Molecule

Molecule, smallest particle of a substance having the specific chemical properties of that substance. If a molecule is broken into anything smaller, the parts differ in nature from the original substance. In principle, a sample of water can be divided into two parts and each part further divided, producing smaller samples of water. The process of division and subdivision ultimately produces a single molecule of water, whose further splitting gives something that is not like water in character: hydrogen and oxygen. Each molecule is pictured as existing independently of other molecules. An encounter of two usually results in their bouncing away from each other without fundamental change. More violent encounters alter the molecular composition, as when a chemical change occurs.

Molecules of compounds are composed of atoms of the constituent elements. A molecule is said to be diatomic if made of two atoms and polyatomic if consisting of a large number of atoms. Some naturally occurring molecules are made of hundreds, thousands, or even millions of atoms. Much of modern chemistry is concerned with determining the composition, structure, and size of molecules. Extremely short bursts of laser light are used to study molecular reactions as they take place.

Simple molecules are the smallest in size. Hydrogen molecules, for example, have a diameter of about 10^-10 m, or about one hundred-millionth of an inch, and a mass near 3 × 10^-27 kg. Other, more complicated molecules take the form of chains, rings, or helices.

The idea of molecules as distinct from atoms was first suggested by the Italian physicist Amedeo Avogadro in 1811. He postulated, in what is now known as Avogadro's law, that under given temperature and pressure conditions, equal volumes of any two gases contain the same number of molecules. This provided a way of comparing the relative weights of molecules and ultimately of obtaining the comparative weights of atoms. Much of modern physics and chemistry depends on these results. See Avogadro's Number.

An extension of Avogadro's molecular law is the kinetic theory formulated by several people, including the British physicist James Clerk Maxwell, the Dutch physicist Johannes Diderik van der Waals, and the Austrian physicist Ludwig Boltzmann. This theory states that molecules are in constant motion, the intensity of the motion increasing with the addition of heat. The motion, if the molecule is made of more than one atom, includes vibration within the molecule and a rotation, much like the rotation of the Moon around the Earth. Information about these internal vibrations and rotations is obtained by several means, including spectroscopy and the measuring of specific heat. In 1989, for the first time, physicists predicted completely the course of the simplest possible molecular reaction (involving hydrogen atoms) in terms of quantum theory.

The weight of a molecule can be determined either by calculation or experiment. The molecular weights of the elementary atoms, such as carbon-12, are the same as their atomic weights, which have already been established (see Atom: Atomic Weight). If the atomic structure of a molecule is known, the molecular weight can be calculated. For example, water (H₂O) has two hydrogen atoms (atomic weight of the hydrogen atom = 1) and one oxygen atom (atomic weight of the oxygen atom = 16), giving water a molecular weight of 18. Complex molecules can have molecular weights ranging into the hundreds of millions. In experimental determination, the molecular weight of a substance is ascertained by calculating the actual weight in grams per mole.