Year 8 Science · Unit 2 · Lesson 22
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Learning Goals
1. Potassium (K) is element 19. State (a) its group in the periodic table, and (b) whether it is a metal or a non-metal, with one property you would expect it to have.
Match each idea to its meaning
Write the matching letter in the middle column. Each letter is used once.
| Idea | Letter | Meaning |
|---|---|---|
| Atomic number | A. Giving up one property to gain another, such as a cheaper but heavier metal. | |
| Isotopes | B. An atom that has gained or lost electrons, so it carries a charge. | |
| Ion | C. An element with some metal and some non-metal properties, such as silicon. | |
| Metalloid | D. The number of protons; it sets the element’s place in the table. | |
| Trade-off | E. Atoms of the same element with different numbers of neutrons. |
Odd one out
In each row, circle the one that does not belong with the other three. Give the reason.
| Row | Choices | Reason |
|---|---|---|
| (a) | Li Na K Mg | |
| (b) | copper silicon iron aluminium | |
| (c) | Cu CO₂ NaCl H₂O |
Scenario: the beach bike club
A school bike club is choosing a metal for a new frame. The frame will be ridden along the beach, in salty air. The club’s two rules: the metal must have a density below 5 g/cm³, and it must not corrode quickly in salty air.
| Metal | Atomic number | Density (g/cm³) | In salty air | Cost (1 = cheapest) |
|---|---|---|---|---|
| Iron (Fe) | 26 | 7.87 | rusts, and the rust flakes off | 1 |
| Aluminium (Al) | 13 | 2.70 | thin oxide layer seals the surface | 2 |
| Magnesium (Mg) | 12 | 1.74 | corrodes quickly | 3 |
| Titanium (Ti) | 22 | 4.51 | hardly corrodes at all | 4 |
Source: densities are standard values for the pure metals (real frames use alloys); corrosion and cost are simplified ratings.
Classify with a reason
For each metal write “passes” or “fails” the club’s two rules. If it fails, name the rule it breaks.
| Metal | Passes or fails? | Reason |
|---|---|---|
| Iron | ||
| Aluminium | ||
| Magnesium | ||
| Titanium |
2. Each frame contains 600 cm³ of metal. Find the mass of an aluminium frame and of an iron frame (mass = density × volume), then the difference between them.
Iron frame is heavier by g
3. The club has a small budget. Two metals pass both rules. Choose one for the club and justify your choice. Describe the trade-off: what does the club give up by not choosing the other one?
4. Hydrogen is the least dense gas of all, and it is cheaper than helium. Yet party balloons and modern airships are filled with helium. In 1937 the hydrogen-filled airship Hindenburg caught fire and was destroyed. Explain the choice of helium. Use each gas’s atomic number, its place in the periodic table and its reactivity, and describe the trade-off.
Warm Up
1. (a) Group 1. (b) A metal; for example it is very reactive (reacts strongly with water), is soft, or conducts electricity. [2 marks: 1 group, 1 metal with a correct property]
Match each idea to its meaning
Atomic number → D; Isotopes → E; Ion → B; Metalloid → C; Trade-off → A.
Odd one out
(a) Mg: it is in Group 2; Li, Na and K are all Group 1 metals. (b) silicon: it is a metalloid; copper, iron and aluminium are metals. (c) Cu: it is an element (one type of atom); CO₂, NaCl and H₂O are compounds.
Classify with a reason
Iron: fails; density 7.87 g/cm³ is above 5 (it also rusts). Aluminium: passes; 2.70 g/cm³ and its oxide layer protects it. Magnesium: fails; it corrodes quickly in salty air. Titanium: passes; 4.51 g/cm³ and it hardly corrodes.
2. Aluminium: 2.70 × 600 = 1620 g. Iron: 7.87 × 600 = 4722 g. Difference: 4722 − 1620 = 3102 g (about 3.1 kg). [3 marks: 1 each mass, 1 difference]
3. Aluminium. It passes both rules and is the cheaper of the two (cost 2 against 4), which suits a small budget; it is also the lighter of the two (2.70 against 4.51 g/cm³). The trade-off: the club gives up titanium’s even better resistance to corrosion. Accept titanium only if the answer explains that the budget rules it out or argues the extra cost is worth it. [3 marks: 1 choice meeting both rules, 1 cost reason, 1 trade-off named]
Show What You Know
4. Marking guide (points expected): 1 hydrogen has atomic number 1 and sits at the top left of the table (it is a non-metal gas); helium has atomic number 2 and is in Group 18; 1 helium is a noble gas, so it is very unreactive and does not burn; 1 hydrogen is reactive: it burns (reacts with oxygen in air), as the Hindenburg fire showed; 1 both gases are much less dense than air, so both make a balloon float; 1 trade-off: helium gives up a little lift and costs more, but it is chosen because safety matters more than the small gain from hydrogen. [5 marks]
Common error to watch for: saying hydrogen is a metal because it sits above Group 1. Hydrogen is a non-metal gas; its place in the table does not make it a metal, and it is its reactivity with oxygen, not its column, that makes it unsafe.