AQA GCSE Chemistry: Chemical Bonds and Ionic Bonding (8462)

This guide covers 4.2.1.1 Chemical Bonds and 4.2.1.2 Ionic Bonding, from AQA GCSE Chemistry (8462), GCSE exams June 2018 onwards, Version 1.1. Both open Topic 2: Bonding, Structure, and the Properties of Matter.

Syllabus coverage

AQA GCSE CHEMISTRY (8462) — 4.2.1.1 CHEMICAL BONDS

There are three types of strong chemical bonds: ionic, covalent and metallic. For ionic bonding the particles are oppositely charged ions. For covalent bonding the particles are atoms which share pairs of electrons. For metallic bonding the particles are atoms which share delocalised electrons. Ionic bonding occurs in compounds formed from metals combined with non-metals; covalent bonding occurs in most non-metallic elements and in compounds of non-metals; metallic bonding occurs in metallic elements and alloys. Students should be able to explain chemical bonding in terms of electrostatic forces and the transfer or sharing of electrons.

4.2.1.2 IONIC BONDING

When a metal atom reacts with a non-metal atom, electrons in the outer shell of the metal atom are transferred: metal atoms lose electrons to become positively charged ions; non-metal atoms gain electrons to become negatively charged ions. Ions produced 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). The electron transfer can be represented by a dot and cross diagram. The charge on ions produced by metals in Groups 1 and 2 and non-metals in Groups 6 and 7 relates to the group number of the element; students should be able to work out this charge from the group number, limited to metals in Groups 1 and 2 and non-metals in Groups 6 and 7.

How to approach it

Learn the three bond types as a matched set from the start, since questions frequently ask you to identify which type of bonding a given compound has and justify it from the elements involved: metal + non- metal signals ionic; non-metal + non-metal signals covalent; metal alone (or an alloy) signals metallic. Being able to justify the identification from the elements’ positions in the periodic table, not just name the bond type, is what separates a full-mark answer.

For dot and cross diagrams, practise the specific convention: dots represent electrons from one atom, crosses represent electrons from the other, and only the outer (valence) shell electrons are shown, with square brackets and the appropriate charge around each ion in the finished diagram. For working out ion charge from group number, the rule is direct: a Group 1 metal loses 1 electron to form a 1+ ion; a Group 2 metal loses 2 electrons to form a 2+ ion; a Group 6 non-metal gains 2 electrons to form a 2− ion; a Group 7 non-metal gains 1 electron to form a 1− ion — in every case the ion formed has the electronic structure of the nearest noble gas.

Worked example: dot and cross diagram logic for magnesium oxide

A question asks candidates to explain, in terms of electron transfer, how magnesium oxide forms.

Magnesium (Group 2):    has 2 electrons in its outer shell; loses both
                         to form a Mg2+ ion with the electronic
                         structure of neon (a noble gas)
Oxygen (Group 6):        has 6 electrons in its outer shell; gains 2
                         electrons (both lost by magnesium) to form an
                         O2- ion, also with the electronic structure
                         of neon
Result:                  the oppositely charged Mg2+ and O2- ions are
                         held together by strong electrostatic forces
                         of attraction, forming the ionic bond in
                         magnesium oxide (MgO)

Explicitly stating both ions’ resulting electronic structures, and naming the noble gas each now matches, demonstrates the full understanding this content requires, not just the final formula.

Key terms to define precisely

Ion — an atom or group of atoms that has lost or gained one or more electrons, giving it an overall electric charge. Electron transfer — the movement of one or more electrons from one atom to another, the mechanism by which ionic bonds form. Electrostatic force of attraction — the force holding oppositely charged ions together in an ionic bond, arising from the attraction between positive and negative charges. Noble gas electronic structure — a full outer electron shell, the stable arrangement that atoms tend towards when losing or gaining electrons to form ions. Dot and cross diagram — a diagram representing the electron transfer involved in ionic bond formation, using dots for one atom’s electrons and crosses for the other’s, showing only the outer shell. Precision about “electrostatic force of attraction” as the specific named force holding an ionic compound together — rather than a vaguer description like “they stick together” — is exactly the kind of terminology GCSE mark schemes are looking for in a full-mark explanation.

Common mistakes

Identifying a bond type without justifying it from the elements involved (metal/non-metal for ionic, non-metal/non-metal for covalent). Forgetting to show the correct charge and square brackets around each ion in a finished diagram (the convention of showing only outer-shell electrons is not itself a requirement – diagrams showing all occupied shells are acceptable provided the charges are shown). Applying the “charge equals group number” rule to elements outside Groups 1, 2, 6 and 7, where the specification does not require (and the rule does not reliably apply to) this simple approach.

Quick revision checklist

• Learn the three bond types matched to the type of elements involved (metal/non-metal, non-metal/non-metal, metal alone).
• Practise drawing dot and cross diagrams showing only outer-shell electrons with correct ion charges.
• Memorise the ion charge produced by Groups 1, 2, 6 and 7, and that each resulting ion matches a noble gas structure.
• Practise explaining ionic bond formation as a two-step process: electron transfer, then electrostatic attraction between the ions formed.

Related resources

• Chemical Bonds and Ionic Bonding revision notes
• Chemical Bonds and Ionic Bonding practice questions

Official syllabus

AQA GCSE Chemistry (8462) specification, Version 1.1, exams June 2018 onwards — aqa.org.uk/8462.