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Atomic Structure

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 O2-, 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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