Writing Complex Chemical Formulae 4


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Some compounds contain more than two elements.of the names of these compounds either end in the letters -ATE or - ITE but there are some exceptions which end in -IDE. eg sodium sulfite contains sodium, sulfur and oxygen. Copper nitrite contains copper, nitrogen and  oxygen. Sodium hydroxide contains sodium, hydrogen and oxygen,

You cannot cross over three valency numbers so a different method is used. In data there is likely to be a list of complex ions as in the table below. Otherwise you will need to learn them.

Sulfate ion has the formula SO42- which means that in each formula unit there is 1 atom of sulfur, 4 atoms of oxygen and because it has added two electrons it has two negative charges. The valency of these ions is the same as the number of charges. So sulfate has a valency of 2.

At this stage, I recommend leaving out the charge when writing the formula and only include it if an ionic formula is asked for,

Ammonium NH4+ 1
Hydroxide OH- 1
Hydrogencarbonate HCO3- 1
Ethanoate CH3COO- 1
Nitrate NO3- 1
Permanganate MnO4- 1
Hydrogensulfite HSO3- 1
Hydrogensulfate HSO4- 1
Sulphate SO32- 2
Sulphite SO42- 2
Dichromate Cr2O72- 2
Chromate CrO42- 2
Carbonate CO32- 2
Phosphate PO43- 3

So to write a formula such as calcium phosphate, find the valency of calcium (Group 2 so VN = 2) and phosphate, PO4, has a VN of 3

Calcium  phosphate                    Ammonium sulphate            Calcium hydroxide

  Ca            PO4                              NH4            SO4                   Ca           OH

  2                 3                                   1                  2                       2              1

   Ca3(PO4)2                                       (NH4)2SO4                           Ca(OH)2  (NOT CaOH2 which is wrong)

Notice a bracket is used if ANY of the 'complex' ions has to be multiplied by a number > 1