molecule [New Lat.,=little mass], smallest particle of a compound that has all the chemical properties of that compound. A single atom is usually not referred to as a molecule, and ionic compounds such as common salt are not made up of molecules. Unlike ions , molecules carry no electrical charge.

Nature of Molecules

Molecules are made up of two or more atoms , either of the same element or of two or more different elements, joined by one or more covalent chemical bonds . According to the kinetic-molecular theory, the molecules of a substance are in constant motion. The state (solid, liquid, or gaseous) in which matter appears depends on the speed and separation of the molecules in the matter. Substances differ according to the structure and composition of their molecules. A molecular compound is represented by its molecular formula ; for example, water is represented by the formula H ₂ O. A more complex structural formula is sometimes used to show the arrangement of atoms in the molecule.

Molecules differ in size and molecular weight as well as in structure. In a chemical reaction between molecular substances, the molecules are often broken apart into atoms or radicals that recombine to form other molecules, i.e., other substances. In other cases two or more molecules will combine to form a single larger molecule, or a large molecule will be broken up into several smaller molecules.

Molecules can assume many shapes and sizes. Molecules of hydrogen gas, H ₂ , are very small; each consists of two atoms of hydrogen. Water molecules, H ₂ O, are much larger, containing an atom of oxygen as well as two of hydrogen. The atoms in a water molecule are arranged at the corners of an isosceles triangle; the oxygen atom is located where the two equal sides meet and the angle between these sides is about 105°. A carbon dioxide molecule, CO ₂ , is linear, with the two oxygen atoms an equal distance on either side of the carbon atom. In methane, CH ₄ , the hydrogen atoms are arranged at the corners of a tetrahedron with the carbon atom in the center. In benzene, C ₆ H ₆ , the carbon atoms form a hexagonal ring with a hydrogen atom joined to each carbon atom. More complex molecules resemble rings, chains, helices, or other forms. Many molecules occurring in living organisms are very complex. RNA and DNA molecules resemble giant helices. By polymerization a large number of small molecules may be joined to form a single large polymer molecule. Typical polymers include synthetic resins, rubbers, and plastics.

Evolution of Molecular Theory

The terms atom and molecule were used interchangeably until the early 19th cent. Initial experimental work with gases led to what is essentially the modern distinction. J. A. C. Charles and R. Boyle had shown that all gases exhibit the same relationship between a change in temperature or pressure and the corresponding change in volume. J. L. Gay-Lussac had shown that gases always combine in simple whole-number volume proportions and had rediscovered the earlier findings of Charles, which had not been published.

Dalton's Theory

One early theorist was John Dalton, best known for his atomic theory. Dalton believed that gases were made up of tiny particles, which he thought were atoms. He thought that these atoms were stationary and in contact with one another and that heat was a material substance, called caloric, that was contained in shells around the atom (these shells of caloric were actually what was in contact). When a gas was heated, the amount of caloric was increased, the shells became larger, and the gas expanded. Dalton did not accept Gay-Lussac's findings about combining volumes of gases, perhaps because it could not be explained by his theory.

Avogadro's Hypothesis

A different theory from Dalton's that could explain the combining volumes of gases was proposed by the Italian physicist Amadeo Avogadro in 1811. According to his theory, under given conditions of temperature and pressure, a given volume of any gas contains a definite number of particles. From the earlier observation that one volume of hydrogen gas and one volume of chlorine gas react to form two volumes of hydrogen chloride gas he deduced that the particles in gaseous hydrogen or chlorine could not be single atoms, but must be some combination of atoms. He called this combination a molecule. He reasoned that the two volumes of hydrogen chloride that are formed must contain twice as many particles as either single volume of hydrogen or chlorine. Thus, if there were 100 particles each of hydrogen and chlorine, there would be 200 particles of hydrogen chloride produced; but there could be only 100 particles produced if the original particles of hydrogen and chlorine were indivisible atoms, since each particle of hydrogen chloride contains both hydrogen and chlorine. An assumption that there are two atoms in a molecule of gaseous hydrogen or chlorine and one atom each of hydrogen and chlorine in a molecule of hydrogen chloride preserves both the hypothesis of indivisible atoms and the hypothesis of equal numbers of particles in equal volumes of gases. Similar reasoning would allow a larger even number of atoms in the molecules of hydrogen or chlorine, but Avogadro favored a rule of simplicity, using the smallest possible number. In the model of gases proposed by Avogadro, the particles were not in contact and much of the volume of the gas was empty space.

Cannizaro's Compromise

Avogadro's theory was not well accepted; most responses were very critical. Meanwhile, Dalton's theory prompted extensive experimentation and especially the determination of combining weights of the elements. Many shortcomings of Dalton's theory were uncovered, and although a number of modifications were suggested, none were very successful. It was not until 1858 that the Italian chemist Stanislao Cannizaro suggested a merging of Avogadro's and Dalton's theories. The acceptance of this revised theory was assisted by the acceptance by physicists at about the same time of the kinetic-molecular theory of gases that was first proposed in 1738 by Daniel Bernoulli.

Molecule

A molecule is a particle consisting of two or more atoms joined to each other by means of a covalent bond. (Electrons are shared in covalent bonds.) There are a number of different ways of representing molecules. One method is called an electron-dot diagram, which shows the atoms included in the molecule and the electron pairs that hold the atoms together. Another method is the ball-and-stick model, in which the atoms present in the molecule are represented by billiard-ball-like spheres; the bonds that join them are represented by wooden sticks. A third method is called a space-filling model, which shows the relative size of the atoms in the molecule and the way the atoms are actually arranged in space (see Figure 1).

Words to Know

Atom: The smallest particle of which an element can exist.

Chemical bond: An electrical force of attraction that holds two atoms together.

Covalent bond: A chemical bond formed when two atoms share a pair of electrons with each other.

Compound: A substance consisting of two or more elements in specific proportions.

Element: A pure substance that cannot be broken down into anything simpler by ordinary chemical means.

Molecular formula: A shorthand method for representing the composition of a molecule using symbols for the type of atoms involved and subscripts for the number of atoms involved.

Molecule: A particle formed when two or more atoms join together.

Structural formula: The chemical representation of a molecule that shows how the atoms are arranged within the molecule.

Formation of compounds

A compound is formed when two atoms of an element react with each other. For example, water is formed when atoms of hydrogen react with atoms of oxygen. The reaction between two atoms always involves the exchange of electrons between the two atoms. One atom tends to lose one or more electrons, and the other atom tends to gain that (or those) electrons.

In general, this exchange of electrons can occur in two ways. First, one atom can completely lose its electrons to the second atom. The first atom, with fewer electrons than usual, becomes a positively charged particle called a cation. The second, with more electrons than usual, becomes a negatively charged particle called an anion. A compound formed in this way consists of pairs of ions, some positive and some negative. The ions stay together because they carry opposite electric charges, and opposite electric charges attract each other.

Sodium chloride is a compound that consists of ions. There is no such thing as a molecule of sodium chloride. Instead, sodium chloride consists of sodium ions and chloride ions.

In many instances, the reaction between two atoms does not involve a complete loss and gain of electrons. Instead, electrons from both atoms are shared between the two atoms. In some cases, the sharing is equal, or nearly equal, with the electrons spending about half their time with each atom. In other cases, one atom will exert a somewhat stronger force on the electrons than the other atom. In that instance, the electrons are still shared by the two atoms—but not equally.

Electrons shared between two atoms are said to form a covalent bond. The combination of atoms joined to each other by means of a covalent bond is a molecule.

Polar and nonpolar molecules

Consider the situation when the electrons that make up a covalent bond spend more time with one atom than with the other. In that case, the atom that has the electrons more often will be slightly more negative than the other atom. The molecule that contains this arrangement is said to be a polar molecule. The term polar suggests a separation of charges, like the separation of magnetic force in a magnet with north and south poles.

But now think of a molecule in which the electrons in a covalent bond are shared equally—or almost equally. In that case, both atoms have the electrons about the same amount of time, and the distribution of negative electrical charge is about equal. There is no separation of charges, and the molecule is said to be nonpolar.

Formulas

Molecular formulas. The structure of a molecule can be represented by a molecular formula. A molecular formula indicates the elements present in the molecule as well as the ratio of those elements. For example, the molecular formula for water is H₂O. That formula tells you, first of all, that two elements are present in the compound, hydrogen (H) and oxygen (O). The formula also tells that the ratio of hydrogen to oxygen in the compound is 2 to 1. (There is no 1 following the O in H₂O. If no number is written in as a subscript, it is understood to be 1.)

Structural formulas. A structural formula gives the same information as a molecular formula—the kind and number of atoms present—plus one more piece of information: the way those atoms are arranged within the molecule. As you'll notice in Figure 2, structural formulas help differentiate between substances that share identical molecular formulas, such as ethyl alcohol and methyl ether.

[See also Atom; Chemical bond; Compound, chemical; Element, chemical; Formula, chemical ]

molecule Smallest particle of a substance (such as a compound) that exhibits the properties of that substance. Molecules consist of two or more atoms held together by chemical bonds. For example, water molecules consist of two atoms of hydrogen bonded to one atom of oxygen (H₂O). A macromolecule can be up to 1000 times greater in diameter. A molecule (unlike an ion) has no electrical charge. Molecules were first hypothesized in 1811 by Italian physicist Amedeo Avogadro and first detected by Scottish botanist Robert Brown.

mol·e·cule / ˈmäləˌkyoōl/ • n. Chem. a group of atoms bonded together, representing the smallest fundamental unit of a chemical compound that can take part in a chemical reaction.

molecule The smallest part of a compound, which is a substance formed by the chemical combination of one or more types of atom. For example, water consists of two atoms of hydrogen and one atom of oxygen, hence it has the formula H₂O.

molecule (mol-i-kewl) n. a particle consisting of two or more atoms held together by chemical bonds. It is the smallest unit of an element or compound capable of existing independently.
—molecular adj.

molecule XVIII. — F. molécule, dim. f. L. mōlēs MOLE3.
Hence molecular XIX.

molecule See COVALENT BOND.

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