Monatomic ions are formed when atoms lose or gain electrons due to a chemical reaction.
Nonmetallic elements form anions.
Anions have more electrons than the parent atom.
Anions are larger than the parent atom.
Cations have fewer electrons than the parent atom.
Cations are smaller than the parent atom.
| atom | electron number (shell occupancy) | ion | electron number (shell occupancy) |
 | 8 in shell 1 and 2 |  | 10 in shells 1 and 2 |
 | 12 in shells 1, 2 and 3 |  | 10 in shells 1 and 2 |
Nonmetallic elements form anions.
Anions have more electrons than the parent atom.
Usually monatomic have gained sufficient electrons to have the same electron configuration as the noble gas in the same row of the periodic table.
Anions are larger than the parent atom.
The anion and the parent atom have the same nuclear charge.
There are additional repulsive forces between the electrons in the outer shell for the anion.
The graphic shows the relative sizes of oxygen atoms and oxide anions.
Metallic elements form cationsThere are additional repulsive forces between the electrons in the outer shell for the anion.
The graphic shows the relative sizes of oxygen atoms and oxide anions.
Cations have fewer electrons than the parent atom.
Commonly monoatomic cations have lost all of the electrons in the valence shell.
Cations are smaller than the parent atom.
The cation and the parent atom have the same nuclear charge.
The cation has one fewer shells of electrons.
The graphic shows the relative sizes of magnesium atoms and magnesium cations.
The cation has one fewer shells of electrons.
The graphic shows the relative sizes of magnesium atoms and magnesium cations.