Edexcel A Level Chemistry: Structure, Bonding and Introductory Organic Chemistry — Revision Notes

Condensed for the final weeks. For the full explanation, use the Structure, Bonding and Organic Chemistry study guide.

Bonding

Bond	Held by
Ionic	Electrostatic attraction between oppositely charged ions
Covalent	Shared pair attracted to both nuclei
Dative covalent	Both electrons from the same atom
Metallic	Positive ions and delocalised electrons

Every definition needs electrostatic attraction.

Polarisation: a small, highly charged cation distorts a large anion’s electron cloud, giving an ionic compound covalent character. Polarising power rises with higher charge and smaller radius; polarisability rises with larger anion radius. This is why AlCl₃ behaves covalently while NaCl does not.

Comparing theoretical and experimental lattice energies measures this: a large discrepancy indicates significant covalent character. (This comparison itself is Unit 4, topic 12 content – Unit 1 goes no further than recognising covalent character from polarisation.)

Shapes — VSEPR

Electron pairs repel and get as far apart as possible; lone pairs repel more strongly than bonding pairs, reducing bond angles by about 2.5° each.

Bond pairs	Lone pairs	Shape	Angle
2	0	Linear	180°
3	0	Trigonal planar	120°
4	0	Tetrahedral	109.5°
3	1	Trigonal pyramidal	107°
2	2	Bent	104.5°
5	0	Trigonal bipyramidal	120° / 90°
6	0	Octahedral	90°

CH₄ → NH₃ → H₂O (109.5 → 107 → 104.5) is examined constantly, and the explanation — one then two lone pairs — is what scores.

Polarity and intermolecular forces

A molecule is polar only if the bond dipoles do not cancel. CO₂ has polar bonds but is linear and symmetrical, so it is non-polar; H₂O is bent, so it is polar. Symmetry decides it.

Force	Present in	Strength
London (induced dipole)	All molecules	Weakest, grows with electron number
Permanent dipole–dipole	Polar molecules	Intermediate
Hydrogen bonding	H bonded to N, O or F plus an acceptor lone pair	Strongest

Hydrogen bonding needs both conditions. HCl does not hydrogen bond.

Ice is less dense than water because each molecule forms four hydrogen bonds in a tetrahedral arrangement, creating an open lattice with holes; melting partly collapses it, so the liquid is denser.

Simple molecular substances melt at low temperatures because the weak intermolecular forces break, not the covalent bonds. That is the most penalised sentence in the topic.

Organic nomenclature and isomerism

Root by longest carbon chain, suffix by functional group, prefix by substituents, numbered to give the lowest locants.

• Structural isomerism — chain, position, functional group.
• Stereoisomerism — same structural formula, different spatial arrangement:
  • E/Z (geometric) — requires a C=C double bond (restricted rotation) and two different groups on each carbon. Both conditions are needed.
  • Optical (Unit 4, topic 15 content) — requires a chiral centre: a carbon with four different groups. The two enantiomers are non-superimposable mirror images and rotate plane-polarised light in opposite directions.

A racemic mixture rotates light not at all, because equal amounts of the two enantiomers cancel. This matters mechanistically: a reaction proceeding through a planar carbocation or planar intermediate produces a racemic mixture, because attack is equally likely from either face. That is direct evidence for the mechanism.

Mechanisms

(The halogenoalkane hydrolysis content below – the mechanism table’s Nucleophilic substitution/Elimination rows, the aqueous-vs-ethanolic KOH rule, and the C-I to C-F rate trend – is Unit 2, topic 10 content, examined after Unit 1.)

Curly arrows show a pair of electrons moving, starting from a bond or lone pair — never from an atom or a positive charge.

Mechanism	Substrate
Free radical substitution	Alkanes + halogen, UV
Electrophilic addition	Alkenes
Nucleophilic substitution	Halogenoalkanes
Elimination	Halogenoalkanes with ethanolic KOH

Aqueous KOH gives substitution to an alcohol; ethanolic KOH gives elimination to an alkene. The solvent decides the product.

Rate of hydrolysis: C–I > C–Br > C–Cl > C–F. This follows bond enthalpy, not electronegativity — C–F is the most polar bond but by far the strongest, so fluoroalkanes are least reactive. That is the standard trap.

Markovnikov addition should be explained through carbocation stability (tertiary > secondary > primary), not stated as a rule about hydrogen.

Exam traps

• Saying covalent bonds break on melting a molecular solid.
• Explaining C–X reactivity by electronegativity.
• Giving only one condition for E/Z isomerism.
• Forgetting that a racemic mixture is optically inactive.
• Arrows starting at an atom or a charge.
• Saying a molecule is polar because its bonds are polar, ignoring symmetry.

Self-test

1. Give the two conditions for E/Z isomerism.
2. What is a chiral centre, and why is a racemic mixture optically inactive?
3. Why does a planar intermediate produce a racemic mixture?
4. Which halogenoalkane hydrolyses fastest and why?
5. Why is ice less dense than water?

Answers: 1. A C=C double bond preventing rotation, and two different groups attached to each of the double-bonded carbons. 2. A carbon atom bonded to four different groups; a racemic mixture contains equal amounts of the two enantiomers, whose equal and opposite rotations cancel. 3. The intermediate is flat, so the nucleophile can attack from either face with equal probability, producing equal amounts of both enantiomers. 4. The iodoalkane — the C–I bond has the lowest bond enthalpy and so breaks most readily; polarity is not the deciding factor. 5. Each water molecule forms four hydrogen bonds in a tetrahedral arrangement, producing an open lattice containing holes, which partly collapses on melting.