Year 8 Science · Unit 2 · Lesson 16
Challenge Worksheet
Learning Goals
Find the errors
This comparison has exactly three errors. Underline them and correct them below.
A student’s comparison of sodium and magnesium
Sodium (atomic number 11) and magnesium (atomic number 12) are both metals in Period 3. Because their atomic numbers are next to each other, they are in the same group and have almost identical properties. Sodium sits to the right of magnesium in the table. Both conduct electricity because they are metals. Each sodium atom has 12 protons.
1. Write the three corrections.
| Element | Atomic number | Period | What is seen in cold water |
|---|---|---|---|
| Beryllium (Be) | 4 | 2 | No visible reaction |
| Magnesium (Mg) | 12 | 3 | A few small bubbles form very slowly |
| Calcium (Ca) | 20 | 4 | Steady fizzing; the water turns cloudy |
| Strontium (Sr) | 38 | 5 | Faster fizzing |
| Barium (Ba) | 56 | 6 | Vigorous fizzing |
Source: descriptions summarised from standard chemistry references.
2. Describe the reactivity pattern in Table 1. Radium (Ra) is the next element down Group 2, in Period 7. Predict how it would behave in cold water, and say how confident you can be.
Choose the evidence
Three different questions are asked about magnesium and barium. For each, name the most useful feature, give the answer, and name one feature that would not help.
| Question | Most useful feature | Answer | A feature that would not help |
|---|---|---|---|
| Which reacts faster with cold water? | |||
| Which has larger atoms? | |||
| Do both conduct electricity? |
Someone claims
“Elements whose atomic numbers are next to each other are always the most alike. So neon (10) and sodium (11) should be more alike than sodium (11) and potassium (19).”
3. Evaluate this claim. Compare neon with sodium, and sodium with potassium, using group, period and category, and describe how each element behaves. End with a judgement about which feature best predicts whether two elements are alike.
Find the errors
1. (i) Neighbouring atomic numbers in a period are in different groups: sodium is in Group 1 and magnesium in Group 2, so they are not almost identical. (ii) Sodium sits to the left of magnesium, not the right. (iii) Each sodium atom has 11 protons, not 12 (atomic number = number of protons). [3 marks: 1 per correction]
Your Turn
2. Reactivity with water increases going down Group 2, from no visible reaction (beryllium) to vigorous fizzing (barium). Radium should react even more vigorously than barium. Fairly confident about the direction, because the pattern holds for all five elements, but not about exactly how vigorous it would be; it would still need testing (accept any cautious wording). [3 marks: 1 pattern, 1 prediction, 1 confidence with reason]
Choose the evidence
Reacts faster: position in Group 2 (barium is lower); barium; not helpful: for example colour or mass number. Larger atoms: period number (barium Period 6, magnesium Period 3, so barium has more electron shells); barium; not helpful: for example category (both metals). Both conduct: category; yes, both are metals; not helpful: for example period or position in the group. Accept any sensible irrelevant feature.
Show What You Know
3. Marking guide (points expected): 1 neon: Group 18, Period 2, non-metal (noble gas); 1 sodium: Group 1, Period 3, metal; potassium: Group 1, Period 4, metal; 1 neon is unreactive, while sodium reacts vigorously with water, so the neighbours 10 and 11 behave very differently; 1 sodium and potassium are both reactive metals that react with water (potassium more vigorously), so they are alike even though their atomic numbers differ by 8; 1 the claim is wrong, because neighbouring atomic numbers can sit in different groups and even different categories; 1 judgement: a shared group is the better predictor of similar properties, although group members are similar, not identical. [6 marks]
Common error to watch for: assuming the table works like a number line, so that close atomic numbers must mean close properties. Similarity runs down the columns, not along the rows.