Edexcel IGCSE Chemistry · Spec 1.38-1.40
Ion Charges, Ionic Formulae and Dot-and-Cross Diagrams
Covers recall the required ion charges, Construct neutral ionic formulae and Draw dot-and-cross diagrams for unequal ion ratios.
Chemistry revision video
Ion Charges, Ionic Formulae and Dot-and-Cross Diagrams
Prefer to watch on YouTube? Open this video on YouTube.
Explained
Ion charges, building formulae, and drawing the diagram
Every ionic formula in the course can be built from two things: the charges on the ions, and the rule that the compound overall must be neutral. Learn the charges and the formulae assemble themselves.
The charges you can predict
For the main groups, the group number tells you. Group 1 forms 1 plus, group 2 forms 2 plus, group 3 forms 3 plus. Group 5 forms 3 minus, group 6 forms 2 minus, group 7 forms 1 minus.
Metals lose electrons and form positive ions. Non metals gain electrons and form negative ions. The number lost or gained is whatever takes the atom to a full outer shell.
Transition metals are the exception, forming more than one possible charge, which is why their compounds are named with a Roman numeral. Iron(II) is Fe two plus and iron(III) is Fe three plus, and the numeral is telling you the charge rather than counting anything.
The ones you have to learn
Polyatomic ions are groups of atoms carrying a single charge between them, and there is no rule for them. Nitrate is NO3 minus. Sulfate is SO4 two minus. Carbonate is CO3 two minus. Hydroxide is OH minus. Ammonium is NH4 plus, the only common positive one. Phosphate is PO4 three minus.
Learn these six as facts. Every salt preparation, precipitation and formula question in the course uses them.
Building the formula
Write the two ions with their charges, then add whichever numbers of each make the charges cancel.
Magnesium chloride: Mg two plus and Cl minus, so two chlorides for each magnesium, giving MgCl2.
Aluminium oxide: Al three plus and O two minus. Three and two have a lowest common multiple of six, so two aluminiums and three oxides, giving Al2O3.
Where you need more than one polyatomic ion, bracket it before adding the number, so that the subscript applies to the whole group. Ammonium sulfate is (NH4)2SO4, meaning two ammonium ions. Written NH42SO4 it would mean forty two hydrogens, which is why the brackets are not optional.
What the mark scheme accepts and rejects
An Edexcel International GCSE Chemistry mark scheme sets a grid with three positive ions down one side and three negative ions along the top, and asks for the formulae in the empty cells. Three marks, and the answers it prints include Mg(NO3)2, Li2SO4 and AlPO4.
Those three are chosen carefully. One needs brackets, one needs a subscript on the metal, and one needs neither because the charges already cancel. A candidate who has learned to always add a number somewhere gets the third one wrong.
On the diagrams, the same paper asks for an electron arrangement and awards two marks: one for a correct diagram with an inner arrangement of 2,8, and one for a correct diagram with eight electrons in the outer shell.
Then the ruling that decides it: 2,8,8 written out with no diagram scores one mark only.
The electron arrangement is correct, and it was asked for as a diagram. Half the credit is for drawing it, so a written arrangement collects half. Where a question says draw, the drawing is part of the answer.
It adds one further instruction: ignore the charge on the ion. So forgetting the square brackets and the charge around an ion diagram is not penalised here, which is worth knowing, and putting them in is not penalised either. The shells are what is being marked.
Drawing dot and cross diagrams for ionic compounds
Show the outer shell of each atom before the transfer, then draw the ions afterwards with square brackets and the charge outside the bracket.
Use dots for one atom's electrons and crosses for the other's, so the transfer can be seen. The metal ion ends with an empty outer shell drawn, or with its full inner shell shown, and the non metal ion ends with eight electrons, one of which came from the metal and is drawn in the other symbol.
Where the ratio is not one to one, draw every ion. Magnesium chloride needs one magnesium and two chlorides, and a diagram showing only one chloride has not accounted for the second electron the magnesium lost.
That accounting is what the diagram exists to show. Count the electrons the metal gives away and check that the same number has arrived somewhere on your page.
Spec 1.38-1.40
What you need to know
- Recall the required ion charges
- Construct neutral ionic formulae
- Draw dot-and-cross diagrams for unequal ion ratios
Active recall
Quick check
Answer each question before opening the answer.
What charge does sulfate carry?
2-
What is the formula of aluminium oxide?
Al2O3
Why are brackets used in (NH4)2SO4?
Because more than one polyatomic ammonium ion is needed
Chemistry revision app
Take this topic further in the app
275 guided topics with questions marked as you answer them, section checkpoints and timed practice papers. Fifteen topics are free to try, with no card needed.