Learning focus
Use chemical formulae and equations accurately, convert measured quantities into moles, apply balanced-equation ratios and present multi-step calculations with units and checks.
Electrical neutrality
An ionic compound has no overall charge. The total positive charge must equal the total negative charge. The formula therefore expresses the lowest whole-number ratio of cations to anions that balances charge.
Systematic method
Write each ion with its charge, determine the least common multiple of charge magnitudes, then choose the number of each ion needed. Remove charges when writing the final formula. Simplify the ratio if both subscripts share a common factor.
Examples
Na+ and O2- form Na2O. Mg2+ and Cl- form MgCl2. Al3+ and O2- form Al2O3. Ca2+ and N3- form Ca3N2. The method works because each final formula has total charge zero.
Transition-metal names
Roman numerals in names state oxidation number and therefore ionic charge. Iron(II) chloride contains Fe2+ and is FeCl2; iron(III) chloride contains Fe3+ and is FeCl3. The numeral is not a subscript copied into the formula.
Worked example
For aluminium sulfate: Al3+ and SO4 2-. The least common charge is 6, so use 2 Al3+ and 3 sulfate ions: Al2(SO4)3.
Practical or data skill
Prepare ion cards showing symbols and charges. Combine cards until positive and negative charges balance, then write the formula.
Examination tip
Check the final formula by calculating total positive and total negative charge.
Review questions and suggested answers
Question 1
Write the formula of magnesium nitride.
Suggested answer
Mg3N2.
Question 2
Write the formula of copper(II) oxide.
Suggested answer
CuO.
Question 3
Why is calcium chloride CaCl2?
Suggested answer
One Ca2+ requires two Cl- ions to balance charge.
