Structure and bonding questions show up in almost every O-Level Chemistry paper, and they are some of the most predictable marks on the whole syllabus. The catch is that markers are strict. You either use the right words or you lose the mark, even if you clearly understand the idea.
Here is the system we teach at Dojo, built from the phrasing that actually scores in the Cambridge mark schemes.
The four structures you must be able to tell apart
Every substance in the O-Level syllabus falls into one of four boxes:
- Simple molecular (e.g. iodine, carbon dioxide, methane, water)
- Giant covalent / giant molecular (e.g. diamond, graphite, silicon dioxide)
- Giant ionic (e.g. sodium chloride, magnesium oxide)
- Giant metallic (e.g. copper, iron, aluminium)
Before you write a single word, decide which box the substance is in. Almost every wrong answer we mark comes from a student who skipped this step and started explaining bonds when the question was about forces between molecules.
The one mistake that costs the most marks
When a simple molecular substance melts or boils, the covalent bonds do not break. Only the weak intermolecular forces of attraction between molecules are overcome.
So never write "the covalent bonds in iodine are weak". Covalent bonds are strong. What is weak is the attraction between the molecules.
Compare the two sentences that earn marks:
- Simple molecular: "Iodine has a low melting point because only weak intermolecular forces of attraction between molecules need to be overcome, which requires little energy."
- Giant covalent: "Silicon dioxide has a high melting point because many strong covalent bonds between atoms must be broken, which requires a large amount of energy."
Notice the pattern: what is overcome + how strong it is + how much energy is needed. Three ingredients, three marks in a three-mark question.
Building your answer: the three-part formula
For any "explain the physical property" question, structure your answer like this:
Structure → particles and forces → property.
Melting and boiling point
- Giant ionic: "There are strong electrostatic forces of attraction between oppositely charged ions in the giant ionic lattice. A large amount of energy is needed to overcome these, so the melting point is high."
- Giant metallic: "There are strong electrostatic forces of attraction between the positive metal ions and the sea of delocalised electrons, so a lot of energy is needed to separate them."
- Simple molecular: as above, weak intermolecular forces.
Electrical conductivity
The golden rule: a substance conducts only if it has mobile charged particles, either free ions or free electrons.
- Ionic solid: does not conduct, because ions are held in fixed positions in the lattice and cannot move.
- Ionic molten or aqueous: does conduct, because the ions are now free to move and carry charge.
- Metal: conducts in solid and liquid state, because of delocalised (mobile) electrons.
- Graphite: conducts because each carbon atom forms only three bonds, leaving one delocalised electron per atom free to move.
- Diamond and simple molecular substances: do not conduct, because there are no free electrons or free ions.
Write "mobile" or "free to move". "Ions can move about" is fine. "Electricity flows through it" is not an explanation, it is a restatement.
Solubility and hardness
- Ionic compounds are generally soluble in water and insoluble in organic solvents. Simple molecular substances are usually the opposite.
- Diamond is hard because of a rigid three dimensional network of strong covalent bonds.
- Graphite is soft and slippery because it has layers held together by weak forces, so the layers can slide over each other.
Working through a typical question
"Substance X has a melting point of 1600 °C, does not conduct electricity when solid but conducts when molten, and dissolves in water. Deduce its structure and explain your answer. [3]"
Model answer:
"X is giant ionic. The high melting point shows strong electrostatic forces of attraction between oppositely charged ions that need a lot of energy to overcome. It does not conduct when solid because the ions are held in fixed positions, but conducts when molten because the ions become free to move and carry charge."
Three pieces of evidence, three explanations. Never state the structure without linking each property back to particles and forces.
Quick self-check before you move on
- Did I name the exact force? (Covalent bond, intermolecular force, electrostatic attraction between ions, attraction to delocalised electrons.)
- Did I say strong or weak, and say why energy is or is not needed?
- For conductivity, did I mention mobile ions or mobile electrons?
- Did I accidentally say covalent bonds break during boiling?
If you can tick all four, this topic becomes a guaranteed source of marks rather than a guessing game.
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