An atom is the smallest unit of matter that retains the properties of an element. It is composed of three types of subatomic particles: protons, neutrons, and electrons.
Protons and neutrons are located in the nucleus at the center of the atom, while electrons orbit the nucleus in regions called electron shells. The number of protons in an atom's nucleus determines the atomic number and identifies the element.
For example, hydrogen has one proton, so its atomic number is 1. Oxygen has 8 protons, so its atomic number is 8.
The number of protons and neutrons in an atom's nucleus also determines its atomic mass. The number of electrons in an atom usually equals the number of protons, making the atom electrically neutral.