AQA GCSE Chemistry 8462: Covalent bonding and metallic bonding – Revision Notes

These notes condense 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). The content is examined on Paper 1, at both Foundation and Higher tier; nothing in these two sections is Higher tier only. For full explanations and worked examples, use the study guide.

Then test yourself with the practice questions. Ionic bonding is in separate revision notes. The course hub is AQA GCSE Chemistry, the printable checklist lists every statement, and the free diagnostics show where your gaps are.

Definitions to learn word for word

• Covalent bond: a shared pair of electrons between two atoms. Covalent bonds are strong.
• Single, double, triple bond: one, two or three shared pairs between the same two atoms.
• Small molecule: a fixed, small number of atoms held together by covalent bonds (H₂O, NH₃, CO₂).
• Polymer: a very large molecule made of many repeating units joined by covalent bonds.
• Giant covalent structure: a huge number of atoms joined by covalent bonds in a continuous network (diamond, silicon dioxide).
• Molecular formula: the actual number of atoms of each element in one molecule.
• Delocalised electrons: outer-shell electrons from metal atoms that are free to move through the whole structure.
• Metallic bond: the strong bond that comes from sharing delocalised electrons among the metal atoms in a giant regular structure.

Bond counts (4.2.1.4)

Atom	Group	Outer electrons	Bonds it forms
H	1	1	1
C	4	4	4
N	5	5	3
O	6	6	2
Cl	7	7	1

For groups 4 to 7: bonds = 8 − group number. Hydrogen needs only 2 electrons, so it forms 1 bond.

The eight dot and cross molecules

Molecule	Line form	Shared pairs	Non-shared pairs
Hydrogen, H₂	H–H	1	0
Chlorine, Cl₂	Cl–Cl	1	3 per Cl
Oxygen, O₂	O=O	2	2 per O
Nitrogen, N₂	N≡N	3	1 per N
Hydrogen chloride, HCl	H–Cl	1	3 on Cl
Water, H₂O	H–O–H	2	2 on O
Ammonia, NH₃	N with three N–H	3	1 on N
Methane, CH₄	C with four C–H	4	0

Method: drawing any dot and cross diagram

1. Write the outer electrons for each atom.
2. Bonds per atom = electrons needed to fill the shell.
3. Overlap the shells; put ONE dot + ONE cross
   in the overlap for each bond.
4. Put the leftover outer electrons in pairs on
   their own atom.
5. Count: H = 2, every other atom = 8.

Worked reminder: ammonia

N: 5 outer, needs 3  ->  3 N–H bonds
Each overlap: 1 dot (N) + 1 cross (H)
Left on N: 5 - 3 = 2  ->  1 non-shared pair
Check N: 3 x 2 shared + 2 = 8   Each H: 2

Three kinds of covalent substance

Kind	How to spot it in a diagram	Examples you must know
Small molecules	Separate groups of a few atoms, space between them	H₂, Cl₂, O₂, N₂, HCl, H₂O, NH₃, CH₄
Polymers	Repeating unit in brackets, bonds through the brackets, n after	poly(ethene)
Giant covalent	Network with bonds running off every edge	diamond, silicon dioxide

Formula trap. CO₂ is a small molecule. SiO₂ is giant covalent. For a giant structure the formula gives the ratio of atoms only. In silicon dioxide each Si bonds to 4 O and each O bonds to 2 Si.

Representing bonds with lines

• One line = one shared pair (a single bond). Two lines = double, three = triple.
• Polymer: draw one repeating unit in brackets, with a line through each bracket and n outside. n is a large number.

   [  H   H  ]
 --[--C---C--]--     poly(ethene)
   [  H   H  ] n     (H atoms bond to each C, above and below)

• Giant structure: draw part of the network, with lines leaving the edge to show it carries on. Diamond: every C bonded to 4 other C.

Method: molecular formula from a model

1. Use the key: which colour/letter is which element.
2. Count each element. Tick atoms as you count them.
3. Write C first, H second, then the rest
   alphabetically (e.g. C2H6O).
4. Check bond totals: C 4, N 3, O 2, H 1, Cl 1.
5. Do NOT cancel to a simpler ratio.

Limitations of each representation

Representation	Main limitation
Dot and cross	Does not show 3D shape; atom sizes not shown; electrons look different when they are identical
2D displayed formula	No 3D shape; no electrons shown, so non-shared pairs are missing
Ball and stick	Bonds are not sticks and atoms are not spaced apart; no electrons; not to scale
3D space-filling	Bonds are hard to see or count
Diagram of a giant structure	Shows only a tiny part of a structure that continues in all directions

Give a specific limitation for the named model. “It is not accurate” scores nothing.

Metallic bonding (4.2.1.5)

• Metals are giant structures of atoms in a regular pattern.
• Outer-shell electrons are delocalised: free to move through the whole structure.
• Sharing the delocalised electrons gives strong metallic bonds: electrostatic attraction between positive metal ions and the delocalised electrons.
• Diagram: regular rows of + ions with e⁻ between them.
• Found in metallic elements and alloys.

Number of delocalised electrons per atom = number of outer electrons:

Metal	Group	Delocalised e⁻ per atom	Ion left
Sodium	1	1	Na⁺
Calcium	2	2	Ca²⁺
Aluminium	3	3	Al³⁺

Must-know distinctions

	Covalent	Metallic
Elements	Non-metals only	Metals and alloys
Electrons	Shared in pairs between two particular atoms	Delocalised across the whole structure
Structure	Small molecule, polymer or giant covalent	Always giant, regular
Bond strength	Strong	Strong

• Covalent bond (strong, inside a molecule) vs forces between molecules (weak, section 4.2.2.4). Melting a small molecular substance does not break covalent bonds.
• Molecular formula (actual numbers, small molecules) vs formula of a giant structure (ratio only).
• Shared pair (in the overlap, one dot + one cross) vs non-shared pair (on one atom, both the same symbol).

Quick self-test

1. How many pairs of electrons are shared in a nitrogen molecule?
2. How many non-shared pairs of electrons are on the oxygen atom in water?
3. What is the total number of electrons shared in a methane molecule?
4. A displayed formula shows two N atoms joined by a single line, with two H atoms on each N. Give its molecular formula.
5. In the molecule in question 4, how many non-shared pairs are on each N atom?
6. Classify each as small molecules or giant covalent: CO₂, SiO₂, diamond, HCl.
7. In a polymer diagram, what does n represent?
8. How many bonds does each carbon atom form in diamond?
9. State what is meant by delocalised electrons in a metal.
10. How many delocalised electrons does each aluminium atom provide?
11. Give one limitation of a ball and stick model.
12. Why is the formula of diamond written simply as C?

Answers

1. 3 (a triple bond, N≡N).
2. 2. Oxygen has 6 outer electrons; 2 are shared, leaving 4, which is 2 pairs.
3. 8 electrons (4 shared pairs).
4. N₂H₄ (do not cancel to NH₂).
5. 1. Each N has 5 outer electrons and uses 3 in bonds (1 N–N and 2 N–H), leaving 2: one pair.
6. CO₂ small molecules; SiO₂ giant covalent; diamond giant covalent; HCl small molecules.
7. A large number: the number of repeating units in the chain.
8. 4, each to another carbon atom.
9. Outer-shell electrons that are not held by one atom and are free to move through the whole structure.
10. 3 (aluminium is in Group 3).
11. Any one: bonds are not really sticks; atoms are not really spaced apart; atoms not drawn to scale; electrons are not shown.
12. Diamond is a giant structure of only carbon atoms, with no fixed number of atoms, so the formula shows just the element.

Where marks are usually lost

• Shared pair drawn as two dots or two crosses, or placed outside the overlap of the two shells.
• Missing non-shared pairs on O, N or Cl, so the atom does not have 8 outer electrons.
• N₂ drawn with fewer than three shared pairs, or O₂ with a single bond.
• Calling covalent bonds weak when the question is about the bond itself.
• Classifying SiO₂ as a small molecule because its formula looks like CO₂.
• A molecular formula cancelled to a ratio, or an atom missed in the count from a model.
• A vague limitation (“it’s only a model”) instead of a specific one for the named diagram.
• Metallic bonding described as “free electrons” without saying they are delocalised outer-shell electrons moving through the whole structure.
• Forgetting the word “regular” (or “pattern”) when describing how metal atoms are arranged.

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.