Year 8 Science · Unit 2 · Lesson 15
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Learning Goals
Match each term to its meaning
Write the matching letter in the middle column. Each letter is used once.
| Term | Letter | Meaning |
|---|---|---|
| Group | A. How big an atom is; it increases going down a group. | |
| Period | B. A statement that says more than the evidence shows, such as “all Group 1 metals explode”. | |
| Atomic radius | C. A column; its elements can share some similar properties. | |
| Cautious wording | D. A row; its atoms get smaller from left to right. | |
| Overclaim | E. Phrases such as “tend to” or “can share” that allow for variation. |
1. State how reactivity changes going down Group 1, and how it changes going down Group 17.
Read the data
| Element | Na | Mg | Al | Si | P | S | Cl |
|---|---|---|---|---|---|---|---|
| Atomic number | 11 | 12 | 13 | 14 | 15 | 16 | 17 |
| Atomic radius (pm) | 186 | 160 | 143 | 117 | 110 | 104 | 99 |
Source: published values for the Period 3 elements, given in picometres (pm).
2. Describe the pattern in Table 1. Then calculate how much smaller a chlorine atom is than a sodium atom.
Difference = pm
3. Ella says: “Magnesium has one more proton than sodium, so a magnesium atom should be bigger.” Use Table 1 and the lesson’s idea about what changes across a period to explain why Ella is wrong.
Scenario: swapping lithium for sodium
Most rechargeable batteries use lithium. Engineers are testing sodium instead, because it is cheap and plentiful (it can be obtained from salt). Both are in Group 1. A student makes four predictions:
a. Sodium can share some properties with lithium. b. A sodium battery will work exactly like a lithium one. c. Sodium atoms are smaller than lithium atoms. d. Sodium metal reacts with water more vigorously than lithium.
Classify with a reason
For each prediction write “supported” (the group pattern backs it), “overclaim” (says more than the pattern shows) or “wrong” (goes against the pattern). Give a reason.
| Prediction | Verdict | Reason |
|---|---|---|
| a | ||
| b | ||
| c | ||
| d |
4. Chlorine (Cl) and bromine (Br) are both in Group 17. Bromine is one place below chlorine. Compare the two elements: predict which is more reactive and which has larger atoms, explain each prediction using their positions, and finish with one cautious sentence about how similar the two elements are.
Match each term to its meaning
Group → C; Period → D; Atomic radius → A; Cautious wording → E; Overclaim → B.
1. Going down Group 1, reactivity increases. Going down Group 17, reactivity decreases. [2 marks: 1 each]
Read the data
2. Atomic radius decreases from left to right across Period 3. 186 − 99 = 87 pm. [2 marks: 1 pattern, 1 calculation]
3. Table 1 shows magnesium is smaller (160 pm compared with 186 pm). Across a period the atoms have the same number of electron shells, but each has more protons, so the nucleus pulls the electrons in more tightly and the atom is smaller. More protons does not mean a bigger atom here. [3 marks: 1 data, 1 same shells, 1 more protons pull electrons in]
Classify with a reason
a: supported; same group, so they can share similar properties. b: overclaim; similar does not mean identical, so it must be tested. c: wrong; sodium is below lithium, so its atoms are larger (186 pm compared with 152 pm). d: supported; reactivity increases down Group 1 and sodium is below lithium (so sodium must be kept dry).
Show What You Know
4. Marking guide (points expected): 1 chlorine is more reactive; 1 because reactivity decreases going down Group 17 and chlorine is higher; 1 bromine has larger atoms; 1 because bromine is lower, so its atoms have more electron shells; 1 a cautious sentence, for example “Chlorine and bromine can share some similar properties because they are in the same group, but they are not identical.” [5 marks]
Common error to watch for: in Q4, saying bromine is more reactive because “lower in the group means more reactive”. That rule is for Group 1 metals; Group 17 runs the other way.