An ion is an atom, or a collection of covalently-bonded atoms, that all together has a charge

An ion is an atom, or a collection of covalently-bonded atoms, that all together has a charge. This is either because it has more electrons than they would have in the neutral state (making it an anion) or fewer electrons (making it a cation).

Monatomic ions are those that consist of only one atom. Examples include the F- and Ca+2 ions.

consist of atoms that are covalently-bonded and have charge. An example is the hydroxide ion, OH-.

Here is a video which discusses how ions form and how to determine their charges. Video from: Noel Pauller