous paints, and as a tracer. The tritium nucleus, or ion, is called the triton; it consists of a proton plus two neutrons. Tritium oxide (T2O) has a melting point (4.49°C) higher than that of deuterium oxide.

Besides being a mixture of three isotopes, hydrogen is a mixture of two forms, an ortho form and a para form, which differ in their electronic and nuclear spins. At room temperature atmospheric hydrogen is about 3/4ortho-hydrogen and 1/4para-hydrogen. The two forms differ slightly in their physical properties.


Under ordinary conditions hydrogen is a colorless, odorless, tasteless gas that is only slightly soluble in water; it is the least dense gas known. It is the first element in Group 1 of the periodic table. Ordinary hydrogen gas is made up of diatomic molecules (H2) that react with oxygen to form water (H2O) and hydrogen peroxide (H2O2), usually as a result of combustion. A jet of hydrogen burns in air with a very hot blue flame. The flame produced by a mixture of oxygen and hydrogen gases (as in the oxyhydrogen blowpipe) is extremely hot and is used in welding and to melt quartz and certain glasses. Hydrogen gas must be used with caution because it is highly flammable; it forms easily ignited explosive mixtures with oxygen or with air (because of the oxygen in the air). At high temperatures hydrogen is a chemically active mixture of monohydrogen (atomic hydrogen) and the normal diatomic hydrogen (see allotropy).

Hydrogen has a great affinity for oxygen and is a powerful reducing agent (see oxidation and reduction). It reacts with nitrogen to form ammonia. With the halogens it forms compounds (hydrogen halides) that are strongly acidic in water solution. With sulfur it forms hydrogen sulfide (H2S), a colorless gas with an odor like rotten eggs; with sulfur and oxygen it forms sulfuric acid. It combines with several metals to form metal hydrides such as calcium hydride. Combined with carbon (and usually other elements) it is a constituent of a great many organic compounds, such as hydrocarbons, carbohydrates, fats, oils, proteins, and organic acids and bases.

It is theoretically possible for hydrogen to exhibit the properties of a metal, such as electrical conductivity. Although researchers have been able to squeeze hydrogen into liquid and crystalline solid states through applications of intense heat, cold, and pressure, the metallic form eluded them until 1996. By compressing liquid hydrogen to nearly 2 million atmospheres pressure and a temperature of 4,400°K, a team at the Lawrence Livermore National Laboratory created metallic hydrogen for a millionth of a second. While there is no practical application for the accomplishment, proof of the existence of a metallic form of hydrogen may have implications for theories of how Jupiter's magnetic field is produced.

Sources and Commercial Preparation

While hydrogen is only about one part per million in the atmosphere, it is the most abundant element in the universe. It is believed that hydrogen makes up about three quarters of the mass of the universe, or over 90% of the molecules. It is found in the sun and in other stars, where it is the major fuel in the fusion reactions (see nucleosynthesis) from which stars derive their energy.

Hydrogen is prepared commercially by catalytic reaction of steam with hydrocarbons, by the reaction of steam with hot coke (carbon), by the electrolysis of water, and by the reaction of mineral acids on metals. Millions of cubic feet of hydrogen gas are produced daily in the United States alone.


Hydrogen was formerly used for filling balloons, airships, and other lighter-than-air craft, a dangerous practice because of hydrogen's explosive flammability; there were disastrous fires, e.g., the immolation of the German airship Hindenburg at its mooring at Lakehurst, N.J., in 1937. Helium is preferable for use in lighter-than-air craft since it is not flammable. Hydrogen is used in the Haber process for the fixation of atmospheric nitrogen, in the production of methanol, and in hydrogenation of fats and oils. It is also important in low-temperature research. It can be liquefied under pressure and cooled; when the pressure is released, rapid evaporation takes place and some of the hydrogen solidifies.

Discovery of Hydrogen and Its Isotopes

Although hydrogen was prepared many years earlier, it was first recognized as a substance distinct from other flammable gases in 1766 by Henry Cavendish, who is credited with its discovery; it was named by A. L. Lavoisier in 1783. Deuterium was discovered by H. C. Urey, F. G. Brickwedde, and G. M. Murphy in 1932, although its existence had been suspected for some years. Deuterium oxide was also discovered by Urey and was first obtained in nearly pure form by G. N. Lewis. Tritium was synthesized by Ernest Rutherford, L. E. Oliphant, and Paul Harteck in 1935.

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Hydrogen is the simplest of all chemical elements. It is a colorless, odorless, tasteless gas that burns in air to produce water. It has one of the lowest boiling points, 252.9°C (423.2°F), and freezing points, 259.3°C (434.7°F), of all elements.

An atom of hydrogen contains one proton and one electron, making it the simplest atom that can be constructed. Because of the one proton in its nucleus, hydrogen is assigned an atomic number of 1. A total of three isotopes of hydrogen exist. Isotopes are forms of an element with the same atomic number but different atomic masses. Protium and deuterium are both stable isotopes, but tritium is radioactive.

Hydrogen is the first element in the periodic table. Its box is situated at the top of Group 1 in the periodic table, but it is not generally considered a member of the alkali family, the other elements that make up Group 1. Its chemical properties are unique among the elements, and it is usually considered to be in a family of its own.

The Hydrogen Economy

Some social scientists have called the last century of human history the Fossil Age. That term comes from the fact that humans have relied so heavily on the fossil fuelscoal, oil, and natural gasfor the energy we need to run our societies. What happens when the fossil fuels are exhausted? Where will humans turn for a new supply of energy?

A new generation of scientists is suggesting the use of hydrogen as a future energy source. Hydrogen burns in air or oxygen with a very hot flame that can be used to generate steam, electricity, and other forms of energy. The only product of that reaction is water, a harmless substance that can be released to the environment without danger. In addition, enormous amounts of hydrogen are available from water. Electrolysis can be used to obtain hydrogen from the world's lakes and oceans.

An economy based on hydrogen rather than the fossil fuels faces some serious problems, however. First, hydrogen is a difficult gas with which to work. It catches fire easily and, under certain circumstances, does so explosively. Also, the cost of producing hydrogen by electrolysis is currently much too high to make the gas a useful fuel for everyday purposes.

Of course, once the fossil fuels are no longer available, humans may have no choice but to solve these problems in order to remain a high-energy-use civilization.


Hydrogen was discovered in 1766 by English chemist and physicist Henry Cavendish (17311810). It was named by French chemist Antoine-Laurent Lavoisier (17431794) from the Greek words for "water-former." Early research on hydrogen was instrumental in revealing the true nature of oxidation (burning) and, therefore, was an important first step in the birth of modern chemistry.


Hydrogen is by far the most abundant element in the universe. It makes up about 93 percent of all atoms in the universe and about three-quarters of the total mass of the universe. Hydrogen occurs both within stars and in the interstellar space (the space between stars). Within stars, hydrogen is consumed in nuclear reactions by which stars generate their energy.

Hydrogen is much less common as an element on Earth. Its density is so low that it long ago escaped from Earth's gravitational attraction. Hydrogen does occur on Earth in a number of compounds, however, most prominently in water. Water is the most abundant compound on Earth's surface.

Hydrogen also occurs in nearly all organic compounds and constitutes about 61 percent of all the atoms found in the human body. Chemists now believe that hydrogen forms more compounds than any other element, including carbon.

The Isotopes of Hydrogen

Atomic Atomic Percent of
Name Nucleus Number Mass Hydrogen Atoms
Protium 1 proton 1 1 99.985
Deuterium 1 proton; 1 neutron 1 2 0.015
Tritium 1 proton; 2 neutrons 3 trace

Properties and uses

Hydrogen is a relatively inactive element at room temperature, but it becomes much more active at higher temperatures. For example, it burns in air or pure oxygen with a pale blue, almost invisible flame. It can also be made to react with most elements, both metals and nonmetals. When combined with metals, the compounds formed are called hydrides. Some familiar compounds of hydrogen with nonmetals include ammonia (NH3), hydrogen sulfide (H2S), hydrogen chloride (or hydrochloric acid, HCl), hydrogen fluoride (or hydrofluoric acid, H2F2), and water (H2O).

The largest single use of hydrogen is in the production of ammonia. Ammonia, in turn, is used in the production of fertilizers and as a fertilizer itself. It is also a raw material for the production of explosives. Large amounts of hydrogen are also employed in hydrogenation, the process by which hydrogen is reacted with liquid oils to convert them to solid fats. Hydrogen is used in the production of other commercially important chemicals as well, most prominently, hydrogen chloride. Finally, hydrogen acts as a reducing agent in many industrial processes. A reducing agent is a substance that reacts with a metallic ore to convert the ore into a pure metal.

[See also Periodic table ]

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melting point: 259.14°C
boiling point: 252.87°C
density: 0.08988 g/L
most common ions: H+, H

Hydrogen was first recognized as a gaseous substance in 1766 by English chemist and physicist Henry Cavendish. The abundance of hydrogen in Earth's crust is 1,520 parts per million. The abundance of hydrogen in the universe by weight is 74 percent and by number of atoms is 90 percent. Hence, hydrogen is the major constituent of the universe. Under ordinary conditions (STP) on Earth, hydrogen is a colorless, odorless, tasteless gas that is only slightly soluble in water. It is the least dense gas known (0.08988 grams per liter at STP). Ordinary hydrogen gas (H2) exists as diatomic molecules. It reacts with oxygen to form its major compound on Earth, water (H2O). It also reacts with nitrogen, halogens , and sulfur, to form ammonia (NH3), hydrogen monohalide compounds (e.g., HCl) and hydrogen sulfide (H2S), respectively. It combines with several metals to form metal hydrides, and carbon to form a great many organic compounds.

Hydrogen is a mixture of three isotopes : protium (1H; atomic mass 1.007822); deuterium, or heavy hydrogen (2H or D; atomic mass 2.0140; 1 atom of 2H to every 6,700 atoms of 1H); and tritium (3H or T; atomic mass 3.016; has a radioactive nucleus). The fusion of protium nuclei (protons) to form helium is believed to be the major source of the Sun's energy. The extreme heat of reaction in hydrogen-oxygen burning is used in high temperature welding and melting processes. Hydrogen molecule addition reactions (hydrogenation) are widely used in industry, for example, for the hardening of animal fats or vegetable oils, for the synthesis of methanol from carbon monoxide, and in petroleum refining.

see also Cavendish, Henry; Explosions; Gases.

Ágúst Kvaran


Rigden, John S. (2002). Hydrogen: The Essential Element. Cambridge, MA: Harvard University Press.

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hydrogen (symbol H) Gaseous, nonmetallic element, first identified as a separate element in 1766 by English chemist and physicist Henry Cavendish. Colourless and odourless, hydrogen is the lightest and most abundant element in the universe (76% by mass), mostly found combined with oxygen in water. It is used to manufacture ammonia by the Haber process and in rocket fuels. Liquid hydrogen is used in cryogenics, including freezing human bodies with the hope of reviving them in the future. Properties: 1; r.a.m. 1.00797; r.d. 0.0899; m.p. −259.1°C (−434.4°F); b.p. −252.9°C (−423.2°F); most common isotope H1 (99.985%).

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hy·dro·gen / ˈhīdrəjən/ • n. a colorless, odorless, highly flammable gas, the chemical element of atomic number 1. (Symbol: H) DERIVATIVES: hy·drog·e·nous / hīˈdräjənəs/ adj.

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"hydrogen." The Oxford Pocket Dictionary of Current English. . 15 Dec. 2017 <>.

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hydrogen (hy-drŏ-jĕn) n. a colourless gas that is combined with oxygen to form water (H2O) and with various other molecules (chiefly carbon and oxygen) to form all organic compounds. Symbol: H. h. ion concentration see pH.

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hydrogen XVIII. — F. hydrogène, f. Gr. húdōr, hudr- WATER; see -GEN.

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hydrogenabrasion, Australasian, equation, Eurasian, evasion, invasion, occasion, persuasion, pervasion, suasion, Vespasianadhesion, cohesion, Friesian, lesion •circumcision, collision, concision, decision, derision, division, elision, envision, excision, imprecision, incision, misprision, precisian, precision, provision, scission, vision •subdivision • television • Eurovision •LaserVision •corrosion, eclosion, erosion, explosion, implosion •allusion, collusion, conclusion, confusion, contusion, delusion, diffusion, effusion, exclusion, extrusion, fusion, illusion, inclusion, interfusion, intrusion, obtrusion, occlusion, preclusion, profusion, prolusion, protrusion, reclusion, seclusion, suffusion, transfusion •Monaghan • Belgian •Bajan, Cajun, contagion, TrajanGlaswegian, legion, Norwegian, region •irreligion, religion •Injun • Harijan • oxygen • antigen •sojourn • donjon • Georgian •theologian, Trojan •Rügen •bludgeon, curmudgeon, dudgeon, gudgeon, trudgen •dungeon • glycogen • halogen •collagen • Imogen • carcinogen •hallucinogen • androgen •oestrogen (US estrogen) •hydrogen • nitrogen •burgeon, sturgeon, surgeon

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