Proton Number

The proton number (atomic number), $Z$, counts the number of protons in the nucleus.

It determines:

• the element (e.g. $Z = 6$ is always carbon)
• the number of electrons in a neutral atom
• the position of the element in the periodic table

In nuclide notation, the proton number is written as a subscript: $^{A}_{Z}X$.

In alpha decay, $Z$ decreases by 2. In beta-minus decay ($\beta^{-}$), $Z$ increases by 1 as a neutron converts to a proton.