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!
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:
Example: How do we work out that the formula of aluminium oxide is Al2O3?
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
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.
For the metal ions (cations) it couldn't be easier, its just the metal name itself e.g. sodium, magnesium, aluminium etc.
However, there is one complication when a metal can form two different ions like copper or iron.
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,
Fluorine forms the fluoride ion, chlorine the chloride ion, bromine the bromide ion and iodine the iodide ion.
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)
Appendix 3. Tests for Cations and Anions
These are all written up in detail on other pages, so see ...
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