# 2.5 Structures, Bonding and Properties

F.4 Chemistry · Topic II Microscopic World (I) · Lesson 3

Explain a property by identifying the structure, the particles and the attractions between them.

## Key points

- **Structure** = particles + arrangement. Molecular structures contain simple molecules (I~2~, CO~2~) or macromolecules (starch). Giant lattices have **no discrete molecules**; noble gases consist of separate atoms.
- **Simple molecular**: strong covalent bonds **within** molecules, weaker **intermolecular forces (分子間作用力)** between them; usually low melting/boiling points and soft solids.
- **Molecular size**: for **similar molecules**, larger size generally gives stronger **dispersion forces**.
- **Solubility**: compare attractions formed and disrupted. Many non-polar molecular substances dissolve in suitable non-polar solvents; **ethanol is miscible with water**.
- **Metals**: **delocalized electrons (離域電子)** carry charge and transfer heat. Most are dense, high-melting solids. Generally **malleable and ductile**: layers slide while non-directional metallic attraction remains.
- **Ionic solids**: strong ionic attractions give hardness and usually high melting points. **Brittle**: shifting layers brings **like charges** together, causing repulsion.
- **Coordination number (配位數)** = nearest neighbours. Each ion has **6** oppositely charged neighbours in **NaCl**, **8** in **CsCl**.
- **Giant covalent structure (巨型共價結構)**: continuous network of strong covalent bonds; much energy needed to break them. **Diamond** (4 C neighbours) and **quartz** (SiO~2~) are hard and insoluble in water/common organic solvents.
- **Graphite**: each C bonds to **3 C neighbours per layer**; one outer electron per C is delocalized. Weak attractions **between layers** let them slide: **soft**. Strong covalent bonds **within layers**: **high thermal stability**.
- **Exam language**: structure → particles/attractions → property. When a molecular solid melts, its molecules move past each other as enough intermolecular attraction is overcome; **covalent bonds remain intact**. Conductivity requires **mobile charged particles**.

## Must know

| Structure | Particles and bonding | Electrical conductivity |
|---|---|---|
| **Simple molecular** | Separate molecules, e.g. I~2~, CO~2~; strong covalent bonds within, weak attractions between | Usually no mobile charged particles; an aqueous solution conducts **if mobile ions form**, e.g. HCl in water |
| **Giant metallic** | Positive metal ions in a **sea of delocalized electrons**; strong metallic bonding | Solid and molten: **mobile electrons** |
| **Giant ionic** | Oppositely charged ions in a regular lattice; strong ionic attractions | Solid: **no**, ions fixed. Molten / dissolved in water: **mobile ions** |
| **Diamond / quartz** | Atoms in a giant network; strong covalent bonds | Ordinary solid state: **no mobile charged particles** |
| **Graphite** | Giant covalent layers; strong bonds within, weak attractions between | **Delocalized electrons move along the layers** |

Name the **charge carrier** and explain whether it is **free to move**; the presence of charge alone is insufficient.

## Common mistakes

- ✗ "Melting iodine breaks I–I covalent bonds." ✓ It overcomes **attractions between I~2~ molecules**; the covalent bonds within them remain intact.
- ✗ "Solid NaCl conducts because it contains charged ions." ✓ Its ions are **fixed** in the lattice. Molten or dissolved NaCl conducts because its ions are **mobile**.
- ✗ "Graphite is soft because its covalent bonds are weak." ✓ Weak attractions **between layers** let them slide; strong covalent bonds **within layers** explain its high thermal stability.
- ✗ "Graphite conducts, so it has a metallic structure." ✓ Graphite has a **giant covalent layer structure**. Its delocalized electrons move **along the layers** and carry charge.
- ✗ "All simple molecular substances are insoluble in water." ✓ **Ethanol is miscible with water**. Compare solute–solvent attractions with the attractions disrupted during mixing.

## Quick check

1. When solid iodine melts, which attractions are overcome? Are its I–I covalent bonds broken?
2. Does NaCl conduct as a solid, when molten and when dissolved in water? Name the charge carriers.
3. Explain why metals are generally malleable while ionic crystals are brittle.
4. How many oppositely charged ions surround each ion in NaCl and in CsCl?
5. Compare diamond and graphite: carbon neighbours, hardness and electrical conductivity.

## Answers

1. Weak intermolecular attractions are overcome; I–I covalent bonds remain intact.
2. Solid: no, ions fixed. Molten / aqueous: yes, mobile Na^+^ and Cl^−^ ions.
3. Metal layers slide while metallic attraction is maintained. In an ionic crystal, shifting layers brings like charges together, causing repulsion.
4. NaCl: 6; CsCl: 8.
5. Diamond: 4 C neighbours, hard, no mobile charge carriers as an ordinary solid. Graphite: 3 C neighbours per layer, soft as layers slide, conducts through delocalized electrons along layers.

## Did you know?

When dry ice is placed in water, the white fog contains tiny water droplets. Carbon dioxide gas itself is colourless.