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2b. How to work out ionic formula and name ionic compounds

Doc Brown's Chemistry: Chemical Bonding and structure GCSE level, IGCSE, O, IB, AS, A level US grade 9-12 level Revision Notes

A note on ionic formulae

For covalent compounds you have a definite formula of a molecule e.g. H2O meaning two atoms of hydrogen combined with one atom of oxygen to form a single particle of three atoms.

However, ionic compounds form a giant ionic lattice of layer after layer of oppositely charged ions e.g.

for sodium chloride NaCl you have a three dimensional array of positive sodium ions Na+ and negative chloride ions Cl-.

There are no NaCl molecules. Therefore an ionic compound formula gives you the ratio of the component ions.

For example: sodium chloride, formula NaCl means a ratio of one Na+ ion to one Cl- ion

magnesium fluoride, formula MgBr2 means a ratio of one Mg2+ ions to two F- ions

aluminium fluoride, formula AlF3 means a ratio of one Al3+ ion to three F- ions.

Unfortunately the names of ionic compounds don't usually include prefixes or suffixes to help you work out the numbers of ions in a formula. So you need to know the charges on the common ions of groups 1, 2, 6 and 7, and this has been explained in detail in the preceding section. If you know how to work out the charge on ions from the periodic table, you can then work out the correct 'ratio' formula. There is section near the end on showing you examples of working out ionic formula.

The electronic dot & cross diagram for the ionic bonding in the ionic compound calcium chloride

Appendix 1. How to work out the formula for an ionic compound

Table 1a Selected ions and charges

Its a dated image! sulfide is now sulfide and sulfate is now called sulfate!

Table 1b. The periodic table pattern of charges on ions

The charge is based on the number of electrons lost (giving positive ions) or gained (giving negative ions) forming a noble gas electron structure, i.e. to make a full outer shell of electrons

CATIONS from metals ANIONS from non–metals
Group 1 Group 2 Group 3 Group 6 Group 7
lithium ion Li+ beryllium ion Be2+   oxide ion O2– fluoride ion F
sodium ion Na+ magnesium ion Mg2+ aluminium ion Al3+ sulfide ion S2– chloride ion Cl
potassium ion K+ calcium ion Ca2+     bromide ion Br
        iodide ion I

In the electrically balanced stable formula, the total positive ionic charge must equal the total negative ionic charge i.e. the total charge is zero.

To work out an ionic formula by combining ion 'A' with ion 'B' the rule is:

number of ion 'A' x charge of ion 'A' = number of ion 'B' x charge of ion 'B'

(you ignore charge sign as long as you realise that A and B have opposite charges)

Example: How do we work out that the formula of aluminium oxide is Al2O3?

As difficult an example as any you will have to work out!

Aluminium oxide consists of aluminium ions Al3+ and oxide ions O2– 

number of Al3+ ions x charge on Al3+ balances the number of  O2– ions x charge on O2– 

the simplest numbers are 2 of Al3+ x 3 balances 3 of  O2– x 2 (total 6+ balances total 6–)

so the simplest whole number formula for aluminium oxide is Al2O3 

More examples of working out the empirical formula of an ionic compound

numerically ion charge = valency of A or B to deduce the formula

valency or ionic charge = the combining power of the ion (see Table 1a above)

'molecular' or ionic style of formula and compound name

1 of K+ balances 1 of Br because 1 x 1 = 1 x 1 gives KBr or K+Br  potassium bromide

2 of Na+ balances 1 of O2– because 2 x 1 = 1 x 2 gives Na2O or (Na+)2O2–  sodium oxide

1 of Mg2+ balances 2 of Cl because 1 x 2 = 2 x 1 gives MgCl2 or Mg2+(Cl)2  magnesium chloride

1 of Fe3+ balances 3 of F because 1 x 3 = 3 x 1 gives FeF3 or   Fe3+(F)3  iron(III) fluoride

1 of Ca2+ balances 2 of NO3 because 1 x 2 = 2 x 1 gives Ca(NO3)2 or Ca2+(NO3)2  calcium nitrate

2 of Fe3+ balances 3 of SO42– since 2 x 3 = 3 x 2 gives Fe2(SO4)3 or (Fe3+)2(SO42–)3  iron(III) sulfate

1 of Mg2+ is balanced by 2 OH giving Mg(OH)2  magnesium hydroxide

2 of K+ is balanced by 1 CO32– giving K2CO3 potassium carbonate

1 of Ca2+ is balanced by 2 of I giving CaI2 calcium iodide

1 Mg2+ is balanced by 2 NO3 giving Mg(NO3)2 magnesium nitrate

1 Zn2+ is balanced by 1 SO42– giving ZnSO4 zinc sulfate

By applying similar logic you can work out the charge on one ion, knowing the formula and charge on the other ion

e.g. supposing a metal M forms an ionic chloride compound MCl2, Cl will be the chloride ion (charge single –), so to balance the two chloride ions, the metal ion must carry a charge of 2+ i.e. the M2+ ion.

If a metal that forms a singly charged positive ion M+, forms an ionic sulfide compound M2S, the charge on the sulfur ion must be 2– i.e. S2–, to balance the two + charges of the metal ion.

Appendix 2 How do you name simple ionic compounds?

How to name ionic compounds: Naming simple ionic compounds isn't difficult and through your course you get used to the names as they crop up.

The name usually consists of two parts, first the name of the positive metal ion, and secondly, the name of the negative ion derived from the non–metal.

For the metal ions (cations) it couldn't be easier, its just the metal name itself e.g. sodium, magnesium, aluminium etc.

For the Na+ ion e.g. sodium chloride

For the Mg2+ ion e.g. magnesium bromide

For the Al3+ ion eg. aluminium oxide etc.

However, there is one complication when a metal can form two different ions like copper or iron.

In these cases the two ions are distinguished in the name by a Roman numerals number in brackets after the name of the metal which corresponds to the numerical value of the positive charge on the metal ion.

e.g. Cu2O copper(I) oxide contains the Cu+ copper(I0 ion, and CuO copper(II) oxide contains the Cu2+ copper(II) ion

and FeCl2 iron(II) chloride contains the Fe2+ iron(II) ion, and FeCl3 contains the Fe3+ iron(III) ion.

However, things are a bit more complicated for the negative ions (anions) because although the name of the ion is derived from the name of the non–metallic element, it is a bit different.

Oxygen forms the oxide ion, sulfur the sulfide ion etc. the names of these anions from group 6 end in ..ide,

e.g. sodium oxide, magnesium oxide, aluminum oxide

Fluorine forms the fluoride ion, chlorine the chloride ion, bromine the bromide ion and iodine the iodide ion.

The names of the anions from group 7 halogens (naming ending ...ine) end in ...ide, the halide ions),

e.g. potassium fluoride, sodium chloride, calcium bromide etc.

The name only ends in ide, when the metal is only combined with one other non–metal element e.g. oxygen forming an oxide, sulfur forming a sulfide, other 'ide's include nitrides from nitrogen and phosphides from phosphorus.

For anions where two or more non–metallic atoms are combined in a single ion, and one of them is oxygen, the name often ends in ..ate (NOT oxide)

e.g. carbonate (C + O, CO3), sulfate (S + O, SO4), nitrate (N + O, NO3), chlorate (Cl + O, ClOx x varies) etc. see their formula in the table 1a. of Appendix 1 above.

Appendix 3. Tests for Cations and Anions

These are all written up in detail on other pages, so see ...

TESTS for Metal cations (positive ions)Group 1 The Alkali Metals  and  Transition Metals

TESTS for Anions (negative ions)  and  Group 7 The Halogens







What next?

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Sub-index for: Part 2 Ionic Bonding: compounds and properties


Index for ALL chemical bonding and structure notes


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