Cambridge IGCSE Chemistry (workbook) · Exercises 4A–4C, pp.28–33

Chemistry — Ch.4 Ionic, Covalent & Metallic Bonding

Graded 2026-06-17 · Sys4Ethan (Claude Opus 4.8 vision, human-verified)
68%
✅ Correct 15 ◐ Partial 5 ✗ Needs fixing 1 ○ Not attempted 1 Total 22
To fix & review (7)Qp29 · Q6 Qp29 · Q7a RbCl Qp29 · Q7b Al₂S₃ Qp30 · Q8b m.p. Qp30 · Q9b Qp33 · Q1(b) conductivity Qp33 · Q2 (SiO₂ structure)
Qp28 · Q1 table ✓ Correct ions & electron config

F⁻, O²⁻, Al³⁺, N³⁻ with the right outer-shell counts (7, 6, 3, 5) and loss/gain — all correct.

Qp28 · Q2 ✓ Correct ionic bond formation (MCQ)

B ✓ — the non-metal gains an electron.

Qp28 · Q3 ✓ Correct ions vs protons (MCQ)

B ✓

Qp28 · Q4 ✓ Correct KI properties (MCQ)

C ✓ — KI (not KI₂); ionic; soluble.

Qp28 · Q5 ✓ Correct define ionic bond

Strong electrostatic force of attraction between oppositely charged ions — textbook-perfect.

Qp29 · Q6 ◐ Partial explain + / − ions
Try this → Sodium part is good (loses 1e⁻ → Na⁺, more protons than electrons). The question also needs chlorine: it GAINS 1e⁻ → Cl⁻, so more electrons than protons → negative.
Qp29 · Q7a RbCl ✗ Needs fixing dot-and-cross (ionic)
Try this → RbCl is IONIC — electron transfer, not sharing. Don't draw overlapping (shared) circles. Show Rb giving its 1 outer electron to Cl, as separate ions in square brackets with charges $[\text{Rb}]^+\,[\text{Cl}\,]^-$. (You DID draw covalent ones correctly later — same idea, but transfer instead of share.)
Qp29 · Q7b Al₂S₃ ◐ Partial dot-and-cross (ionic)
Try this → Formula $\text{Al}_2\text{S}_3$ is right — but no diagram. Show 2 Al each losing 3e⁻ to 3 S (each gaining 2e⁻), drawn as ions with their charges.
Qp29 · Q8a ✓ Correct label NaCl lattice

Cation / anion / ionic bond labelled.

Qp30 · Q8b m.p. ○ Not attempted melting point vs bonding
Try this → Left blank. Compare the charges: MgO is $\text{Mg}^{2+}/\text{O}^{2-}$ vs NaCl's $\text{Na}^+/\text{Cl}^-$ — bigger charges → stronger attraction → more energy to break → higher melting point.
Qp30 · Q9a ✓ Correct state from conductivity

Solid ✓ — a solid ionic compound can't conduct, so the bulb stays off.

Qp30 · Q9b ◐ Partial make it conduct
Try this → Your method is muddled. Give ONE clear way: either melt it (heat until molten) OR dissolve it in water — each frees the ions to move. Don't mix 'heat' with 'dissolve in water'.
Qp30 · Q9c ✓ Correct why ionic conducts

Right — ionic compounds conduct only when molten or in solution because the ions are then free to move.

Qp30 · Q1 (4B, T/F a–f) ✓ Correct true/false + correct

All six T/F correct, with the false ones corrected (sharing, not transfer).

Qp31 · Q2 (dot-and-cross) ✓ Correct covalent diagrams

Br₂ (single bond), HCl (shared pair) and SO₂ (double bonds) all drawn correctly.

Qp31 · Q3 volatility ✓ Correct define high volatility

'Low boiling point' is the key idea (high volatility = evaporates easily).

Qp31 · Q4 table ✓ Correct identify simple molecules

B ✓ — only X (low m.p./b.p., poor conductor in all states).

Qp32 · Q5 (bonding from data) ✓ Correct deduce bonding type

D ✓ — and you reasoned it: W high m.p. → ionic; X low b.p., insoluble, non-conductor → simple covalent.

Qp32 · Q6 (GeH₄) ✓ Correct molecular hydride + dot-cross

(a) 'does not conduct electricity' ✓; (b) GeH₄ dot-and-cross drawn correctly (Ge sharing with 4 H).

Qp33 · Q1 (diamond vs graphite) ✓ Correct giant covalent structures

Both diagrams drawn; similarities (both carbon, both giant covalent) and differences (diamond 4 bonds/hard, graphite 3 bonds/layered) all correct.

Qp33 · Q1(b) conductivity ◐ Partial why graphite conducts

Diamond explained well (all 4 outer electrons bonded → no free electrons).

Try this → You explained diamond but stopped — now do graphite: each carbon uses only 3 of its 4 outer electrons for bonds, leaving 1 free/delocalised electron per atom that can move and carry charge.
Qp33 · Q2 (SiO₂ structure) ◐ Partial giant covalent SiO₂

(a) black = oxygen ✓; (b) white = silicon ✓.

Try this → (c) Re-count: each black (oxygen) atom bridges how many silicon atoms in the structure? You wrote 3 — look again at how many bonds an oxygen forms here.
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