Predicting ion charges (and those Roman numerals)

By the end you can predict the charge on a simple ion from its group in the Periodic Table, name simple ions, and read Roman numerals such as iron(III).

Atoms of the main-group elements in this table form ions by losing or gaining electrons until they have the electronic configuration of a noble gas. How many electrons that takes depends only on the group.

group	electrons	ion charge	examples	ion name
1	lose 1	1+	$\text{Li}^+$ , $\text{Na}^+$ , $\text{K}^+$	lithium, sodium…
2	lose 2	2+	$\text{Mg}^{2+}$ , $\text{Ca}^{2+}$	magnesium, calcium…
13	lose 3	3+	$\text{Al}^{3+}$	aluminium
15	gain 3	3−	$\text{N}^{3-}$ , $\text{P}^{3-}$	nitride, phosphide
16	gain 2	2−	$\text{O}^{2-}$ , $\text{S}^{2-}$	oxide, sulfide
17	gain 1	1−	$\text{F}^-$ , $\text{Cl}^-$ , $\text{Br}^-$ , $\text{I}^-$	fluoride, chloride…

Metals (left) lose electrons and form positive ions, which keep the element's name.
Non-metals (right) gain electrons and form negative ions, which end in -ide.
Group 14 would need to lose or gain four electrons, which costs too much energy, so carbon and silicon bond covalently. (Tin and lead, lower down, are metals that form 2+ ions, like lead(II), $\text{Pb}^{2+}$ .) Group 18 is already a noble gas.

Sodium ( $2,8,1$ ) could gain seven electrons or lose one. Losing one is far cheaper, so it becomes $\text{Na}^+$ ( $2,8$ , like neon). Chlorine ( $2,8,7$ ) gains one to become $\text{Cl}^-$ ( $2,8,8$ , like argon).

Where it breaks: "atoms want a full shell" is a handy story, not a reason. Forming $\text{Na}^+$ on its own actually costs energy; ionic compounds form because the attraction between oppositely charged ions in the lattice releases more.

Elements in group 2 form ions with a charge of . Elements in group 16 form ions with a charge of .

Nitrogen is in group 15. What is the formula of the nitride ion?

Name the ion $\text{S}^{2-}$ .

The Periodic Table is a cheat sheet