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Nonmetals

NONMETALS

CONCEPT

Nonmetals, as their name implies, are elements that display properties quite different from those of metals. Generally, they are poor conductors of heat and electricity, and they are not ductile: in other words, they cannot be easily reshaped. Included in this broad grouping are the six noble gases, the five halogens, and eight "orphan" elements. Two of these eighthydrogen and carbonare so important that separate essays are devoted to them. Two more, addressed in this essay, are absolutely essential to human life: oxygen and nitrogen. Hydrogen and helium, a nonmetal of the noble gas family, together account for about 99% of the mass of the universe, while Earth and the human body are composed primarily of oxygen, with important components of carbon, nitrogen, and hydrogen. Indeed, much of lifehuman, animal, and plantcan be summed up with these four elements, which together make Earth different from any other known planet. Among the other "orphan" nonmetals are phosphorus and sulfur, the "brimstone" of the Bible, as well as boron and selenium.

HOW IT WORKS

Defining the Nonmetals

The majority of elements on the periodic table are metals: solids (along with one liquid, mercury) which are lustrous or shiny in appearance. Metals are ductile, or malleable, meaning that they can be molded into different shapes without breaking. They are excellent conductors of heat and electricity, and tend to form positive ions by losing electrons. The vast majority of elements87 in allare metals, and these occupy the left, center, and part of the right-hand side of the periodic table.

With the exception of hydrogen, placed in Group 1 above the alkali metals, the nonmetals fill a triangle-shaped space in the upper right corner of the periodic table. All gaseous elements are nonmetals, a group that also includes some solids, as well as bromine, the only element other than mercury that is liquid at room temperature. In general, the nonmetals are characterized by properties opposite those of metals. They are poor conductors of heat and electricity (though carbon, a good electrical conductor, is an exception), and they tend to form negative ions by gaining electrons. They are not particularly malleable, and whereas most metals are shiny, nonmetals may be dull-colored, black (in the case of carbon in some forms or allotropes), or invisible, as is the case with most of the gaseous nonmetals.

Grouping the Nonmetals

Whereas the metals can be broken down into five families, along with seven "orphans"elements that do not fit into a family groupingthe nonmetals are arranged in two families, as well as eight "orphans." Hydrogen, because of its great abundance in the universe and its importance to chemical studies, is addressed in a separate essay within this book. The same is true of carbon: though not nearly as abundant as hydrogen, carbon is a common element of all living things, and therefore it is discussed in the essay on Carbons and Organic Chemistry.

The other six "orphan" nonmetals, which will be examined in detail later in this essay, are listed below by atomic number:

  • 5. Boron (B)
  • 7. Nitrogen (N)
  • 8. Oxygen (O)
  • 15. Phosphorus (P)
  • 16. Sulfur (S)
  • 34. Selenium (Se)

THE NOBLE GASES.

The noble gases, discussed in detail within an essay devoted to that subject, are listed below by atomic number:

  • 2. Helium (He)
  • 10. Neon (Ne)
  • 18. Argon (Ar)
  • 36. Krypton (Kr)
  • 54. Xenon (Xe)
  • 86. Radon (Rn)

Occupying Group 8 of the North American periodic table, the noble gaseswith the exception of heliumhave valence electron configurations of ns2np6. This means that they have two valence electrons (the electrons involved in chemical bonding) on the orbital designated as s, and six more on the p orbital. As for n, this designates the energy level, a number that corresponds to the period or row that an element occupies on the periodic table.

Most elements bond according to what is known as the octet rule, forming a shell composed of eight electrons. Since the noble gases already have eight electrons on their shell, they tend not to bond with other elements: hence the name "noble," which in this context means "set apart." Helium, on the other hand, has a valence shell of s2; however, it too is characterized by a lack of reactivity, and therefore is included in the noble gas family. Noble gases are all monatomic, meaning that they exist as individual atoms, rather than in molecules. To put it another way, their molecules are single atoms. (By contrast, atoms of oxygen, for instance, usually combine to form a diatomic molecule, designated O2.)

Due to their apparent lack of reactivity, the noble gasesalso known as the rare gaseswere once called the inert (inactive) gases. Indeed, helium, neon, and argon have not been found to combine with other elements to form compounds. However, in 1962, English chemist Neil Bartlett (1932-) succeeded in preparing a compound of xenon with platinum and fluorine (XePtF6), thus overturning the idea that the noble gases were entirely "inert." Since that time, numerous compounds of xenon with other elements, most notably oxygen and fluorine, have been developed. Fluorine has also been used to form simple compounds with krypton and radon.

THE HALOGENS.

Next to the noble gases, in Group 7, are the halogens, a family discussed in a separate essay. These are listed below, along with atomic number and chemical symbol. Note that astatine is sometimes included with the metalloids, elements that display characteristics both of metals and nonmetals.

  • 9. Fluorine (F)
  • 17. Chlorine (Cl)
  • 35. Bromine (Br)
  • 53. Iodine (I)
  • 85. Astatine (At)

The halogens all have valence electron configurations of ns2np5: in other words, at any energy level n, they have two valence electrons in the s orbital, and 5 in the p orbital. In terms of phase of matter, the halogens are the most varied family on the periodic table. Fluorine and chlorine are gases, iodine is a solid, and bromine (as noted earlier) is one of only two elements existing at room temperature as a liquid. As for astatine, it is a solid too, but so highly radioactive it is hard to determine much about its properties. (The only other nonmetal that has a large number of radioactive isotopes is the noble gas radon, considered highly dangerous.)

Though they are "next door" to the noble gases, the halogens could not be more different. Whereas their neighbors to the right are the least reactive elements on the periodic table, the halogens are the most reactive. Indeed, fluorine has the highest possible value of electronegativity, the relative ability of an atom to attract valence electrons. It is therefore one of the only elements that will bond with a noble gas.

All of the halogens tend to form salts, compoundsformed, along with water, from the reaction of an acid and basethat bring together a metal and a nonmetal. Due to this tendency, the first of the family to be isolatedchlorine, in 1811was originally named "halogen," a combination of the Greek words halos, or salt, and gennan, "to form or generate." In their pure form, halogens are diatomic, and in contact with other elements, they form ionic bonds, which are the strongest form of chemical bond. In the process of bonding, they become negatively charged ions, or anions.

Abundance of the Nonmetals

IN THE UNIVERSE.

As is stated many times throughout this book, humans may be created equal, but the elements are not. Though 88 elements exist in nature, this certainly does not mean that each occupies a 1/88 share of the total pie. Just two elementsthe nonmetals hydrogen and helium, which occupy the first two positions on the periodic tableaccount for 99.9% of the matter in the entire universe, yet the percentage of matter they make up on Earth is very small.

The reason for this disparity is that, whereas our own planet (as far as we know) is unique in sustaining liferequiring oxygen, carbon, nitrogen, and other elementsthe vast majority of the universe is made up of stars, composed primarily of hydrogen and helium.

ON EARTH AND IN THE ATMOSPHERE.

Nonmetals account for a large portion of Earth's total known elemental massthat is, the composition of the planet's crust, waters, and atmosphere. Listed below are figures ranking nonmetals within the overall picture of elements on Earth. The numbers following each element's name indicate the percentage of each within the planet's total known elemental mass.

  • 1. Oxygen (49.2%)
  • 9. Hydrogen (0.87%)
  • 11. Chlorine (0.19%)
  • 12. Phosphorus (0.11%)
  • 14. Carbon (0.08%)
  • 15. Sulfur (0.06%)
  • 17. Nitrogen (0.03%)
  • 18. Fluorine (0.03%)

Some of the figures above may seem rather small, but in fact only 18 elements account for all but 0.49% of the planet's known elemental mass, the remainder being composed of numerous other elements in small quantities. In Earth's atmosphere, the composition is all nonmetallic, as one might expect:

  • 1. Nitrogen (78%)
  • 2. Oxygen (21%)
  • 3. Argon (0.93%)
  • 4. Various trace gases, including water vapor, carbon dioxide, and ozone or O3(0.07%)

IN THE HUMAN BODY.

As noted earlier, carbon is a component in all living things, and it constitutes the second-most abundant element in the human body. Together with oxygen and hydrogen, it accounts for 93% of the body's mass. Listed below are the most abundant nonmetals in the human body, by ranking.

  • 1. Oxygen (65%)
  • 2. Carbon (18%)
  • 3. Hydrogen (10%)
  • 4. Nitrogen (3%)
  • 6. Phosphorus (1.0%)
  • 9. Sulfur (0.26%)
  • 11. Chlorine (0.14%)

REAL-LIFE APPLICATIONS

Boron

The first "orphan" nonmetal, by atomic number, is boron, named after the Arabic word buraq or the Persian burah. (It is thus unusual in being one of the only elements whose name is not based on a word from a European language, orfor the later elementsthe name of a person or place.) It was discovered in 1808 by English chemist Sir Humphry Davy (1778-1829), a man responsible for identifying or isolating numerous elements; French physicist and chemist Joseph Gay-Lussac (1778-1850), known in part for the gas law named after him; and French chemist Louis Jacques Thénard (1777-1857). These scientists used the reaction of boric acid (H3BO3) with potassium to isolate the element.

Sometimes classified as a metalloid because it is a semiconductor of electricity, boron is applied in filaments used in fiber optics research. Few of its other applications, however, have much to do with electrical conductivity. Boric or boracic acid is used as a mild antiseptic, and in North America, it is applied for the control of cockroaches, silverfish, and other pests. A compound known as borax is a water softener in washing powders, while other compounds are used to produce enamels for the coating of refrigerators and other appliances. Compounds involving boron are also present in pyrotechnic flares, because they emit a distinctive green color, and in the igniters of rockets.

Nitrogen

Though most people think of air as consisting primarily of oxygen, in factas noted abovethe greater part of the air we breathe is made up of nitrogen. Scottish chemist David Rutherford (1749-1819) is usually given credit for discovering the element: in 1772, he identified nitrogen as the element that remained when oxygen was removed from air. Several other scientists around the same time made a similar discovery.

Because of its heavy presence in air, nitrogen is obtained primarily by cooling air to temperatures below the boiling points of its major components. Nitrogen boils (that is, turns into a gas) at a lower temperature than oxygen: 320.44°F (195.8°C), as opposed to 297.4°F (183°C). When air is cooled to 328°F (200°C) and then allowed to warm slowly, the nitrogen boils first, and therefore evaporates first. The nitrogen gas is captured, cooled, and liquefied once more.

Nitrogen can also be obtained from compounds such as potassium nitrate or saltpeter, found primarily in India; or sodium nitrate (Chilean saltpeter), which comes from the desert regions of Chile. To isolate nitrogen, various processes are undertaken in a laboratoryfor instance, heating barium azide or sodium azide, both of which contain nitrogen.

REACTIONS WITH OTHER ELEMENTS.

Existing as diatomic molecules, nitrogen forms very strong triple bonds, and as a result tends to be fairly unreactive at low temperatures. Thus, for instance, when a substance burns, it reacts with the oxygen in the air, but not with the nitrogen. At high temperatures, however, nitrogen combines with other elements, reacting with metals to form nitrides; with hydrogen to form ammonia; with O2 to form nitrites; and with O3 to form nitrates.

Nitrogen and oxygen, in particular, react at high temperatures to form numerous compounds: nitric oxide (NO), nitrous oxide (N2O), nitrogen dioxide (NO2), dinitrogen trioxide (N2O3), and dinitrogen pentoxide (N2O5). In reaction with halogens, nitrogen forms unstable, explosive compounds such as nitrogen trifluoride (NF3), nitrogen trichloride (NCl3), and nitrogen triiodide (NI3). One thing is for sure: never mix ammonia with bleach, which involves the halogen chlorine. Together, these two produce chloramines, gases with poisonous vapors.

USES FOR NITROGEN.

One of the most striking scenes in a memorable film, "Terminator 2: Judgment Day" (1991), occurs near the end of the movie, when the villainous T-1000 robot steps into a pool of liquid nitrogen and cracks to pieces. As noted, nitrogen must be at extremely low temperatures to assume liquid form. Liquid nitrogen, which accounts for about one-third of all commercial uses of the element, is applied for quick-freezing foods, and for preserving foods in transit. Liquid nitrogen also makes it possible to process materials, such as some forms of rubber, that are too pliable for machining at room temperature. These materials are first cooled in liquid nitrogen, and then become more rigid.

In processing iron or steel, which forms undesirable oxides if exposed to oxygen, a blanket of nitrogen is applied to prevent this reaction. The same principle is applied in making computer chips and even in processing foods, since these items too are detrimentally affected by oxidation. Because it is far less combustible than air (magnesium is one of the few elements that burns nitrogen in combustion), nitrogen is also used to clean tanks that have carried petroleum or other combustible materials.

As noted, nitrogen combines with hydrogen to form ammonia, used in fertilizers and cleaning materials. Ammonium nitrate, applied primarily as a fertilizer, is also a dangerous explosive, as shown with horrifying effect in the bombing of the Alfred P. Murrah Federal Building in Oklahoma City on April 19, 1995a tragedy that took 168 lives. Nor is ammonium nitrate the only nitrogen-based explosive. Nitric acid is used in making trinitrotoluene (TNT), nitroglycerin, and dynamite, as well as gunpowder and smokeless powder.

NITROGEN, THE ENVIRONMENT, AND HEALTH.

The nitrogen cycle is the process whereby nitrogen passes from the atmosphere into living things, and ultimately back into the atmosphere. Both through the action of lightning in the sky, and of bacteria in the soil, nitrogen is converted to nitrates and nitritescompounds of nitrogen and oxygen. These are then absorbed by plants to form plant proteins, which convert to animal proteins in the bodies of animals who eat the plants. When an animal dies, the proteins are returned to the soil, and denitrifying bacteria break down these compounds, returning elemental nitrogen to the atmosphere.

Not all nitrogen in the atmosphere is healthful, however. Oxides of nitrogen, formed in the high temperatures of internal combustion engines, pass into the air as nitric oxide. This compound reacts readily with oxygen in the air to form nitrogen dioxide, a toxic reddish-brown gas that adds to the tan color of smog over major cities.

Another health concern is posed by sodium nitrate and sodium nitrite, added to bacon, sausage, hot dogs, ham, bologna and other food products to inhibit the growth of harmful microorganisms. Many researchers believe that nitrites impair the ability of a young child's blood to carry oxygen. Furthermore, nitrites often combine with amines, a form of organic compound, to create a variety of toxins known as nitrosoamines. Because of concerns about these dangers, scientists and health activists have called for a ban on the use of nitrites and nitrates as food additives.

Oxygen

Discovered independently by Swedish chemist Carl W. Scheele (1742-1786) and English chemist Joseph Priestley (1733-1804) in the period 1773-1774, oxygen was named by a third scientist, French chemist Antoine Lavoisier (1743-1794). Believing (incorrectly) that all acids contained the newly discovered element, Lavoisier called it oxygen, which comes from a French word meaning "acid-former."

Like many elements, oxygen has been in use since the beginning of time. But this is quite different from saying, for instance, that iron has been in use since the early stages of human history. A person can live without iron (except for the necessary quantities in the human body), and can survive for weeks or even months without food. One can live for a few days without water (the most famous and plentiful of all oxygen-containing compounds); but one cannot survive for more than a few minutes without the oxygen in air.

Oxygen appears in three allotropes (different versions of the same element, distinguished by molecular structure): monatomic oxygen (O); diatomic oxygen or O2; and triatomic oxygen (O3), better known as ozone. The diatomic form dominates the natural world, but in the upper atmosphere, ozone forms a protective layer that keeps the Sun's harmful ultraviolet radiation from reaching Earth. Concerns that chlorofluorocarbons (CFCs) may be depleting the ozone layer by converting these triatomic molecules to O2 has led to a reduction in the output of CFCs by industrialized nations.

OBTAINING OXYGEN.

Oxygen, of course, is literally "in the air," mixed with larger quantities of nitrogen. Higher up in the atmosphere, it occurs as a free element. Through electrolysis,, it can be obtained from water; however, this process is prohibitively expensive for most commercial applications.

Oxygen-containing compounds are also sources of oxygen for commercial use, but generally oxygen is obtained by the fractional distillation of liquid air, described above with regard to nitrogen. After the nitrogen has been separated, argon and neon (which also have lower boiling points than oxygen) also boil off, leaving behind an impure form of oxygen. This is further purified by a process of cooling, liquefying, and evaporation, which eliminates traces of noble gases such as krypton and xenon.

Many millions of years ago, when Earth was first formed, there was no oxygen on the planet. The growth of oxygen on Earth coincided with the development of organisms that, as they evolved, increasingly needed oxygen. The present concentration of oxygen in the atmosphere, oceans, and the rocks of Earth's crust was reached about 580 million years ago, and is sustained today by biological activity. When plants undergo photosynthesis, carbon dioxide and water react in the presence of chlorophyll to produce carbohydrates and oxygen.

OXIDES AND OTHER COMPOUNDS.

Though the bond in diatomic oxygen is strong, once it is broken, monatomic oxygen reacts readily with other elements to form a seemingly limitless range of compounds: oxides, silicates, carbonates, phosphates, sulfates, and other more complex substances.

The process known as oxidation results in the formation of numerous oxides. Sometimes oxygen and another element form several oxides, as for example in the case of nitrogen, whose five oxides are listed above. Water is an oxide; so too are carbon dioxide and carbon monoxide. When animals and plants die, the organic materials that make them up react with oxygen in the air, resulting in a complex form of oxidation known as decayor, in common language, "rotting."

Oxygen, reacting with compounds such as hydrocarbons, produces carbon dioxide and water vapor at high temperatures. If the oxidation process is extremely rapid, and takes place at high temperatures, it is usually identified as combustion. In addition, oxygen reacts with iron and other metals to form oxides. Many of these oxides, commonly known as rust, are undesirable.

Every year, millions upon millions of dollars are spent on painting metal structures, or for other precautions to protect against the formation of metallic oxides. On the other hand, metallic oxides may be produced deliberately for applications in materials such as mortar color, to enhance the appearance of a brick building.

USES OF OXYGEN.

Aside from the obvious application of oxygen for breathing, there are four major fields that make use of this element: medicine, metallurgy, rocketry, and the field of chemistry concerned with chemical synthesis. The medical application is closest to how we normally use oxygen in our daily lives. In oxygen therapy, a patient having difficulty breathing is given doses of pure, or nearly pure, oxygen. This is used during surgical procedures, and to treat patients who have had heart attacks, as well as those suffering from various infectious or respiratory diseases.

The use of oxygen in metallurgy involves refining coke, which is almost pure carbon, to make carbon monoxide. Carbon monoxide, in turn, reduces iron oxides to pure metallic iron. Oxygen is also used in blast furnaces to convert pig iron to steel by removing excess carbon, silicon, and metallic impurities. In addition, oxygen is applied in torches for welding and cutting. In the form of liquefied oxygen, or LOX, oxygen is used in rockets and missiles. The space shuttle, for instance, carries a huge internal tank containing oxygen and hydrogen, which, when they react, give the vehicle enormous thrust.

In chemical synthesisthe preparation of compounds (especially organic ones) from easily available chemicalscommercial chemists use oxygen, for instance, to loosen the bonds in hydrocarbons. If this is done too quickly, it results in combustion; but at a controlled rate, the chemical synthesis of hydrocarbons can generate products such as acetylene, ethylene, and propylene.

Oxygen can be used to produce synthetic fuels, as well as for water purification and sewage treatment. Airplanes carry oxygen supplies in case of depressurization at high altitudes; in addition, divers carry tanks in which oxygen is mixed with helium, rather than nitrogen, to prevent the dangerous condition known as "the bends."

As early as the 1960s, smog-ridden cities such as Tokyo and Mexico City were equipped with coin-operated oxygen boothsa sort of "phone booth for the lungs." After inserting the appropriate amount of money, a person received a dose of oxygen for inhaling. This idea, spawned by necessity, is the likely inspiration for a rather bizarre fad that took hold in the trendier cities of North America during the mid-1990s: oxygen bars. Popular in Los Angeles, New York, and Toronto, these are establishments in which patrons pay up to a dollar per minute to inhale pure or flavored oxygen. Enthusiasts have touted the health benefits of this practice, but some physicians have warned of oxygen toxicity and other dangers.

The Other "Orphan" Nonmetals

PHOSPHORUS.

Some elements, such as iron or gold, were known from ancient or even prehistoric timesmeaning that the identity of the discoverer is unknown. Phosphorus was the first element whose discoverer is known: German chemist Hennig Brand (c. 1630-c. 1692), who identified it in 1674.

Highly reactive with oxygen, phosphorus is used in the production of safety matches, smoke bombs, and other incendiary devices. It is also important in fertilizers, and in various industrial applications. Phosphorus forms a number of important compounds, most notably phosphates, on which animals and plants depend.

Phosphorus pollution, created by the use of household detergents containing phosphates, raised environmental concerns in the 1960s and 1970s. It was feared that high phosphate levels in rivers and creeks would lead to runaway, detrimental growth of plants and algae near bodies of water, a condition known as eutrophication. These concerns led to a ban on the use of phosphates in detergents.

SULFUR.

On its own, sulfur has no smell, but in combination with other elements, it often acquires a foul odor, which has given it an unpleasant reputation. The element's smell, combined with its combustibility, led to the association of "brimstone"the ancient name for sulfurwith the fires of hell.

Because it is not usually combined with other elements in nature, the discovery of sulfur was relatively easy. Pure, or nearly pure, sulfur is mined on the Gulf Coast of the United States, as well as in Poland and Sicily. Sulfur compounds also appear in a number of ores, such as gypsum (calcium sulfate), or magnesium sulfate, better known as Epsom salts.

Applications of the aforementioned compounds are discussed in the Alkaline Earth Metals essay. In addition, sulfates are used as agricultural insecticides, and for killing algae in water supplies. Potassium aluminum sulfate, a gelatinous solid that sinks to the bottom when dropped into water, is also used in water purification. Sulfur is sometimes applied in pure form as a fungicide, or in matches, fireworks, and gunpowder. More often, however, it is found in compounds such as the sulfates or the sulfidesincluding sulfuric acid and the evil-smelling gas known as hydrogen sulfide.

Rotten eggs and intestinal gas are two examples of hydrogen sulfide, which, though poisonous, usually poses little danger, because the smell keeps people away. Another sulfur compound is mercaptan, an ingredient in the skunk's distinctive aroma. Tiny quantities of mercaptan are added to natural gas (which has no odor) so that dangerous gas leaks can be detected by smell.

SELENIUM.

When Swedish chemist Jons Berzelius (1779-1848) first discovered selenium in 1817, in deposits at the bottom of a tank in a sulfuric acid factory, he thought it was tellurium, a metalloid discovered in 1800. A few months later, he reconsidered the evidence, and realized he had found a new element. Because tellurium, which lies just below selenium on the periodic table, had been named for the Earth (tellus in Latin), he named his new discovery after the Greek word for the Moon, selene.

Found primarily in impurities from sulfide ores, selenium is often obtained commercially as a by-product of the refining of copper by electrolysis. It occurs in at least three allotropic forms, variously black and red in color. Plants and animals, including humans, need small amounts of selenium to survive, but larger quantities can be toxic. This was demonstrated in the late 1970s, when waterfowl in the area of Kesterson Reservoir in northern California began turning up with birth defects. The cause was later traced to the dumping of selenium from agricultural wastes and industrial plants.

Because selenium is photovoltaic (able to convert light directly into electricity) and photoconductive (meaning that its resistance to the flow of electric current decreases in the presence of light), it has applications in photocells, exposure meters, and solar cells. It is also used for the conversion of alternating current to direct current, and is applied as a semiconductor in electronic and solid-state appliances. Photocopiers use selenium in toners, and compounds containing selenium are used to tint glass red, orange, or pink.

WHERE TO LEARN MORE

Beatty, Richard. Phosphorus. New York: Benchmark Books, 2001.

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

Fitzgerald, Karen. The Story of Oxygen. New York: F. Watts, 1996.

"Group Trends: Selected Nonmetals" (Web site). <http://wine1.sb.fsu.edu/chm1045/notes/Periodic/NMTrends/Period08.htm> (May 29, 2001).

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

"Nonmetals" Rutgers University Department of Chemistry (Web site). <http://chemistry.rutgers.edu/genchem/nonmet.html> (May 29, 2001).

"Nonmetals, Semimetals, and Their Compounds" (Web site). <http://ull.chemistry.uakron.edu/genchem/21/title.html> (May 29, 2001).

WebElements (Web site). <http://www.webelements.com> (May 22, 2001).

KEY TERMS

ALLOTROPES:

Different versions of the same element, distinguished by molecular structure.

DIATOMIC:

A term describing an element that exists as molecules composed of two atoms. This is in contrast to monatomic elements.

ELECTROLYSIS:

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

HALOGENS:

Group 7 of the periodic table of elements, with valence electron configurations of ns2np5. In contrast to the noble gases, the halogens are known for high levels of reactivity.

ION:

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

ISOTOPES:

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 ofmass. Isotopes may be either stable or unstable. The unstable type, known as radioisotopes, are radioactive.

METALS:

Elements that are lustrous or shiny in appearance; malleable, meaning that they can be molded into different shapes without breaking; and excellent conductors of heat and electricity. Metals, which constitute the vast majority of all elements, tend to form positive ions by losing electrons.

MONATOMIC:

A term describing an element that exists as single atoms. This in contrast to diatomic elements.

NOBLE GASES:

Group 8 of the periodic table of elements, all of whom (with the exception of helium) have valence electron configurations of ns2np6. The noble gases are noted for their extreme lack of reactivityin other words, they tend not to react to, or bond with, other elements.

NONMETALS:

Elements that have a dull appearance; are not malleable; are poor conductors of heat and electricity; and tend to gain electrons to form negative ions. They are thus opposite of metals in most regards, as befits their name. In addition to hydrogen, in Group 1 of the periodic table, the other 18 nonmetals occupy the upper right-hand side of the chart. They include the noble gases, halogens, and seven "orphan" elements: boron, carbon, nitrogen, oxygen, phosphorus, sulfur, and selenium.

ORBITAL:

A pattern of probabilities regarding the position of an electron for anatom in a particular energy state. The higher the principal energy level, the more complex the pattern of orbitals.

RADIOACTIVITY:

A term describing a phenomenon whereby certain isotopes known as radioisotopes are subject to a form of decay brought about by the emission of high-energy particles. "Decay" does not mean that the isotope "rots"; rather, it decays to form another isotope until eventually (though this may take a long time), it becomes stable.

REACTIVITY:

The tendency for bonds between atoms or molecules to be made or broken in such a way that materials aretransformed.

SHELL:

The orbital pattern of the valence electrons at the outside of an atom.

VALENCE ELECTRONS:

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

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nonmetal

nonmetal, chemical element possessing certain properties by which it is distinguished from a metal. In general, this distinction is drawn on the basis that a nonmetal tends to accept electrons and form negative ions and that its oxide is acidic. Nonmetals are poor conductors of heat and electricity (see conduction) and do not have the luster of metals. Arsenic, antimony, selenium, and tellurium exhibit both nonmetallic and metallic properties and are called metalloids. Unlike the metals, which are all solids (with the exception of mercury) under ordinary conditions of temperature and pressure, the nonmetals appear in all three states. Argon, chlorine, fluorine, helium, hydrogen, krypton, neon, nitrogen, oxygen, and xenon are normally gases. Bromine is a liquid. Boron, carbon, iodine, phosphorus, silicon, and sulfur are solids. Certain of them, e.g., boron, carbon, iodine, silicon, and sulfur, form crystals, as do the metals. In hardness they vary considerably. Carbon in its allotropic form, the diamond, is the hardest element known. With the exception of carbon, sulfur, nitrogen, oxygen, and the inert gases—argon, helium, krypton, neon, and xenon—the nonmetals do not occur uncombined in nature, but exist in numerous relatively abundant compounds, among which are the oxides, halides (binary halogen compounds), sulfides, carbonates, nitrates, phosphates, silicates, and sulfates. With a few exceptions, the nonmetallic elements are important chiefly for their compounds. For the properties and uses of specific nonmetals, see the separate articles on these elements.

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