Challenge questions test the same content you already know — atoms, ions, bonding, structures — but they ask about it in ways you haven't seen before. They combine two or three ideas in one question, give you an unfamiliar substance, or ask you to work backwards from a property to a structure. You already have the knowledge. This guide teaches you to spot the tricks examiners like to hide in the wording, so that on the day, nothing catches you off guard. Work through it slowly — click to reveal each step, try the practice questions properly before checking the answer, and read every examiner's note. That's where the marks are won.
Twelve mistakes IGCSE students make again and again on Topic 2 challenge questions. Read the trap, then the truth, then see exactly how examiners exploit it.
These questions combine several ideas at once. Click through each step yourself before revealing the next — that's how you build the habit examiners are testing for.
It looks like a question about electronic configuration, but it's really three questions chained together: (1) work out which group each element is in, (2) work out what ion each one forms, and (3) work out the formula that balances the charges. You have to do all three, in order, to get the mark.
X has configuration 2,8,3 — 3 electrons in its outer shell, so X is in Group III. It will lose those 3 outer electrons to get a full outer shell, forming an X³⁺ ion. Y has configuration 2,6 — 6 electrons in its outer shell, so Y is in Group VI. It will gain 2 electrons to complete its outer shell, forming a Y²⁻ ion.
A common wrong answer is simply "XY" — pairing them 1:1 without checking the charges balance. That can't be right here: +3 and −2 don't cancel out. The overall compound must have no net charge, so we need to find the lowest common multiple of 3 and 2, which is 6. We need enough X³⁺ ions to give +6, and enough Y²⁻ ions to give −6.
We need 2 × X³⁺ (giving +6 total) and 3 × Y²⁻ (giving −6 total), so the charges cancel exactly. That gives the ratio X : Y = 2 : 3.
Normally you're given the percentage abundances and asked to calculate Ar. Here it's the opposite — you're given Ar and asked to work backwards to find the abundances. This means you need to set up an algebraic equation rather than just plugging numbers into the usual formula.
Let x = percentage abundance of boron-10. Since there are only two isotopes and the percentages must add to 100, the abundance of boron-11 must be (100 − x)%.
The weighted-average formula for Ar is: Ar = (mass₁ × abundance₁ + mass₂ × abundance₂) ÷ 100. Substituting what we know:
(10 × x + 11 × (100 − x)) ÷ 100 = 10.8
10x + 11(100 − x) = 1080
10x + 1100 − 11x = 1080
−x + 1100 = 1080
−x = −20
x = 20
So boron-10 makes up 20% and boron-11 makes up 100 − 20 = 80%.
You're given a set of physical properties and asked to work backwards to identify the bonding and structure — the reverse of the usual "given the bonding, predict the properties" direction.
Very high melting point → suggests a giant structure (lots of strong bonds/forces to break). Does not conduct as a solid → rules out metallic bonding (metals conduct as solids). Conducts when dissolved → something inside becomes free to move and carry charge when placed in water.
Not simple molecular — those have low melting points, and this one is very high. Not giant covalent — giant covalent structures (like diamond or SiO₂) don't have charged particles at all, so they can't start conducting just because they dissolve. Not metallic — metals conduct even as solids, but Z doesn't.
The only structure left is ionic. A giant ionic lattice explains the very high melting point (strong electrostatic forces between many ions), the lack of conduction as a solid (ions fixed in place), and the conduction when dissolved (ions become free to move in solution).
You've probably never drawn HCN before — and that's the point. Challenge papers deliberately use unfamiliar molecules to check that you know the method, not just memorised diagrams. The rules never change: every atom wants a full outer shell (2 for hydrogen, 8 for everything else in this course), achieved by sharing electron pairs.
H has 1 outer electron and needs 1 more (to reach 2). C has 4 outer electrons and needs 4 more (to reach 8). N has 5 outer electrons and needs 3 more (to reach 8). H shares its 1 electron with C, forming one shared pair — a single bond. That satisfies H completely, but only gives C 1 of the 4 electrons it needs, so C must share its remaining 3 electrons with N.
C shares 3 of its own electrons with N, and N shares 3 of its own electrons back with C — that's three shared pairs, a triple bond between C and N. Check the totals: C now has 1 (from H-C bond) + 3 (from C≡N triple bond) = 4 shared pairs = 8 electrons around it. ✓ N has 3 shared pairs (6 electrons) plus its own unshared pair (2 electrons) = 8 electrons around it. ✓ H has 1 shared pair = 2 electrons around it. ✓
The skeletal structure is H–C≡N, with a lone (unshared) pair of electrons on the nitrogen atom. See the diagram below.
| Row | Solid copper | Solid NaCl | Molten NaCl | Diamond |
|---|---|---|---|---|
| A | Conducts | Conducts | Conducts | Does not conduct |
| B | Conducts | Does not conduct | Conducts | Does not conduct |
| C | Conducts | Does not conduct | Does not conduct | Conducts |
| D | Does not conduct | Conducts | Conducts | Conducts |
This question packs all three bonding types — metallic, ionic, and giant covalent — into a single table. You need to test each substance against what you know, then check which row matches on all four columns, not just one or two.
Solid copper — metallic bonding, delocalised electrons free to move even as a solid → conducts. Solid NaCl — ionic, ions fixed in the lattice → does not conduct. Molten NaCl — ionic, but now the ions are free to move → conducts. Diamond — giant covalent, no delocalised electrons at all → does not conduct.
Row A: solid NaCl shown as "conducts" — wrong. Row C: molten NaCl shown as "does not conduct" and diamond shown as "conducts" — both wrong. Row D: solid copper shown as "does not conduct" — wrong. Only Row B has all four correct: copper conducts, solid NaCl doesn't, molten NaCl does, diamond doesn't.
Row B matches every column correctly, and it's the only row that does.
These pairs of questions look almost identical — but the correct answers are different. Challenge papers exploit exactly this kind of near-identical wording. Read both questions carefully before checking the key difference.
This is the single most useful mental model for Topic 2 challenge questions. Learn to run it in both directions.
These are real-style student answers that look plausible but contain a mistake examiners see constantly. Read the question and the student's answer first, decide for yourself what's wrong, then reveal the flaw.
Ten challenge-style multiple choice questions. Pick an answer, then check it — every option is explained, right or wrong, so you learn from every attempt.