Year 9 Science · Unit 2 · Lesson 10
Challenge Worksheet
Learning Goals
1. State what “delocalised” means for the electrons in a metal, and name two properties of metals that it explains.
Find the mistake: a student wrote this answer
“All three types of bonding, ionic, covalent and metallic, are basically the same: they all involve electrons being shared between atoms. The only difference is how many electrons are shared. Ionic bonds share 1 electron, covalent bonds share 2, and metallic bonds share all the electrons. This is why metallic bonds are the strongest.”
2. Identify three scientific errors in the student’s answer. For each, say what is wrong.
3. Rewrite the answer correctly, in no more than four sentences. Say clearly what happens to the electrons in each type of bonding.
Read the data
An alloy is a metal mixed with small amounts of other elements. Table 1 compares two pure metals with three of their alloys.
| Material | Made of | Conductivity (millions of S/m) | Compared with pure metal |
|---|---|---|---|
| Copper | pure copper | 59.6 | soft, easily bent |
| Brass | 70% copper, 30% zinc | 16 | harder |
| Phosphor bronze | 95% copper, 5% tin | 8.7 | harder, springier |
| Iron | pure iron | 10.0 | soft, easily bent |
| Stainless steel | iron, 18% chromium, 8% nickel | 1.4 | much harder, rust-proof |
Source: handbook conductivities for annealed copper, cartridge brass, phosphor bronze, pure iron and grade 304 stainless steel.
4. Calculate how many times better pure copper conducts than phosphor bronze. Use the metal lattice to explain why adding other atoms makes an alloy both harder and a poorer conductor.
5. The metal contacts inside a power socket must carry current and must spring back each time a plug is pushed in. An engineer chooses phosphor bronze rather than pure copper. Evaluate this choice using Table 1.
6. Tungsten (W) is a metal that melts at 3422 °C, the highest of any pure metal. Magnesium oxide, which is held together by ionic bonding, melts at 2852 °C. A student claims: “Substances with ionic bonding always melt at higher temperatures than metals, because ionic bonding is stronger.” Evaluate this claim.