This set has ten original questions, ordered from easier to harder, covering all five lessons in bonding and structure. Questions 1 to 3 cover ions, 4 and 5 cover covalent bonds, 6 to 8 link structure to properties, and 9 and 10 mix skills.
Attempt each question on paper first, writing full sentences where it asks you to explain. Then open the answer. Any data marked as invented is made up for practice and does not describe real substances.
Questions
1. Sodium has proton number 11. Give the electron arrangement of the sodium atom and of the Na⁺ ion, and state the number of protons and electrons in the ion.
Show answer
Sodium atom: 2,8,1. It loses 1 outer electron, so Na⁺ is 2,8.
Na⁺ has 11 protons and 10 electrons. Charge = 11 − 10 = 1+, which matches.
2. Sulfur has proton number 16. Explain how a sulfide ion, S²⁻, forms.
Show answer
Sulfur atom: 2,8,6. It gains 2 electrons and becomes 2,8,8, a full outer shell. It has 16 protons and 18 electrons, so the charge is 16 − 18 = 2−.
S²⁻ (2,8,8)
3. Give the formulas of magnesium chloride and aluminium oxide, using the ions Mg²⁺, Cl⁻, Al³⁺ and O²⁻.
Show answer
Magnesium chloride: Mg²⁺ needs 2 Cl⁻ to balance (2+ and 2 × 1−), so MgCl₂.
Aluminium oxide: the lowest common multiple of 3 and 2 is 6, so 2 Al³⁺ (total 6+) and 3 O²⁻ (total 6−), giving Al₂O₃.
4. Ammonia is NH₃. How many shared pairs and how many lone pairs are in the molecule? Show that the electron count is correct.
Show answer
Nitrogen has 5 outer electrons and needs 3 more, so it shares one pair with each of 3 hydrogen atoms: 3 shared pairs. Nitrogen’s remaining 2 electrons form 1 lone pair.
Count check: total outer electrons = 5 + 3 × 1 = 8. In the diagram, 3 bonding pairs (6) + 1 lone pair (2) = 8.
5. In carbon dioxide, each carbon to oxygen link is a double bond. Explain why, using the outer electrons of each atom.
Show answer
Carbon has 4 outer electrons and needs 4 more, which it gets by sharing 2 pairs with each of 2 oxygen atoms. Each oxygen has 6 outer electrons and needs 2 more, which one double bond provides. Each oxygen also keeps 2 lone pairs.
Count check: 4 + 6 + 6 = 16 outer electrons. Diagram: 4 bonding pairs (8) + 4 lone pairs (8) = 16.
6. Four substances are tested. The data are invented for practice.
| Substance | Melting point / °C | Conducts when solid | Conducts when molten |
|---|---|---|---|
| A | 1610 | no | no |
| B | 710 | no | yes |
| C | −50 | no | no |
| D | 1540 | yes | yes |
Identify the structure of each: giant covalent, giant ionic, simple molecular or metallic.
Show answer
A: very high melting point and no conduction in either state: giant covalent.
B: high melting point, conducts only when molten: giant ionic.
C: low melting point and no conduction: simple molecular.
D: high melting point and conducts when solid: metallic, because delocalised electrons move in the solid.
7. Explain why graphite conducts electricity and diamond does not.
Show answer
In graphite, each carbon atom forms 3 covalent bonds in a layer, so the fourth outer electron is delocalised and free to move along the layer and carry charge. In diamond, each carbon forms 4 covalent bonds, so all outer electrons are used in bonds, and no charged particles are free to move.
8. Explain why sodium chloride crystals are brittle.
Show answer
Sodium chloride is a lattice of alternating positive and negative ions. If a force slides a layer of ions by one position, ions of the same charge become neighbours and repel each other. The layers push apart and the crystal splits.
9. Iodine is a solid that melts easily, while sodium chloride needs a far higher temperature. Explain the difference in melting point, and say whether either conducts when molten.
Show answer
Iodine is simple molecular. Only weak intermolecular forces between I₂ molecules are overcome, so little energy is needed. Sodium chloride is a giant ionic lattice, so many strong electrostatic attractions between ions must be overcome, needing much more energy.
Molten iodine does not conduct, as it has only neutral molecules. Molten sodium chloride conducts, because its ions are free to move.
10. A student says: “When water boils, the covalent bonds break, which is why it needs so much heat.” Correct the statement and explain what actually happens.
Show answer
When water boils, the covalent bonds inside each H₂O molecule do not break. The water vapour is still H₂O. What is overcome are the intermolecular forces between molecules, so that they separate into a gas. Covalent bonds break only in a chemical reaction, such as when water is split into hydrogen and oxygen.
If you got these wrong
Use the pattern of your errors to choose the next lesson.
- Wrong electron counts or charges (1 to 3): go back to explain ion formation using electrons.
- Missing lone pairs or bonds (4 and 5): review represent a simple covalent bond.
- Properties of diamond or graphite (6, 7): review relate a giant structure to a property.
- Melting point answers that name the wrong bond (9, 10): review contrast molecular and ionic melting behaviour.
- Conductivity answers that name the wrong carrier (6, 7, 9): review explain conductivity in solid and molten states.
Log each error in the mistake log and retest queue so you can try a fresh question of the same kind a few days later. The mole and equation-ratio tutor and equation balance reasoning trainer become useful once you move on to quantities and equations.
Some students need help separating “knowing the fact” from “writing the chain of reasoning”. That difference is what a teacher looks for in online one-to-one Chemistry tuition.