AQA GCSE Chemistry 8462: Covalent bonding and metallic bonding – Study Guide

This study guide teaches sections 4.2.1.4 Covalent bonding and 4.2.1.5 Metallic bonding of the AQA GCSE Chemistry (8462) specification, for teaching from September 2016 with exams from June 2018 (version 1.1). Both sit in Topic 2, Bonding, structure, and the properties of matter, which is examined on Paper 1 (1 hour 45 minutes, 100 marks, 50% of the GCSE) and can also be drawn on in Paper 2. Neither section contains any content marked Higher tier only, so everything here is for both Foundation and Higher tier. No required practical is attached to these two sections.

Use it with the revision notes and practice questions. Ionic bonding has its own study guide, revision notes and practice questions. The course hub is AQA GCSE Chemistry, and the printable checklist lists every statement.

What these sections cover

Spec	What you must be able to do	Tier
4.2.1.4	State that covalent bonds form when atoms share pairs of electrons, and that these bonds are strong	Both
4.2.1.4	Recognise common substances made of small molecules from their chemical formula	Both
4.2.1.4	Recognise substances as small molecules, polymers or giant covalent structures (for example diamond and silicon dioxide) from diagrams	Both
4.2.1.4	Draw dot and cross diagrams for H₂, Cl₂, O₂, N₂, HCl, H₂O, NH₃ and CH₄	Both
4.2.1.4	Use a line for a single bond in small molecules, polymer repeating units and parts of giant structures	Both
4.2.1.4	Describe the limitations of dot and cross, ball and stick, 2D and 3D diagrams	Both
4.2.1.4	Deduce a molecular formula from a model or diagram	Both
4.2.1.5	Describe metals as giant structures of atoms in a regular pattern with delocalised outer electrons, and explain why metallic bonds are strong	Both
4.2.1.5	Recognise metallic giant structures from diagrams	Both

Covalent bonding: what it is (4.2.1.4)

A covalent bond forms when two atoms share a pair of electrons. Each atom gives one electron to the pair. Covalent bonding occurs in most non-metallic elements and in compounds of non-metals.

The bond is strong. The negative shared pair sits between the two positive nuclei, and both nuclei are attracted to it by electrostatic attraction.

For the non-metals here, the number of covalent bonds an atom forms equals the number of electrons it needs to fill its outer shell.

Atom	Outer electrons	Electrons needed	Covalent bonds formed
H	1	1 (to reach 2)	1
Cl	7	1	1
O	6	2	2
N	5	3	3
C	4	4	4

One shared pair is a single bond (drawn as one line). Two shared pairs is a double bond. Three shared pairs is a triple bond.

The three kinds of covalent substance

• Small molecules. A fixed, small number of atoms joined by covalent bonds, for example H₂O, CO₂, NH₃, CH₄, HCl, H₂, O₂, N₂ and Cl₂.
• Very large molecules (polymers). Long chains of repeating units joined by covalent bonds, for example poly(ethene).
• Giant covalent structures. Huge numbers of atoms, each joined to its neighbours by covalent bonds, in a network that continues in all directions, for example diamond and silicon dioxide.

Recognising small molecules from a formula

A formula of only non-metal symbols with small numbers usually means small molecules. Two traps:

• A formula with a metal (NaCl, MgO) is ionic, not covalent.
• SiO₂ looks like CO₂, but silicon dioxide is a giant covalent structure. For a giant structure the formula gives only the ratio of atoms, not the number in a molecule. Learn diamond (C) and silicon dioxide (SiO₂) as the named giant covalent examples.

Recognising the type from a diagram

• Separate, identical small groups of atoms with space between them: small molecules.
• A chain drawn inside brackets with a small n after the bracket: a polymer.
• A network where the bonds run off the edge of the diagram in every direction: a giant covalent structure.

Dot and cross diagrams for the eight molecules

In a dot and cross diagram, the electrons of one atom are dots and those of the other atom are crosses. Show the outer shell of each atom. Each shared pair (one dot and one cross) sits in the overlap between the two atoms. Electrons that are not shared stay in pairs on their own atom.

Molecule	Shared pairs	Bond(s)	Non-shared pairs
H₂	1	H–H single	none
Cl₂	1	Cl–Cl single	3 on each Cl
HCl	1	H–Cl single	3 on Cl
O₂	2	O=O double	2 on each O
N₂	3	N≡N triple	1 on each N
H₂O	2	two O–H single	2 on O
NH₃	3	three N–H single	1 on N
CH₄	4	four C–H single	none

Check every diagram: each hydrogen has 2 electrons in its shell, and every other atom has 8 in its outer shell.

Worked example 1: water

Step 1. Outer electrons: oxygen has 6, each hydrogen has 1.

Step 2. Oxygen needs 2 more electrons, so it forms 2 bonds, one with each hydrogen.

Step 3. Draw the O outer shell overlapping each H shell. In each overlap put one dot (from O) and one cross (from H).

Step 4. Oxygen has 6 − 2 = 4 electrons left. Draw them as 2 pairs of dots on the oxygen, outside the overlaps.

Step 5. Check: oxygen has 2 + 2 shared + 4 unshared = 8. Each hydrogen has 2.

Line form:   H–O–H
Molecular formula: H₂O

Worked example 2: nitrogen

Step 1. Each nitrogen atom has 5 outer electrons and needs 3 more.

Step 2. The atoms share three pairs: a triple bond.

Step 3. In the overlap draw 3 dots and 3 crosses, arranged as three dot-cross pairs.

Step 4. Each nitrogen has 5 − 3 = 2 electrons left: one non-shared pair on each atom.

Step 5. Check: each N has 6 shared + 2 unshared = 8.

Line form:   N≡N

O₂ works the same way: each O needs 2, so a double bond, with 2 non-shared pairs on each atom.

Representing covalent bonds with lines

The specification asks you to use a line for a single bond in three settings.

Small molecules. Methane is drawn with C in the centre and four lines to four H atoms. Double and triple bonds are two and three lines (O=O, N≡N).

Polymer repeating units. A polymer is drawn as one repeating unit inside brackets, with a bond line passing through each bracket and n written after it. For poly(ethene):

   [  H   H  ]
   [  |   |  ]
 --[--C---C--]--
   [  |   |  ]
   [  H   H  ] n

Here n is a large number: the number of repeating units in one chain. The lines crossing the brackets show the chain continues.

Part of a giant covalent structure. You draw a few atoms with lines between them, and lines leaving the edge to show the structure continues. In diamond each carbon atom is bonded to four other carbon atoms. In silicon dioxide each silicon atom is bonded to four oxygen atoms and each oxygen atom to two silicon atoms. Since each oxygen is shared between two silicons, there are 4 ÷ 2 = 2 oxygen atoms for every silicon atom, which is why the formula is SiO₂.

Deducing a molecular formula from a model

A molecular formula shows the actual number of each type of atom in one molecule. To deduce it from a model or diagram, count each type of atom and write the counts as subscripts.

Worked example 3: a ball and stick model

A ball and stick model shows: two black balls joined to each other; three white balls on the first black ball; two white balls on the second black ball; and one red ball on the second black ball, which carries one more white ball. (Black = carbon, white = hydrogen, red = oxygen.)

Step 1. Count carbon: 2.

Step 2. Count hydrogen: 3 + 2 + 1 = 6.

Step 3. Count oxygen: 1.

Step 4. Write carbon, then hydrogen, then others: C₂H₆O.

Step 5. Check: each C has 4 sticks, the O has 2, each H has 1.

     H   H
     |   |
 H – C – C – O – H
     |   |
     H   H

Do not cancel the numbers: a molecular formula describes one whole molecule, not a ratio.

Limitations of the different diagrams

Learn one specific limitation of each model.

Representation	What it shows well	Limitation
Dot and cross	Which atom each electron came from; shared and unshared pairs	Usually 2D, so no shape; atoms not to scale; electrons from different atoms are really identical
2D displayed (lines)	Which atoms are bonded, and single/double/triple bonds	No 3D shape; no unshared electrons; no electrons at all
Ball and stick	3D shape and bond directions	Bonds are not sticks; atoms are drawn far apart, but really touch; atoms not to scale; no electrons
3D space-filling	Relative sizes of atoms and the overall shape	Hard to see the bonds or count them; no electrons
Any diagram of a giant structure	How the atoms are linked	Shows only a tiny part; the real structure continues in all directions with an enormous number of atoms

Metallic bonding (4.2.1.5)

Metals are giant structures of atoms arranged in a regular pattern. The electrons in the outer shell of each metal atom become delocalised: they are no longer attached to one atom and are free to move through the whole structure.

What remains of each atom is a positive ion. The sharing of the delocalised electrons between all the atoms gives strong metallic bonds. The bond is an electrostatic attraction between the positive metal ions and the negative delocalised electrons, and it acts throughout the structure.

The usual diagram shows rows of + circles in a regular arrangement with electrons (e⁻) in the spaces between them: a metallic giant structure.

Metallic bonding occurs in metallic elements and in alloys.

Worked example 4: describing the bonding in magnesium

Describe the structure and bonding in magnesium.

Point 1. Magnesium is a giant structure of atoms arranged in a regular pattern.

Point 2. Magnesium is in Group 2, so each atom has 2 outer electrons. These become delocalised, leaving Mg²⁺ ions.

Point 3. The delocalised electrons are free to move through the whole structure.

Point 4. There is strong electrostatic attraction between the positive ions and the delocalised electrons. This sharing of delocalised electrons is the strong metallic bond.

The links between metallic bonding and properties such as conductivity and malleability are taught in 4.2.2.7 and 4.2.2.8.

Common errors

• Writing that covalent bonds are weak. The bonds are strong. Small molecular substances have low melting points because of weak forces between molecules, which is section 4.2.2.4, not because the covalent bonds break.
• Putting the shared pair outside the overlap, or drawing two dots (or two crosses) as a shared pair between two different atoms.
• Forgetting the non-shared pairs on O, N or Cl, so the outer shell does not total 8.
• Drawing N₂ with a single or double bond. It needs a triple bond.
• Calling silicon dioxide a small molecule because its formula looks like CO₂.
• Cancelling a molecular formula. Ethane (H₃C–CH₃) is C₂H₆; writing CH₃ describes no real molecule.
• Saying metals have “free electrons” without saying the electrons are delocalised and come from the outer shell.
• Describing metallic bonding as attraction between “atoms and electrons” with no mention of electrostatic attraction or of sharing through the whole structure.

Next steps

1. Go through the revision notes and do their quick self-test.
2. Attempt the practice questions, then mark them against the worked answers.
3. Find wider gaps with the free 10-minute diagnostics.

Official syllabus

AQA GCSE Chemistry (8462) specification, for teaching from September 2016, GCSE exams June 2018 onwards, version 1.1, published by AQA – sections 4.2.1.4 Covalent bonding and 4.2.1.5 Metallic bonding.