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 $$23 - 11 = 12$$.

Isotopes

Atoms belong to the same element if their nuclei contain the same number of protons. However, atoms of that element can have different numbers of neutrons. These different forms of the same element are called isotopes.

The nucleon number is the total number of protons and neutrons in a nucleus. Isotopes of an element have the same proton number but different neutron numbers, so they also have different nucleon numbers.

For example, carbon-12 and carbon-14 are isotopes of carbon. Both have 6 protons, but carbon-12 has 6 neutrons while carbon-14 has 8. Their nucleon numbers are therefore 12 and 14, respectively.