The atom is the smallest unit of a chemical element having the properties of that element. An atom contains several kinds of particles. Its central core, the nucleus, consists of positively charged particles, called protons, and uncharged particles, called Neutrons. Surrounding the nucleus and orbiting it are negatively charged particles, called Electrons. Each atom has an equal number of protons and electrons.

The nucleus occupies only a tiny fraction of an atom's volume but contains almost all of its mass. Electrons in the outermost orbits determine the atom's chemical and electrical properties.

The number of protons in an atom's nucleus is called the Atomic Number. All atoms of an element have the same atomic number and differ in atomic number from atoms of other elements.

The total number of protons and neutrons combined is the atom's mass number. Atoms containing the same number of protons but different numbers of neutrons are different forms, or isotopes of the same element.