The basic unit of an element that can enter into chemical combination is called an atom. An atom is the smallest unit of matter that still retains the properties of an element. Atoms are made up of three subatomic particles: protons, neutrons, and electrons. Protons and neutrons are found in the nucleus of the atom, while electrons orbit the nucleus. The number of protons in an atom determines its atomic number and its chemical properties. The number of neutrons can vary, giving rise to different isotopes of an element.