Atomic Structure & the Periodic Table

Everything is made of atoms; their structure explains the periodic table.

Sub-atomic particles

Proton: relative mass $1$, charge $+1$ (in the nucleus).
Neutron: relative mass $1$, charge $0$ (in the nucleus).
Electron: negligible mass, charge $-1$ (in shells).

The atomic number is the number of protons; the mass number is protons + neutrons.