Atomic Mass and Isotope Basics

Atomic mass and isotopes are linked by the number of protons and neutrons in an atom. The number of protons sets an element’s atomic number, or Z. For example, every atom with eight protons is oxygen.

Atoms of the same element can still differ because they can have different numbers of neutrons. ...

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....