Ions and Charge showed what an ion is; Valence Electrons and Dot Symbols showed how many outer electrons each atom has. Put them together and you can predict the charge most main-group atoms take.
Key idea
Main-group atoms gain or lose electrons to reach the electron arrangement of the nearest noble gas. Metals on the left lose electrons: group 1 forms $+1$ ions, group 2 forms $+2$, aluminum $+3$. Nonmetals on the right gain them: group 15 forms $-3$, group 16 $-2$, group 17 $-1$.
Why it works
It is about energy, not wanting. Removing sodium's one outer electron costs 496 kJ/mol, but removing a second one, from the full inner shell much closer to the nucleus, costs 4562 kJ/mol, far more than any energy the reaction can repay. So sodium stops at $\mathrm{Na^+}$, with neon's arrangement. A chlorine atom releases energy when it gains one electron into its nearly full shell, but a second electron would have to start a new shell farther out, which costs energy. So chlorine stops at $\mathrm{Cl^-}$, with argon's arrangement.
Worked examples
Worked example: Calcium
What ion does calcium (group 2) form?
- Calcium has 2 valence electrons ($\mathrm{[Ar]\,4s^2}$).
- Losing both leaves argon's arrangement.
Answer: $\mathrm{Ca^{2+}}$
Worked example: Nitrogen
What ion does nitrogen (group 15) form, and how many electrons does it have?
- Nitrogen has 5 valence electrons; gaining 3 completes the shell like neon.
- Ion: $\mathrm{N^{3-}}$, with $7 + 3 = 10$ electrons.
Answer: $\mathrm{N^{3-}}$, 10 electrons
Common mistake
Predicting that sodium gains seven electrons instead of losing one, or writing the sulfide ion as $\mathrm{S^{2+}}$. Count which way is shorter to a noble gas: sodium is 1 past neon, sulfur is 2 short of argon. Then check the sign: losing electrons gives a positive ion, gaining gives a negative one.