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Atomic Structure

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.

AlF_3, AlCl_3, AlBr_3, AlI_3,       3⁺ with 1^- gives 1 to 3.

Al₂O_3, Al₂S_3,                           3⁺ with 2^- gives 2 to 3.

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