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K324 / 2027
Bonding and Structure

Lesson 6 of 6

Work backwards from the evidence

Use melting point and conductivity together to identify and explain a likely structure.

O-Level 2026 SEC G3 2027. Extensions are labelled Pure only.

  • I can reason from properties to structure and back.Syllabus

    Syllabus K324, 3.4(g). Reason from properties to structure and back

Work backwards from the evidence

Use several properties together, then explain the particles, attractions and charge carriers.

Worked exampleIdentify a likely structure

An unknown solid melts at a high temperature. It does not conduct when solid, but conducts after melting. What structure is most likely?

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  1. The high melting point suggests strong attractions extending through the solid.
  2. The molten material has mobile charged particles.
  3. The solid lacks mobile charge carriers. That makes a metal unlikely.
  4. An ionic lattice fits both observations: fixed ions in the solid, mobile ions after melting.
Answer

A giant ionic structure. Strong electrostatic attractions require much energy to overcome; ions carry charge only when free to move.

Pure only

Worked examplePredict a compound from electron arrangement

Element X has electron arrangement 2,8,2. Predict its bonding and formula when it reacts with chlorine.

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  1. X can lose two outer electrons to form X2+, with a full outer shell.
  2. Each chlorine atom gains one electron to form Cl-.
  3. Two chloride ions balance one X2+ ion.
Answer

XCl2, with ionic bonding between X2+ and Cl-. The electron arrangement helps predict chemical behaviour as well as physical properties.

Check your understanding

A pure substance melts at a low temperature and does not conduct as a solid or liquid. Which structure fits?

  1. Giant ionic: its ions are held weakly, so the lattice melts easily.
  2. Giant covalent: its few strong bonds break at a low temperature.
  3. Simple molecular: melting overcomes only weak forces between molecules.
Show the answer

Simple molecular: melting overcomes only weak forces between molecules.

Low melting point means little energy is needed, which fits weak intermolecular forces. The covalent bonds inside the molecules stay intact. No ions or free electrons means no conduction.

Exam-style question

Magnesium oxide melts at 2852 °C. Chlorine is a gas at room temperature. Explain this difference in terms of structure and bonding. [3]

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Exam definitions: learn these in mark-scheme words
TermDefinition to write
ElementA substance that cannot be broken down into simpler substances by chemical methods. It is made of only one type of atom.
CompoundA substance made of two or more elements chemically combined in a fixed ratio.
MixtureTwo or more substances that are not chemically combined. They can be separated by physical methods.
Ionic bondThe electrostatic attraction between oppositely charged ions.
Covalent bondThe electrostatic attraction between a shared pair of electrons and the nuclei of the two bonded atoms.
AlloyA mixture of a metal with one or more other elements, usually other metals.
Pure only
Pure only: more exam definitions
TermDefinition to write
Metallic bondThe electrostatic attraction between a lattice of positive metal ions and a sea of delocalised electrons.