When an atom loses or gains an electron, and therefore has a charge.

Positive Ions
Smaller than its original atom, because it loses an electron/volume.

Negative Ions
Bigger than its original atom, because it gains an electron/volume.

Monatomic ion:
Just a normal ion from an atom

Polyatomic ion:
An ion with more than one atom, like SO4, NO3

When an ion has the same amount of valence electrons as a noble gas because it’s more stable
BUT not the same amount of protons.

A few examples of the most common ions: