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S4 Chemistry Bonding, Paper 4: electron counting and comparing structures
S4 Chemistry Bonding, Paper 4: electron counting and comparing structures
Ten graded items moving from shared pairs and lone pairs to formula mass and the properties of giant structures.
Focus Count outer electrons before sharing them out; to compare melting points, name the structure type first, then the force between particles.
01 Paper details (check answers afterwards)
| Item | Detail |
|---|---|
| Questions | 10 |
| Suggested time | 25 |
| Difficulty mix | Foundation 4 · Intermediate 3 · Challenge 2 · Error redo 1 |
| Chapter | Chapter: S4 Chemistry: Bonding and Structure |
| Level | Four basic, three advanced, two challenge and one error-repair item |
02 Questions (10 items)
- FoundationHow many shared pairs of electrons are there in one methane molecule, CH4?
- FoundationHow many lone pairs are there on the nitrogen atom of ammonia, NH3?
- FoundationFind the molar mass of calcium carbonate, CaCO3, with Ca as 40.1, C as 12.0 and O as 16.0.
- FoundationWhy do metals conduct electricity?
- IntermediateMagnesium oxide and sodium fluoride are both ionic. Which has the higher melting point?
- IntermediateGraphite conducts electricity while diamond does not. Explain why.
- IntermediateWhat is the electron arrangement of the chloride ion, Cl-?
- ChallengeHow many oxygen atoms are there in 0.50 mol of carbon dioxide, with the Avogadro constant as 6.0 x 10^23 mol^-1?
- ChallengeIodine melts at 114 degrees but sodium chloride at 801 degrees. Explain the difference.
- Error redoA student says a covalent bond can only form between atoms of the same non-metal. Give the correct statement.
03 Answers and one-line notes
| # | Answer | One-line note |
|---|---|---|
| 1 | 4 pairs | Carbon has four outer electrons and shares one pair with each hydrogen, giving four pairs. |
| 2 | 1 pair | Nitrogen has five outer electrons, three are used in bonds to hydrogen and the remaining two form one lone pair. |
| 3 | 100.1 g mol^-1 | 40.1 plus 12.0 plus three times 16.0 gives 100.1. |
| 4 | They contain delocalised electrons that can move through the whole structure | Metal atoms release outer electrons into a sea of delocalised electrons, and mobile charged particles are what conduction needs. |
| 5 | Magnesium oxide | Magnesium is 2+ and oxide is 2-, so the charge product is 4 against 1 for sodium fluoride, giving far stronger attraction. |
| 6 | Graphite has delocalised electrons between its layers; every diamond electron is locked in a bond | Both are giant covalent, but each graphite carbon bonds to only three neighbours, leaving delocalised electrons free to move. |
| 7 | 2,8,8 | A chlorine atom is 2,8,7, so gaining one electron fills the outer shell to eight. |
| 8 | 6.0 x 10^23 | Each carbon dioxide molecule holds two oxygen atoms, so 0.50 times 2 times 6.0 times 10 to the 23 gives 6.0 times 10 to the 23. |
| 9 | Iodine is simple molecular so only weak forces between molecules break; sodium chloride needs a strong ionic lattice broken | The covalent bonds stay intact during melting of iodine; only the weak forces between molecules break, unlike the strong ionic lattice of sodium chloride. |
| 10 | Different non-metals also form covalent bonds, for example hydrogen chloride and water | Different non-metals also share electrons, as in hydrogen chloride, water and carbon dioxide, so the test is shared pairs, not identity. |
04 What to do after marking
- After marking, redo only the wrong items and write down which step went wrong (formula, unit, or misread number).
- Repeat the same item two days later; move to the next paper only when both attempts are correct.
- Parents only need the answer and note columns; there is no need to explain every item.
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