What are the three types of chemical bond, and when does each form?
The three types of chemical bond (ionic, covalent and metallic); when each forms based on the elements involved; the link between bonding and the particles transferred or shared.
A focused answer to AQA GCSE Chemistry 4.2.1, covering the three types of chemical bond, when ionic, covalent and metallic bonding occur, and how electrons are transferred or shared in each.
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What this dot point is asking
AQA wants you to name the three types of chemical bond, state which combinations of elements form each type, and explain that bonding involves either the transfer or the sharing of electrons in the outer shells of atoms. The decisive clue in any question is whether the elements are metals or non-metals.
The three types of bond
When each type forms
The combination of elements tells you the bond type. A metal with a non-metal gives ionic bonds; two or more non-metals give covalent bonds; a metal on its own (an element or alloy) has metallic bonding. So you can predict the bonding in a compound just from where its elements sit in the periodic table.
Why atoms bond
Atoms react to reach a full outer shell. Metals can do this by losing electrons (forming positive ions), and non-metals by gaining or sharing electrons. Which route happens decides whether the bond is ionic, covalent or metallic. A non-metal reacting with a metal gains the metal's electrons (ionic); two non-metals share because neither will give up electrons (covalent); metal atoms together pool their outer electrons (metallic).
How the bond type sets the structure and properties
The bond type is the first thing to identify because it determines the whole structure of the substance and therefore its properties. Ionic bonding gives a giant lattice of alternating positive and negative ions, which makes ionic compounds hard, brittle solids with high melting points that conduct only when the ions are free to move (molten or dissolved). Covalent bonding gives either small molecules (with low melting points, held to each other by weak intermolecular forces) or giant covalent lattices such as diamond (with very high melting points). Metallic bonding gives a giant lattice of positive ions in a sea of delocalised electrons, which makes metals good conductors, malleable and high melting. So once you know the elements involved, you can predict the bond type, then the structure, then the bulk properties in a single chain of reasoning.
Representing bonds
Chemists show bonding with dot and cross diagrams, in which the electrons from one atom are drawn as dots and those from the other as crosses. For ionic compounds the diagram shows electrons transferred and the charge on each ion in square brackets; for covalent molecules it shows the shared pairs in the overlap between the atoms. These diagrams make clear, at a glance, whether electrons have been transferred (ionic) or shared (covalent).
Try this
Q1. State the type of bonding between magnesium and oxygen. [1 mark]
- Cue. Ionic, because magnesium is a metal and oxygen a non-metal.
Q2. State the type of bonding in chlorine gas, . [1 mark]
- Cue. Covalent, because both atoms are non-metals.
Q3. State the type of bonding in iron, and what holds it together. [2 marks]
- Cue. Metallic bonding; positive ions attracted to a sea of delocalised electrons.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA 20193 marksFor each of the following substances, state the type of chemical bonding present and justify your choice from the elements involved: magnesium oxide (), oxygen gas (), and copper ().Show worked answer →
A 3-mark Paper 1 question testing the link between elements and bond type.
Magnesium oxide (1 mark): ionic bonding, because magnesium is a metal and oxygen a non-metal, so electrons are transferred. Oxygen gas (1 mark): covalent bonding, because both atoms are non-metals, so they share electrons. Copper (1 mark): metallic bonding, because it is a metal element, so positive ions sit in a sea of delocalised electrons.
Markers reward each justification being tied to whether the elements are metals or non-metals.
AQA 20212 marksExplain why a metal and a non-metal form an ionic bond rather than a covalent bond, referring to what happens to the electrons.Show worked answer →
A 2-mark question on why ionic bonding forms.
A metal has few outer electrons and loses them easily, while a non-metal has a nearly full outer shell and gains electrons easily (1 mark). So electrons are transferred from the metal to the non-metal, forming oppositely charged ions held by electrostatic attraction (an ionic bond), rather than being shared as in a covalent bond (1 mark).
Markers want the transfer (not sharing) of electrons identified as the reason.
Related dot points
- Ionic bonding; the transfer of electrons to form ions; dot and cross diagrams; the giant ionic lattice; and how the structure explains melting points and conductivity.
A focused answer to AQA GCSE Chemistry 4.2.1, covering how ions form by electron transfer, drawing dot and cross diagrams, the giant ionic lattice, and how the structure explains the high melting points and conductivity of ionic compounds.
- Covalent bonding; shared pairs of electrons; small molecules; giant covalent structures such as diamond, graphite and silicon dioxide; and how structure explains properties.
A focused answer to AQA GCSE Chemistry 4.2.1 and 4.2.2, covering how covalent bonds form by sharing electrons, small molecules, polymers and giant covalent structures such as diamond, graphite and silicon dioxide, and how structure explains their properties.
- Metallic bonding; positive ions in a sea of delocalised electrons; the properties of metals; why alloys are harder than pure metals.
A focused answer to AQA GCSE Chemistry 4.2.1 and 4.2.2, covering metallic bonding as positive ions in a sea of delocalised electrons, how this explains conductivity and malleability, and why alloys are harder than pure metals.
- The three states of matter; the particle model; changes of state; state symbols; and the limitations of the particle model.
A focused answer to AQA GCSE Chemistry 4.2.2, covering the three states of matter, the particle model, melting, boiling and the energy needed for changes of state, state symbols, and the limitations of the simple particle model.
- Linking structure and bonding to properties; the four main structures; allotropes of carbon including graphene and fullerenes; and predicting properties from structure.
A focused answer to AQA GCSE Chemistry 4.2.2 and 4.2.3, linking the four main structures (ionic, simple molecular, giant covalent and metallic) to their properties, and covering the allotropes of carbon including graphene and fullerenes.
Sources & how we know this
- AQA GCSE Chemistry (8462) specification — AQA (2016)