GCSE CHEMISTRY - How can you tell the Formula of Ionic Compounds from the Charges on the Ions?

What are the Formulae of Ionic Compounds?

Why is the formula of sodium oxide Na₂O
and not NaO or NaO₂?

The ionic solid must have the same number
of plus and minus charges.
If Na⁺ forms a compound with O^2-,
then the formula will be Na₂O.
Two sodium ions (each 1⁺) will be needed
to balance the charge on one oxygen ion (2^-).
Sodium has valency 1, oxygen has valency 2.

Examples of formulae of other ionic compounds
from the elements on the previous page are

LiF, LiCl, LiBr, LiI,                                1⁺ with 1^-
NaF, NaCl, NaBr, NaI,                               gives
KF, KCl, KBr, KI,                                     1 to 1.

Li₂O, Na₂O, K₂O, 1⁺ with 2^-
Li₂S, Na₂S, K₂S, gives 2 to 1.

MgF₂, MgCl₂, MgBr₂, MgI₂, 2⁺ with 1^-
CaF₂, CaCl₂, CaBr₂, CaI₂, gives 1 to 2.

MgO, CaO, MgS, CaS, 2⁺ with 2^- gives 1 to 1.

AlF₃_, AlCl₃_, AlBr₃_, AlI₃_, 3⁺ with 1^- gives 1 to 3.

Al₂O₃_, Al₂S₃_, 3⁺ with 2^- gives 2 to 3.

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