2.12 Valences of typical ions
The chart seen here shows the most common ions for different columns in the Periodic Table. Common cations are shaded blue and common anions are shaded red. This table is not to suggest that all the elements always form ions. But, when they do, for most elements, their likely ionic form is predictable as shown.
The red numbers below the columns are typical ionic charges. Elements in groups on the left side of the table have “extra” electrons in outer shells, and readily give up those electrons to become cations. Elements in group 1 (alkali elements) generally have an ionic charge of +1. Those in group 2 (alkaline earth elements) usually have an ionic charge of +2.
On the other side of the chart, helium and other elements in Group 18 of the Periodic Table have completely filled outer shells and, consequently, do not ionize. Elements in Groups 16 and 17 have room for a small number of additional electrons in their outer shells and consequently accept extra electrons to become anions. Elements in group 17 (halogens) are lacking a single electron to fill their outer shells completely. So, they typically become monovalent anions (charge of -1) because they acquire an extra electron to fill the shell. Oxygen and other elements in Group 16 typically form divalent anions (-2). Elements in Group 13, Group 14, and Group 15 typically ionized to form cations with charge of +3, +4, and +5, respectively, although other valences are possible.
Ionization of the transition metals in the central portion of the table (groups 3 through 12) is less predictable than ionization of elements near the table’s sides. When ionized, elements in groups 3, 4, and 5 commonly have valences of +3, +4, and +5, respectively. The rest of the transition metals exist in a number of ionic states, typically +2 or +3.
The three most abundant elements in the Earth’s crust (oxygen, silicon, and aluminum) are on the right side of the Periodic Table, in groups 13, 14, and 16. Oxygen typically has a charge of -2, silicon of +4, and aluminum of +3.