# What are "atomic number", and "mass number", and how do these terms relate to nuclear structure?

The nucleus contains either protons, fundamental particles with positive charge, or neutrons, fundamental particles with neutral charge. The number of protons, $Z$, is the atomic number, and this number determines elemental identity: $Z = 1$, hydrogen; $Z = 2$, helium; $Z = 3$, lithium; $Z = 6$, rubidium; $Z = 37$, rubidium. Do you have to remember these numbers? No, because in every test in Chemistry and Physics you should and must be supplied with a Periodic Table of elements.