Atoms are the elementary building blocks of material substances. Although the term atom, derived from the Greek word atomos, meaning indivisible, would seem inappropriate for an entity that, as science has established, is divisible, the word atom still makes sense, because, depending on the context, atoms can still be regarded as indivisible. Namely, once split, the atom loses its identity. For example, an atom of gold is the building block of gold. If we split a gold atom, there is no more gold. As Carl H. Snyder has written, an atom is "the smallest particle of an element that we can identify as that element."

Atoms share many characteristics with other material objects: they can be measured, and they also have mass and weight. For example, a gold atom, which is could be regarded as an atom of average size, measures approximately 3_10^10_8 in diameter, and its mass is...