Year 8 Science · Unit 2 · Lesson 15
Challenge Worksheet
Learning Goals
Find the errors
This summary has exactly three errors. Underline them and correct them below.
A student’s summary
Going down Group 1, the atoms get larger and the metals become less reactive. Potassium reacts more vigorously with water than sodium. Going across a period from left to right, the atoms gain protons and get larger. In Group 17, fluorine at the top is the least reactive element. Group 1 and Group 17 show reactivity changing in opposite directions down the group.
1. Write the three corrections.
| Element | Atomic radius (pm) | Increase from the element above (pm) |
|---|---|---|
| Fluorine (F) | 71 | – |
| Chlorine (Cl) | 99 | |
| Bromine (Br) | 114 | |
| Iodine (I) | 133 |
Data: real halogen sizes, to the nearest whole picometre.
2. Complete the last column of Table 1. Then calculate how many times larger an iodine atom is than a fluorine atom, to one decimal place.
Iodine radius ÷ fluorine radius = times
3. Kai says: “Each step down Group 17 adds the same amount to the radius, so astatine (below iodine) must be exactly 152 pm.” Use your completed column to evaluate Kai’s claim, and write a better prediction for astatine’s radius and its reactivity compared with iodine.
Plan an investigation
Reactivity also increases down Group 2. Plan a fair test: does magnesium (Period 3) or calcium (Period 4) react faster with cold water?
Hypothesis (If …, then …, because …)
Variables
Method (teacher demo, eye protection on)
4. Evaluate: “Group patterns are useless. Lithium only fizzes gently, the Group 17 radius steps are all different, and you still have to test every element.” Use evidence from Group 1 and Table 1, and end with a cautious judgement.
Find the errors
1. (i) Going down Group 1 the metals become more reactive, not less. (ii) Going across a period the atoms get smaller, not larger. (iii) Fluorine is the most reactive element in Group 17, not the least. [3 marks: 1 per correction]
Your Turn
2. Increases: chlorine 28 pm, bromine 15 pm, iodine 19 pm. 133 ÷ 71 = 1.87, so about 1.9 times larger. [3 marks: 2 column (1 if two correct), 1 ratio]
3. Kai is wrong: the steps are 28, 15 and 19 pm, so the increase is not the same each time (152 pm came from assuming another 19 pm step). The pattern gives a direction, not an exact value. Better prediction: astatine’s atoms are larger than 133 pm, and astatine is less reactive than iodine. [3 marks: 1 steps differ, 1 larger than 133 pm, 1 less reactive]
Plan an investigation
Hypothesis: if calcium is lower in Group 2, then it will react faster with water than magnesium, because metal reactivity increases down the group. Independent: the metal (magnesium or calcium). Dependent: rate of reaction, for example bubbles counted in one minute or volume of gas collected in two minutes. Controlled: size or mass of the metal piece; volume and temperature of the water; same container. Method: add equal masses of each metal to the same volume of cold water in separate test tubes; count bubbles (or collect gas) for the same time; repeat each test and average. Safety: small pieces only, safety glasses, teacher supervision.
Show What You Know
4. Marking guide (points expected): 1 lithium reacting gently fits the pattern, because lithium is at the top of Group 1 and is the least reactive; sodium and potassium react more vigorously, as predicted; 1 the Group 17 radius steps differ (28, 15, 19 pm), but every step is an increase, so the direction of the pattern holds; 1 a pattern with variation or exceptions is still useful, it just needs careful wording; 1 patterns let you predict relative properties (for example potassium more reactive than sodium) before testing; 1 experiments still confirm predictions, so testing is not wasted; 1 judgement, for example “The claim is not justified: group patterns tend to give reliable predictions of direction, although they do not give exact values.” [6 marks]
Common error to watch for: in Q3, agreeing with Kai because the numbers “go up”. A trend in direction is not the same as equal steps; students should use the calculated column.