Matter is composed of exceedingly small particles called atoms. An atom is the smallest unit of an element that can participate in a chemical change.

An element consists of only one type of atom, which has a mass that is characteristic of the element and is the same for all atoms of that element. A macroscopic sample of an element contains an incredibly large number of atoms, all of which have identical chemical properties.

A copper penny contains approximately 3 × 10²² copper atoms, each of which has the same chemical properties.

Atoms of one element differ in properties from atoms of all other elements.

A compound consists of atoms of two or more elements combined in a small, whole-number ratio. In a given compound, the numbers of atoms of each of its elements are always present in the same ratio.

Copper(II) oxide, a powdery, black compound, results from the combination of two types of atoms—copper (pink spheres) and oxygen (red spheres)—in a 1:1 ratio.

Atoms are neither created nor destroyed during a chemical change but rearranged to yield substances that are different from those present before the change.