# Structure and Properties

> The four main structures, diamond and graphite, and how to identify a mystery substance from its properties.

From HSC Chemistry on WhizzNotes, free study notes for NSW students: https://whizznotes.com/chemistry/structure-and-properties/. Updated 3 November 2026.
Year 11 and Year 12 boxes go deeper than the Year 10 text around them.

### Story time: The pencil and the diamond ring
Kai's aunt showed him her diamond ring. "This is the hardest natural material on Earth," she said. "It can cut glass."

Kai held up his pencil. "So what's this made of?"

"Graphite. It's so soft it rubs off onto paper."

Here is the surprise: both are made of **nothing but carbon atoms**. The only difference is how the atoms are joined together. In chemistry, **structure decides properties**. Change how the particles are arranged and bonded, and you change everything about the substance.

**In short:** A substance's structure decides its properties. Covalent molecular substances melt easily and never conduct, covalent networks like diamond are very hard with very high melting points, ionic lattices conduct only when molten or dissolved, and metals conduct as solids.

## Four types of structure

Almost every solid fits one of four structures.

| Structure | Particles | Bonding | Examples |
|---|---|---|---|
| **Ionic lattice** | Positive and negative ions | Ionic | Sodium chloride, magnesium oxide |
| **Covalent molecular** | Small separate molecules | Covalent inside molecules, weak forces between them | Water, carbon dioxide, sugar, iodine |
| **Covalent network** (giant covalent) | Atoms | Covalent bonds everywhere, in a huge 3D or layered network | Diamond, graphite, silicon dioxide (sand) |
| **Metallic lattice** | Positive ions in a sea of electrons | Metallic | Copper, iron, aluminium |

## The property table

This table is one of the most useful in all of chemistry. Learn it well.

| Property | Ionic | Covalent molecular | Covalent network | Metallic |
|---|---|---|---|---|
| Melting point | High | **Low** | **Very high** | Usually high |
| Conducts as a solid | No | No | No (graphite yes) | **Yes** |
| Conducts when molten | **Yes** | No | No | Yes |
| Conducts dissolved in water | **Yes** | Usually no | Does not dissolve | Does not dissolve |
| Hard or soft | Hard, brittle | Soft | Very hard (graphite soft) | Malleable, ductile |
| State at room temperature | Solid | Often gas or liquid | Solid | Solid (except mercury) |

## Diamond and graphite

Diamond and graphite are **allotropes** of carbon: different forms of the same element.

*Diagram: Diamond: each carbon bonded to four others in a 3D network. Graphite: flat layers of hexagons with free electrons between layers*
*Diamond is a rigid 3D network. Graphite is flat layers held together weakly, with free electrons (red).*

| | Diamond | Graphite |
|---|---|---|
| Bonds per carbon | **4**, in a 3D tetrahedral network | **3**, in flat sheets of hexagons |
| Hardness | Hardest natural substance. Used in drill tips and cutting tools | Soft and slippery: layers slide. Used in pencils and as a lubricant |
| Conducts electricity | No: every electron is in a bond | **Yes**: the fourth electron of each carbon is delocalised and moves along the layers |
| Melting point | Very high (about 3500 °C) | Very high (over 3600 °C) |

**Silicon dioxide** (quartz, sand) has a similar 3D network to diamond, so it is also hard with a very high melting point.

### Key idea: Melting means breaking what holds the particles together
- Covalent molecular: only weak forces **between** molecules break. Low melting point.
- Covalent network: strong covalent bonds **must** break. Very high melting point.
- Ionic and metallic: strong attractions in every direction. High melting point.

## Identifying a mystery substance

Work through the properties like a detective.

1. **Low melting point?** It is **covalent molecular**.
2. **Conducts as a solid?** It is **metallic** (or graphite).
3. **Conducts only when melted or dissolved?** It is **ionic**.
4. **Very high melting point, never conducts, does not dissolve?** It is a **covalent network**.

### Worked example: Substance X
Substance X melts at 1600 °C. It does not conduct when solid or when melted, and it does not dissolve in water.

Very high melting point rules out covalent molecular. No conduction when melted rules out ionic and metallic. So X is a **covalent network** solid. (It is in fact silicon dioxide.)

### Watch out: Molecular substances do not break their bonds when they melt
When ice melts, water molecules move apart. The O−H covalent bonds inside each molecule stay whole. Melting and boiling never turn water into hydrogen and oxygen.

**Mind map**

```text
Structure and properties
  Ionic lattice
    High melting point
    Conducts molten or dissolved
    Brittle
  Covalent molecular
    Low melting point
    Never conducts
  Covalent network
    Very high melting point
    Diamond, graphite, SiO₂
    Graphite conducts
  Metallic
    Conducts as solid
    Malleable
  Detective steps
    Melting point first
    Then conductivity
```

### Year 11: Intermolecular forces and boiling points
For covalent molecular substances, boiling points depend on the forces **between** molecules.

- **Bigger molecules** have stronger dispersion forces. Down group 17: $\mathrm{F_2}$ (gas), $\mathrm{Cl_2}$ (gas), $\mathrm{Br_2}$ (liquid), $\mathrm{I_2}$ (solid).
- **Polar molecules** have extra dipole-dipole forces.
- **Hydrogen bonding** (H attached to N, O or F) is the strongest. Water (18 g/mol) boils at 100 °C, while methane (16 g/mol) boils at $-162\,^\circ\text{C}$.

**Solubility rule**: "like dissolves like". Polar and ionic substances dissolve in polar solvents like water. Non-polar substances like oil and wax dissolve in non-polar solvents like hexane.

### Year 12: New carbon materials
**Graphene** is a single layer of graphite, one atom thick. It is about 200 times stronger than steel, conducts electricity extremely well, and is almost transparent.

**Fullerenes** are hollow cages or tubes of carbon. Buckminsterfullerene, $\mathrm{C_{60}}$, is shaped like a soccer ball and is a molecular substance, so it has a much lower melting point than diamond. **Carbon nanotubes** are rolled-up graphene: very strong, light, and conducting, used in sports equipment and electronics.

These show again that the arrangement of atoms, not just the element, controls properties.

## Quick summary

- Four structures: **ionic lattice**, **covalent molecular**, **covalent network**, **metallic lattice**.
- Covalent molecular: low melting point, never conducts.
- Covalent network: very high melting point, hard, does not conduct (except graphite).
- Ionic: high melting point, brittle, conducts only when molten or dissolved.
- Metallic: conducts as a solid, malleable.
- Diamond (4 bonds, hard, no conduction) and graphite (3 bonds, layers, soft, conducts) are allotropes of carbon.

## Quiz

Try these questions from the chapter quiz.

1. Which structure has the lowest melting points?
   - A. Ionic lattice
   - B. Covalent molecular
   - C. Covalent network
   - D. Metallic lattice
   - Answer: B. Covalent molecular. Only weak forces between molecules need to be overcome.

2. A substance conducts electricity as a solid and is malleable. It is most likely:
   - A. Metallic
   - B. Ionic
   - C. Covalent molecular
   - D. Covalent network
   - Answer: A. Metallic. Conducting as a solid and bending are signs of metallic bonding.

3. A substance melts at 801 °C and conducts only when molten or dissolved. It is:
   - A. Metallic
   - B. Ionic
   - C. Covalent molecular
   - D. Covalent network
   - Answer: B. Ionic. Ions are fixed in the solid but free to move when molten or dissolved.

The whole chapter quiz is on the page, for signed-in students: https://whizznotes.com/chemistry/structure-and-properties/#quiz