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S4 Chemistry: Atomic Structure and the Periodic Table – Error-Correction Practice (10 Questions)
S4 Chemistry: Atomic Structure and the Periodic Table – Error-Correction Practice (10 Questions)
Ten items target the usual slips: mass number taken as proton number, ion electrons not adjusted, isotopes mistaken for different electron counts, shell capacity misused, and a simple average used for relative atomic mass.
Quick answer 1. 27 and 13; 2. 10 electrons, mass number 16; 3. the proton number; 4. 6.93; 5. 2,8,6 in period 3 group 6; 6. 2,8,8,1 in period 4 group 1; 7. 20 protons, mass number 40; 8. 75 and 25 per cent; 9. 17 protons, mass number 35; 10. same protons, different mass numbers.
01 Paper details (check answers afterwards)
| Item | Detail |
|---|---|
| Questions | 10 questions |
| Suggested time | about 20 minutes |
| Difficulty mix | Foundation 3 · Intermediate 3 · Challenge 2 · Error redo 2 |
| Chapter | Chapter: S4 Chemistry | Atomic Structure and the Periodic Table: proton number, nucleon number and electron arrangement |
| Level | S4 |
02 Questions (10 items)
- FoundationA neutral atom has 13 protons and 14 neutrons. What are its mass number and electron number?
- FoundationAn oxide ion O2- has 8 protons and 8 neutrons. Find its electron number and mass number.
- FoundationA student says the mass number decides which element an atom is. What is wrong?
- IntermediateLithium has 6Li and 7Li with abundances of 7.5% and 92.5%. Find its relative atomic mass.
- IntermediateAn atom has 16 electrons. Write its electron arrangement and state its position in the periodic table.
- IntermediateAn atom has 19 electrons. Give its electron arrangement, period and group.
- ChallengeA Y2+ ion has 18 electrons and 20 neutrons. Find its proton number and mass number.
- ChallengeChlorine has a relative atomic mass of 35.5 and consists of 35Cl and 37Cl. Find the two abundances.
- Error redoA Z- ion has 18 electrons and 18 neutrons. Find its proton number and mass number.
- Error redoA student says isotopes share the same mass number and differ only in electrons. What is wrong?
03 Answers and one-line notes
| # | Answer | One-line note |
|---|---|---|
| 1 | Mass number 27 and 13 electrons. | Mass number adds protons to neutrons; the electron number equals the proton number. |
| 2 | 10 electrons and mass number 16. | A negative ion has extra electrons, while the mass number counts protons and neutrons only. |
| 3 | The proton number decides the element; the mass number varies between isotopes. | One element can have several mass numbers but only one proton number. |
| 4 | 6.93, since 6 times 0.075 plus 7 times 0.925 is 6.925. | Relative atomic mass is an abundance-weighted mean, not the simple average of the two mass numbers. |
| 5 | 2, 8, 6, so it is in period 3 group 6. | Fill shells with capacities 2 and 8 in turn; the outermost count gives the group. |
| 6 | 2, 8, 8, 1, so period 4 group 1. | The third shell holds 8, so the leftover electron opens a fourth shell. |
| 7 | 20 protons and mass number 40. | A positive ion has lost electrons, so add them back to get the proton number. |
| 8 | 35Cl is 75 per cent and 37Cl is 25 per cent. | Let the 35Cl abundance be x and solve 35x plus 37(1 minus x) equals 35.5. |
| 9 | 17 protons and mass number 35. | A negative ion gained one electron, so subtract one from 18 rather than adding. |
| 10 | Isotopes share the proton number but differ in mass number, and their electron counts are the same. | Confusing isotopes with ions is the most common conceptual slip here. |
04 What to do after marking
- After marking, redo only the wrong items and write down which step went wrong (formula, unit, or misread number).
- Repeat the same item two days later; move to the next paper only when both attempts are correct.
- Parents only need the answer and note columns; there is no need to explain every item.
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