The stable isotope oxygen-16 has an atomic number of 8 and a mass number, also known as an atomic mass, of approximately 16. The atomic number indicates how many protons are in the nucleus of the atom, while the atomic mass designates the number of protons and neutrons.
Subtracting the atomic mass from the atomic number yields the number of neutrons in the nucleus. For stable isotopes such as oxygen-16, the number of electrons is equal to the number of protons.
Oxygen mostly occurs as a diatomic molecule on Earth; this means that an atom of the element is commonly found bonded to one other atom. Another allotrope of oxygen is ozone, which is made of three bonded atoms of oxygen; it is formed by ultraviolet light or electrical discharges.