Year 8 Science · Unit 2 · Lesson 13
Challenge Worksheet
Learning Goals
Find the errors
This paragraph contains exactly three errors. Underline each one and write the correction below.
A student’s revision notes
The periodic table lists the elements in alphabetical order. Each vertical column is called a group. In groups 1, 2 and 13–17, elements in the same group have the same number of outer-shell electrons. Magnesium (atomic number 12) is in period 2, because it has two outer electrons. Helium has only 2 electrons, so it belongs in group 2 with beryllium.
1. Write the three corrections.
Decode the electron arrangement
In groups 13 to 17, group number = outer-shell electrons + 10. Show your working for row 1 beside the table.
| Electron arrangement | Atomic number | Element | Period | Group |
|---|---|---|---|---|
| 2,8,5 | ||||
| 2,4 | ||||
| 2,8,6 |
2. Element X has one more proton than argon (2,8,8). Write the electron arrangement of X, name it, and give its group and period. Explain why adding just one proton moves the element to the far left of a new row.
How long is each period?
| Period | Number of elements | Atomic numbers in this period |
|---|---|---|
| 1 | 2 | 1 to 2 |
| 2 | 8 | 3 to 10 |
| 3 | 8 | 11 to 18 |
| 4 | 18 | 19 to 36 |
| 5 | 18 |
Source: element counts from the current table of 118 elements.
3. Complete Table 1. Then find the period and group of elements 53 and 38. (The last element of every period is in group 18.)
53: period group 38: period group
4. A student claims that sodium and chlorine must have similar chemical properties, because both are in period 3. Is the student right? Say what period 3 really tells you about the two atoms, and name an element that sodium does behave like.
5. The periodic table is sometimes called “the chemist’s address book”. Evaluate this description. Explain what an element’s address (group and period) tells a chemist, use evidence from this sheet, describe one thing the address does not tell you exactly, and finish with a judgement.
Find the errors
1. (i) The elements are in order of atomic number, not alphabetical order. (ii) Magnesium (2,8,2) is in period 3: it has three shells; its two outer electrons put it in group 2. (iii) Helium is in group 18, because its only shell is full, like neon and argon. [3 marks: 1 per correction]
Decode the electron arrangement
2,8,5: 15, phosphorus, period 3, group 15. 2,4: 6, carbon, period 2, group 14. 2,8,6: 16, sulfur, period 3, group 16. Working for row 1: 2 + 8 + 5 = 15 electrons = 15 protons in a neutral atom; three shells, so period 3.
2. 2,8,8,1; potassium (atomic number 19); group 1, period 4. The first three shells are full at argon, so the 19th electron must start a fourth shell (new period); with only one outer electron it is in group 1, at the left. [4 marks: 1 arrangement, 1 name, 1 group and period, 1 explanation]
Go Deeper
3. Period 5 is 37 to 54. Element 53: period 5; 54 is the last element, in group 18, so 53 is one box to its left: group 17 (iodine). Element 38: period 5; 37 is the first element (group 1), so 38 is group 2 (strontium). [4 marks: 1 table, 1 each for 53 and 38, 1 reasoning]
4. Wrong. Period 3 only means both atoms have three shells. Sodium is in group 1 (one outer electron) and chlorine in group 17 (seven), so they behave very differently (reactive metal, reactive non-metal). Sodium behaves like lithium or potassium (same group). [3 marks: 1 period = shells, 1 different groups so different behaviour, 1 Li or K]
Show What You Know
5. Marking guide (points expected): 1 group plus period points to exactly one box (group 1, period 3 is only sodium); 1 group gives the outer electrons and so the likely behaviour; 1 period gives the number of shells; 1 strength: e.g. Li, Na and K share group 1 and all react with water, as position predicts; 1 limit: the address gives the kind of behaviour, not how strong (Li, Na and K react with different vigour); 1 supported judgement. [6 marks]
Common error to watch for: using magnesium’s two outer electrons (2,8,2) as its period. Outer electrons give the group; the number of shells gives the period.