Isotopes

Atoms of the same element always have the same number of protons, but they can have different numbers of neutrons. Atoms of the same element with different numbers of neutrons are called isotopes.

The number of protons is the atomic number, so all isotopes of an element have the same atomic number. The total number of protons and neutrons is the mass number. Because isotopes have different numbers of neutrons, they have different mass numbers.

For example, carbon-12 and carbon-14 are isotopes of carbon. Both have 6 protons, but carbon-12 has 6 neutrons and carbon-14 has 8. They are both carbon because their proton numbers are the same, even though their masses differ.

Mass number and atomic number

An atom has a central nucleus containing protons and neutrons. These particles account for nearly all the atom’s mass.

The mass number is the total number of protons and neutrons in the nucleus. It is a count of these particles, not the measured mass of the atom.

The atomic number is the number of protons in the nucleus. It identifies the element: every atom of an element has the same number of protons.

Nuclide notation

In nuclide notation, the element’s chemical symbol is written with the mass number at the top left and the atomic number at the bottom left. For example, sodium can be written as:

The atomic number, $$11$$, shows that the nucleus contains $$11$$ protons. The mass number, $$23$$, shows that it contains $$23$$ protons and neutrons in total. The number of neutrons is therefore $$23 - 11 = 12$$.