To connect bonding with a property, read the data first, name the structure second, and only then explain the property. This reasoning chain works for any substance, including ones you have never seen named.
It sits at the start of integrated chemical reasoning because every later lesson in the module assumes you can say what the particles are and how they are held together.
What is the reasoning chain?
Properties are consequences of structure. If you know which particles are present and what holds them, you can predict how the substance behaves, and the reverse also works.
- Pick out the clues: melting point, hardness, electrical conduction (in solid, molten and dissolved states) and solubility.
- Match clues to a structure: giant ionic, giant covalent, simple molecular or metallic.
- Name the particles and the force: ions with electrostatic attraction, atoms with covalent bonds, molecules with weak forces between them, or positive ions in a sea of delocalised electrons.
- Explain the property by saying what must be overcome or what is free to move.
Worked example
The data below is invented for practice. Three unfamiliar solids, X, Y and Z, were tested.
| Substance | Melting point | Conducts as solid? | Conducts when molten? |
|---|---|---|---|
| X | 1,710 °C | No | No |
| Y | 801 °C | No | Yes |
| Z | −7 °C | No | No |
Step 1, clues for X: very high melting point and no conduction in any state. Nothing carries charge, so there are no free ions or electrons.
Step 2, structure for X: a giant covalent structure. A very large number of strong covalent bonds must all be broken to melt it, which needs a lot of energy.
Step 3, Y: high melting point, no conduction as a solid, conduction when molten. This is a giant ionic lattice. In the solid the ions are fixed in place. When molten they are free to move and carry charge.
Step 4, Z: a melting point below room temperature and no conduction. This is a simple molecular substance. Only weak forces between the molecules are overcome, so little energy is needed. The molecules are uncharged, so nothing conducts.
Answer in one line each: X is giant covalent, Y is ionic, Z is simple molecular, and each property follows from what the particles are.
The mistake to watch for
A frequent error is to say that Z melts because its covalent bonds break.
Mistaken answer: “Z has a low melting point because the covalent bonds are weak.”
The covalent bonds inside each molecule are still strong, and they are not broken on melting. The correction: “Z has a low melting point because the forces between molecules are weak, so little energy is needed to separate the molecules.” Always ask which particles move apart when a substance melts.
Check yourself
All data below is invented.
1. Substance A is a shiny solid that conducts electricity as a solid and can be hammered into sheets. Name the structure and the particles responsible for conduction.
Show answer
A metallic structure. Delocalised electrons are free to move through the lattice of positive ions and carry charge. Layers of ions can slide over each other, so it can be shaped without breaking.
2. Substance B has a melting point of 1,050 °C. It does not conduct as a solid but conducts when dissolved in water. Explain the conduction.
Show answer
B is ionic. In the solid, ions are held in fixed positions. In solution the ions are separated and free to move, so they carry charge.
3. Substance C boils at 78 °C and does not conduct in any state. Is it more likely to be giant covalent or simple molecular? Give one reason.
Show answer
Simple molecular. A giant covalent structure would need a very high temperature to change state, but C changes state easily because only weak forces between molecules need to be overcome.
Where this leads next
Once you can name a structure from data, try the numerical side in combining an equation ratio with a rate dataset, then test every lesson in the mixed practice set. The scientific investigation critic helps you check whether a dataset really supports your conclusion.
Some students know every bonding fact yet lose marks because they skip the naming step. That is the kind of habit our teachers look for in online one-to-one Co-ordinated Sciences tuition.