Atomic Number, Mass Number and Isotopes

Two numbers pin down an atom:

• Atomic number ($Z$) = number of protons. It defines which element the atom is — every carbon atom has $Z = 6$.
• Mass number ($A$) = number of protons + neutrons.

So number of neutrons = $A - Z$. We write an atom as $^{A}_{Z}\text{X}$, for example $^{23}_{11}\text{Na}$: 11 protons, $23 - 11 = 12$ neutrons.

Isotopes are atoms of the same element (same $Z$) with different numbers of neutrons (different $A$). Chlorine has two common isotopes, $^{35}\text{Cl}$ and $^{37}\text{Cl}$ — both have 17 protons, but 18 and 20 neutrons.