Ionic, covalent and metallic bonding

Three ways atoms achieve a stable arrangement of electrons:

• Ionic — electrons are transferred; the bond is the electrostatic attraction between oppositely charged ions. Giant ionic lattices have high melting points (many strong attractions to overcome) and conduct only when molten or dissolved, because only then are the ions free to move.
• Covalent — a shared pair of electrons, attracted to both nuclei. In a co-ordinate (dative) bond both electrons come from the same atom (e.g. NH₄⁺, or a ligand bonding to a metal ion). Once formed it is identical to any other covalent bond; it is drawn as an arrow.
• Metallic — positive metal ions in a sea of delocalised electrons. Strength rises with more delocalised electrons per atom and a smaller ionic radius: Na < Mg < Al.

Four structures: giant ionic (NaCl) · giant metallic (Mg) · giant covalent/macromolecular (diamond, SiO₂ — very high m.p., strong covalent bonds broken) · simple molecular (I₂, CO₂ — low m.p., only weak intermolecular forces broken).

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Shapes of molecules and ions (VSEPR)

Valence Shell Electron Pair Repulsion: electron pairs around a central atom repel each other and arrange themselves as far apart as possible. Crucially:

Method: count the electron pairs around the central atom → that gives the basic geometry; then subtract lone pairs to name the shape, and reduce the angle by ~2.5° per lone pair.

• 2 pairs, 0 lone → linear, 180° (BeCl₂, CO₂)
• 3 pairs, 0 lone → trigonal planar, 120° (BF₃)
• 4 pairs, 0 lone → tetrahedral, 109.5° (CH₄, NH₄⁺)
• 4 pairs, 1 lone → trigonal pyramidal, 107° (NH₃)
• 4 pairs, 2 lone → bent / non-linear, 104.5° (H₂O)
• 5 pairs, 0 lone → trigonal bipyramidal, 120° and 90° (PCl₅)
• 6 pairs, 0 lone → octahedral, 90° (SF₆)

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Electronegativity, polar bonds and polar molecules

Electronegativity is the power of an atom to attract the pair of electrons in a covalent bond. It increases across a period (more nuclear charge, same shielding, smaller radius) and decreases down a group. The most electronegative elements are F > O > N ≈ Cl.

• Two identical atoms → electrons shared equally → non-polar bond.
• Different electronegativities → electrons pulled towards the more electronegative atom → a permanent dipole, δ+ and δ−.
• A very large difference (roughly > 1.8 on the Pauling scale) → the electron is effectively transferred → ionic.

The distinction that earns marks: a molecule can have polar bonds but be non-polar overall if the dipoles cancel by symmetry. CO₂ (linear) and CCl₄ (tetrahedral) are non-polar; H₂O (bent) and CHCl₃ are polar because the dipoles do not cancel.

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Van der Waals, dipole–dipole and hydrogen bonding

Three intermolecular forces, weakest to strongest. Melting and boiling a simple molecular substance breaks these — never the covalent bonds.

• Van der Waals (induced dipole–dipole / London) forces — present between all molecules. Electrons move randomly, giving an instantaneous dipole that induces a dipole in a neighbour. They get stronger with more electrons (larger M_r) and with a larger contact surface — which is why straight-chain alkanes boil higher than their branched isomers.
• Permanent dipole–dipole forces — between polar molecules, in addition to van der Waals.
• Hydrogen bonding — the strongest. It requires H bonded directly to N, O or F, and a lone pair on the N, O or F of a neighbouring molecule.

Two classic consequences of hydrogen bonding: (1) H₂O, NH₃ and HF have anomalously high boiling points compared with the other hydrides in their groups. (2) Ice is less dense than water — hydrogen bonds hold the molecules in an open tetrahedral lattice, further apart on average than in the liquid, so ice floats.

Sort by the STRONGEST intermolecular force present

Tap a substance, then tap the strongest intermolecular force it has. (Every molecule has van der Waals forces — so choose the strongest one present.)

Match the shape to its bond angle

Tap a shape on the left, then its bond angle on the right.

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The four bonding errors that cost the most marks

• "Boiling breaks the covalent bonds." It does not. Boiling a simple molecular substance breaks only the intermolecular forces. The covalent bonds inside each molecule survive intact.
• Polar bond ≠ polar molecule. CO₂ and CCl₄ contain very polar bonds but are non-polar overall, because the dipoles cancel by symmetry.
• Hydrogen bonding needs H bonded DIRECTLY to N, O or F — and a lone pair to bond to. HCl does not hydrogen bond; nor does CH₃Cl, however polar it is.
• Ionic solids do not conduct when solid. The ions are held in fixed positions in the lattice; they only become mobile when molten or dissolved.

Bond angle method, every time: count the electron pairs → get the basic geometry → subtract the lone pairs to name the shape → reduce the angle by about 2.5° per lone pair. Show that working and you earn the marks even if you misremember the number.

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The big ideas to know

That is the whole of AQA 3.1.3. Press Finish to see your score.