Isotopes

Elements have a set number of protons that determines their atomic number (Z). For example, all atoms with eight protons are oxygen; however, the number of neutrons can vary for atoms of the same element. The sum of the number of protons and the number of neutrons is the mass number (A). Atoms with the same atomic number but differ...

Isotopes are forms of an element where the number of neutrons in the nucleus varies, but the number of protons — the atomic number — is the same. This results in different atomic masses but does not affect the atom's charge.

Most elements have more than one isotope.

For example, elemental carbon has three naturally existing isotopes....