Year 9 Science · Unit 2 · Lesson 10
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Learning Goals
1. Complete the table to compare the three types of bonding.
| Feature | Ionic | Covalent | Metallic |
|---|---|---|---|
| Types of atoms involved | |||
| What the electrons do | |||
| Conducts as a solid? | |||
| Bends or shatters? |
2. Gold can be beaten into gold leaf about 0.1 µm thick, roughly a thousand times thinner than a sheet of paper. Explain (a) why a metal can be shaped like this and (b) why a crystal of sodium chloride could never be.
3. A metal spoon left in a pot of hot soup soon feels hot at the handle, but a wooden spoon does not. Use the sea-of-electrons model to explain why metals conduct heat so well.
Scenario: four unlabelled substances
A laboratory technician finds four jars with worn-off labels. She tests each substance and records its properties.
| Jar | Melting point (°C) | Conducts electricity? | Other observations |
|---|---|---|---|
| W | 1455 | Yes, as a solid and as a liquid | Shiny; bends when hit |
| X | 734 | Only when melted or dissolved | White crystals; shatter when hit |
| Y | −23 | No | Colourless liquid |
| Z | about 1700 | No | Very hard; does not dissolve |
Source: the properties of nickel, potassium bromide, tetrachloromethane and quartz, in a jumbled order.
4. Identify the type of bonding and structure in W, X, Y and Z. Give one piece of evidence for each.
Read the data
Overhead power lines in Australia are made mostly of aluminium, not copper. Table 2 helps explain why.
| Metal | Electrical conductivity (millions of S/m) | Density (g/cm³) |
|---|---|---|
| Silver | 63.0 | 10.5 |
| Copper | 59.6 | 8.96 |
| Aluminium | 37.7 | 2.70 |
Source: data book values. S/m (siemens per metre) measures how well a material conducts.
5. Divide conductivity by density for copper and for aluminium. How many times larger is aluminium’s value?
Answer times
6. Use your answer to explain why aluminium is chosen for power lines hung between towers, even though copper conducts better. Why is copper still used for wiring inside houses?
7. A computer circuit board has three parts: copper tracks that carry current, a board of glass fibre and epoxy resin that holds everything apart, and blobs of solder, a tin alloy that melts at about 220 °C (copper melts at 1085 °C), that join parts to the tracks. For each part, name the type of bonding and explain why it suits the job.