AQA GCSE Combined Science (8464), Higher tier · Chemistry › Bonding, structure, and the properties of matter › Chemical bonds, ionic, covalent and metallic
Practise Ionic bonding. 20 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
How metal atoms and non-metal atoms form ions by transferring electrons, how to show this with a dot and cross diagram, and how to work out the charge on an ion from its group. Expect 'describe what happens, in terms of electrons' questions (3 to 4 marks), dot and cross diagrams to draw or complete, and formulae to work out.
Grade by grade
What you need to be able to do, from the first marks up to the top grade.
3
State that metals lose and non-metals gain electronsMetal atoms lose outer electrons to form positive ions; non-metal atoms gain electrons to form negative ions.
4
Work out the charge from the group numberGroup 1 → 1+, Group 2 → 2+, Group 6 → 2−, Group 7 → 1−.
5
Write the electronic structure of an ionFor example, Na (2,8,1) becomes Na+ (2,8) and O (2,6) becomes O2− (2,8).
5
Draw dot and cross diagrams for ionic compoundsShow each ion in square brackets with its charge, using dots and crosses for electrons from different atoms.
6
Work out the formula of an ionic compoundBalance the charges so the total is zero, e.g. Ca2+ and Cl− give CaCl2.
7
Describe electron transfer when the ratio is 2 : 1For example, in lithium oxide two lithium atoms each transfer one electron to one oxygen atom.
Notes
Forming ions
When a metal reacts with a non-metal, electrons in the outer shell of the metal atom are transferred to the non-metal atom.
The metal atom loses electrons and becomes a positive ion. The non-metal atom gains electrons and becomes a negative ion.
An ion is charged because it now has unequal numbers of protons and electrons. The number of protons never changes.
The ions formed by metals in Groups 1 and 2 and by non-metals in Groups 6 and 7 have the electronic structure of a noble gas (Group 0): a full outer shell.
Charge from the group number
Group 1 atoms lose 1 electron → 1+ ions, e.g. Na+, K+.
Group 2 atoms lose 2 electrons → 2+ ions, e.g. Mg2+, Ca2+.
Group 6 atoms gain 2 electrons → 2− ions, e.g. O2−, S2−.
Group 7 atoms gain 1 electron → 1− ions, e.g. F−, Cl−.
Negative ions made from one atom end in -ide: oxide, sulfide, fluoride, chloride.
Dot and cross diagrams
Use dots for the electrons of one atom and crosses for the electrons of the other, so you can see where each electron came from.
Sodium chloride: Na (2,8,1) transfers its outer electron to Cl (2,8,7). You get [Na]+ (2,8) and [Cl]− (2,8,8). If sodium's electrons are crosses, the outer shell of the chloride ion has 7 dots and 1 cross.
Draw each ion in square brackets with its charge at the top right. If you draw outer shells only, show the metal ion with an empty outer shell (or with the full shell below it) and the non-metal ion with 8 outer electrons.
Magnesium oxide: Mg (2,8,2) transfers two electrons to O (2,6), forming Mg2+ and O2−.
When the ratio is not 1 : 1, draw every ion: calcium chloride has one Ca2+ and two Cl− ions; sodium oxide has two Na+ ions and one O2−. grade 6+
Working out the formula
A compound has no overall charge, so the total positive charge must equal the total negative charge.
Two K+ balance one O2−, so potassium oxide is K2O. One Mg2+ balances two F−, so magnesium fluoride is MgF2.
Cheatsheet
Metal atoms lose electrons → positive ions; non-metal atoms gain electrons → negative ions
Group 1: 1+ Group 2: 2+ Group 6: 2− Group 7: 1−
Ions from Groups 1, 2, 6 and 7 have a noble gas electronic structure (full outer shell)
Na 2,8,1 → Na+ 2,8 Cl 2,8,7 → Cl− 2,8,8
Mg 2,8,2 → Mg2+ 2,8 O 2,6 → O2− 2,8
Dot and cross: each ion in square brackets, charge at the top right
Formula: total positive charge = total negative charge
How to answer each type of question
Work out the charge on an ion or the formula of a compound
1 to 2 marks5
Find the group of each element: Groups 1 and 2 form positive ions, Groups 6 and 7 negative ions.
Write the charge on each ion.
Use enough of each ion to make the total charge zero, and write the numbers as subscripts.
Example. Barium is in Group 2. Bromine is in Group 7. (a) Give the charge on a barium ion. (b) Give the formula of barium bromide.
Show the model answer
(a) 2+ (1) (b) BaBr2 (1)
Draw or complete a dot and cross diagram
2 to 3 marks5
Work out the ions formed and how many of each you need.
Draw the outer shell of each ion: the non-metal ion has 8 electrons, with the transferred electrons shown as a different symbol.
Put square brackets round each ion and write its charge at the top right.
Example. Potassium has the electronic structure 2,8,8,1. Chlorine has the electronic structure 2,8,7. Draw a dot and cross diagram for potassium chloride. Show the outer shell electrons only.
Show the model answer
The diagram should show: the potassium ion with no electrons in its outer shell (or a full shell of 8) (1) the chloride ion with 8 outer electrons: 7 of one symbol and 1 of the other (1) both ions in brackets with the correct charges, [K]+ and [Cl]− (1)
Explain why an ion has a particular charge
2 marks6
Say how many outer electrons the atom has (from its group or electronic structure).
Say how many electrons it loses or gains to get a full outer shell, and so whether it has more protons or more electrons.
Example. Explain why a sulfide ion has a charge of 2−.
Show the model answer
A sulfur atom has 6 electrons in its outer shell (1). It gains 2 electrons to get a full outer shell, so it has 2 more electrons than protons (1).
Describe how the ions form, in terms of electrons
3 to 4 marks7
Say which atom loses electrons, how many, and that they are transferred to the non-metal.
Say which atom gains electrons and how many each atom gains.
Name the ions formed, with their charges.
Say that the ions have full outer shells (the electronic structure of a noble gas).
Example. Lithium reacts with oxygen to form lithium oxide, Li2O. Describe what happens when lithium atoms react with oxygen atoms. Answer in terms of electrons.
Show the model answer
Each lithium atom loses one electron, which is transferred to oxygen (1). Each oxygen atom gains two electrons, so two lithium atoms are needed for each oxygen atom (1). Li+ ions and O2− ions are formed (1). Both ions have a full outer shell / the electronic structure of a noble gas (1).
Shortcuts and memory tricks
Metals lose (positive ions); non-metals gain (negative ions).
Charge rule: Groups 1 and 2, charge = group number, positive. Groups 6 and 7, charge = 8 − group number, negative.
Swap and drop for formulae: write the charges, swap the numbers over as subscripts, then cancel. Ca2+ Cl− → CaCl2; Mg2+ O2− → Mg2O2 → MgO.
Check: Na+, Mg2+, O2− and F− are all 2,8, the same as neon.
Where marks are lost
Saying a metal atom becomes positive by gaining protons. Ions form by losing or gaining electrons; the number of protons does not change.
Saying atoms share electrons in ionic bonding. Electrons are transferred.
Getting the sign wrong: losing electrons gives a positive ion; gaining electrons gives a negative ion.
Leaving out the brackets or charges on a dot and cross diagram, or drawing a shared pair between the ions.
Drawing only one ion of each type when two are needed, e.g. one Cl− for calcium chloride.
Writing 'chlorine ion' instead of 'chloride ion'. Use the -ide name for the negative ion.
Exam technique
In 'answer in terms of electrons' questions, give numbers: 'each magnesium atom loses two electrons' scores; 'magnesium loses electrons' may not.
Always give the direction of transfer: from the metal atom to the non-metal atom.
Use the electronic structures given in the question and check that each ion ends with a full outer shell.
In a dot and cross diagram, the number of transferred electrons shown in the negative ions must equal the number the metal atoms lost.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
Ionic compounds form when metals react with non-metals. What is the charge on an oxide ion?
2−
Give the electronic structures of the lithium ion and the fluoride ion.
Li+: 2; F−: 2,8
Give the electronic structure of a magnesium ion and of an oxide ion.
both 2,8
Give the electronic structures of the sodium ion and the oxide ion.
Na+: 2,8; O2−: 2,8
Give the charge on the ion formed by potassium and the charge on the ion formed by sulfur.
potassium 1+; sulfur 2−
Identify two mistakes in the student's description. Correct each mistake.
Electrons are transferred, not shared; the aluminium ion is 3+ and the fluoride ion is 1−.
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
Ionic compounds form when metals react with non-metals.
(a) When a metal atom reacts with a non-metal atom, what happens to the electrons in the outer shell of the metal atom? Tick (✓) one box.[1]
They are transferred to the non-metal atom.
They are shared with the non-metal atom.
They are gained from the non-metal atom.
They become delocalised.
(b) What is the charge on a sodium ion? Tick (✓) one box.[1]
1−
1+
2+
2−
(c) What is the charge on an oxide ion?[1]
(d) Complete the sentence. Tick (✓) one box. The ions formed by Group 1 metals have the same electronic structure as ................[1]
a noble gas
a halogen
a transition metal
an alkali metal
Show the answer and mark scheme
(a)Answer: They are transferred to the non-metal atom.
(b)Answer: 1+
(c)Answer: 2−
2− / −2
(d)Answer: a noble gas
Question 2Medium6 marks
Lithium reacts with fluorine to form lithium fluoride, LiF. The electronic structure of lithium is 2,1. The electronic structure of fluorine is 2,7.
(a) Describe what happens when a lithium atom reacts with a fluorine atom. Answer in terms of electrons.[3]
(b) Give the electronic structures of the lithium ion and the fluoride ion.[2]
(c) Which noble gas has the same electronic structure as the fluoride ion?[1]
Show the answer and mark scheme
(a)Answer: The lithium atom loses its outer electron, which is transferred to the fluorine atom; Li+ and F− ions form.
the lithium atom loses one electron
the electron is transferred to / gained by the fluorine atom
Li+ and F− ions are formed
(b)Answer: Li+: 2; F−: 2,8
Li+: 2
F−: 2,8
(c)Answer: neon
neon / Ne
Question 3Hard6 marks
Calcium reacts with chlorine to form calcium chloride. The electronic structure of calcium is 2,8,8,2. The electronic structure of chlorine is 2,8,7.
Describe how calcium chloride forms from calcium atoms and chlorine atoms. Your answer should include the electronic structures of the ions formed and the formula of calcium chloride.[6]
Show the answer and mark scheme
Answer: Each Ca atom loses 2 electrons, one to each of two Cl atoms; Ca2+ (2,8,8) and 2Cl− (2,8,8) form, with noble gas structures; formula CaCl2; ions held by strong electrostatic attraction.
each calcium atom loses its two outer electrons
one electron is transferred to each of two chlorine atoms
each chlorine atom gains one electron
a calcium ion, Ca2+, forms with electronic structure 2,8,8
two chloride ions, Cl−, form, each with electronic structure 2,8,8
all the ions have the electronic structure of a noble gas (argon) / a full outer shell
the formula is CaCl2 because two Cl− ions balance the charge on one Ca2+ ion
the oppositely charged ions are held together by strong electrostatic forces of attraction (in a giant lattice)
Marked with levels of response: the full level descriptors are in the app.