When an atom loses or gains an electron, and therefore has a charge.
Smaller than its original atom, because it loses an electron/volume.
Bigger than its original atom, because it gains an electron/volume.
Just a normal ion from an atom
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: