Formulae of ionic compounds
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The teaching part (2 min) is free to watch. A free account opens the whole lecture: 5 worked examples, 7 practice questions, 2 real past-paper questions with the answers explained, the mistakes that lose marks, the full-marks tip, a one-card summary and a quick quiz.
What this lecture covers
- Why is it MgCl₂?
- The rule
- The ions to know
- Roman numerals
- The method
Read the transcript of the teaching part
Last time
Before we start: last time, in Empirical and molecular formulae, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.
Why is it MgCl₂?
Sodium chloride is NaCl, but magnesium chloride is MgCl₂. Why the extra chlorine? Because an ionic compound has no overall charge. The positive and negative charges must cancel exactly, and that decides the formula.
The rule
An ionic compound is electrically neutral. The total positive charge equals the total negative charge. So the formula is simply the smallest numbers of each ion that make the charges add up to zero. The formula shows the ratio of the ions, written with the positive ion first.
The ions to know
Learn these. Positive: hydrogen, sodium, potassium, silver and ammonium are all one plus. Magnesium, calcium, zinc and copper two are two plus. Aluminium and iron three are three plus. Negative: chloride, bromide, hydroxide and nitrate are one minus. Oxide, sulfide, sulfate and carbonate are two minus.
Roman numerals
Some metals form more than one ion. A Roman numeral in the name tells you the charge. Iron(II) chloride contains Fe²⁺, so the formula is FeCl₂. Iron(III) chloride contains Fe³⁺, so it needs three chloride ions: FeCl₃.
The method
First, write the symbol and charge of each ion. Second, find how many of each are needed so the charges cancel. Use the lowest common multiple. Third, write the formula, with the numbers as subscripts. A subscript of one is not written. Fourth, if you need more than one of a group of atoms like OH or SO₄, put the group in brackets.
How often it comes up
How often Cambridge has asked each part, counted by ExamYard from every past paper in its question bank: Formulae of ionic compounds · Properties of ionic compounds · Formulae.
The whole chapter: Stoichiometry, Stoichiometry.
Chapter 3: Stoichiometry
- Relative formula mass 6 min
- Formulae of ionic compounds 6 min
- Writing and balancing equations 7 min
- Ionic equations 7 min
- The mole and the Avogadro constant 6 min
- Moles: amount, mass and molar mass 6 min
- Reacting masses 7 min
- Gas volumes at r.t.p. 7 min
- Concentration and titration calculations 9 min
- Percentage composition by mass 6 min
- Empirical and molecular formulae 7 min
- Water of crystallisation 7 min
Also for IGCSE Chemistry 0620
Which topics come up most in IGCSE Chemistry 0620 past papers, counted from every paper since 2004.