Nuclear composition and nuclide notation

An atomic nucleus contains protons and neutrons. Together, these particles are called nucleons. Electrons are found outside the nucleus and are not nucleons.

Protons have relative charge $$+1$$, neutrons have relative charge $$0$$, and electrons have relative charge $$-1$$.

Proton number and nucleon number

The proton number, or atomic number, $$Z$$, is the number of protons in the nucleus. It identifies the element: every atom of a given element has the same proton number.

The nucleon number, or mass number, $$A$$, is the total number of protons and neutrons in the nucleus. Subtracting the number of protons from this total gives the number of neutrons:

Reading nuclide notation

A particular nucleus can be represented using nuclide notation:

Here, $$\mathrm{X}$$ is the element symbol. The upper number, $$A$$, is the nucleon number; the lower number, $$Z$$, is the proton number.

For example, $$ {}^{14}_{6}\mathrm{C} $$ represents a carbon nucleus with 6 protons and 14 nucleons in total. It therefore contains $$14 - 6 = 8$$ neutrons.