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S4 Chemistry: Atomic Structure and the Periodic Table – Challenge Practice (10 Questions)
S4 Chemistry: Atomic Structure and the Periodic Table – Challenge Practice (10 Questions)
Ten multi-step items: proton number from the charge, weighted masses from three isotopes, or abundances solved backwards from the relative atomic mass.
Quick answer 1. proton number 15, arrangement 2, 8, 5; 2. nucleon number 39, 19 electrons, arrangement 2, 8, 8, 1; 3. period 4, group 2, ion with 18 electrons; 4. 35.5; 5. 63.62; 6. Br-79 at 55 per cent and Br-81 at 45 per cent; 7. proton number 17, 18 neutrons, arrangement 2, 8, 8; 8. 28.11; 9. Mg2+, two electrons lost; 10. neutrons equal nucleon number minus proton number.
01 Paper details (check answers afterwards)
| Item | Detail |
|---|---|
| Questions | 10 questions |
| Suggested time | about 30 minutes |
| Difficulty mix | Foundation 3 · Intermediate 3 · Challenge 3 · Error redo 1 |
| Chapter | Chapter: S4 Chemistry | Atomic Structure and the Periodic Table: proton number, nucleon number and electron arrangement |
| Level | S4 |
02 Questions (10 items)
- FoundationAn ion X3- has 18 electrons. Find its proton number and the electron arrangement of its atom.
- FoundationAn atom has 20 neutrons and proton number 19. Find its nucleon number, electrons and electron arrangement.
- FoundationAn element has the electron arrangement 2, 8, 8, 2. Find its period, group and the electron count of its stable ion.
- IntermediateNatural chlorine is 75.8 per cent Cl-35 and 24.2 per cent Cl-37. Calculate its relative atomic mass to three significant figures.
- IntermediateA sample is 69.2 per cent Cu-63 and 30.8 per cent Cu-65. Calculate its relative atomic mass to four significant figures.
- IntermediateBromine has only Br-79 and Br-81 and a relative atomic mass of 79.9. Find the percentage abundance of each.
- ChallengeAn ion X- has nucleon number 35 and 18 electrons. Find its proton number, neutrons and electron arrangement.
- ChallengeNatural silicon is 92.2 per cent Si-28, 4.7 per cent Si-29 and 3.1 per cent Si-30. Calculate its relative atomic mass to four significant figures.
- ChallengeAn ion has 10 electrons while its element has proton number 12. Give the ion symbol and explain the electron transfer.
- Error redoA student wrote that neutrons equal the nucleon number, which only works for hydrogen. What is the correct method?
03 Answers and one-line notes
| # | Answer | One-line note |
|---|---|---|
| 1 | Proton number 15, which is phosphorus, with the arrangement 2, 8, 5. | For an anion, subtract the extra electrons from the electron count to get the proton number. |
| 2 | Nucleon number 39, 19 electrons and the arrangement 2, 8, 8, 1. | For a neutral atom the proton number is also the electron count. |
| 3 | Period 4 and group 2; it loses two electrons, leaving 18. | Four shells means period 4, and two outer electrons means group 2. |
| 4 | 35.5, since 35 times 75.8 plus 37 times 24.2 gives 3548.4, and dividing by 100 gives 35.484. | Multiply by the percentages and divide once at the end to avoid place-value slips. |
| 5 | 63.62, since 63 times 69.2 plus 65 times 30.8 gives 6361.6, and dividing by 100 gives 63.616. | A sample value may differ slightly from the natural figure, which is expected. |
| 6 | Br-79 is 55 per cent and Br-81 is 45 per cent. With x for Br-79, 79x plus 81 times (100 – x) equals 7990, so x is 55. | One unknown is enough: write the second abundance as 100 minus x. |
| 7 | Proton number 17, 18 neutrons and the arrangement 2, 8, 8. | A 1- charge means one extra electron, so the proton number is 18 minus 1. |
| 8 | 28.11, since the weighted sum is 2810.9 and dividing by 100 gives 28.109. | Keep every decimal until the final division, then round once. |
| 9 | Mg2+. A magnesium atom has 12 electrons and loses two, leaving the stable arrangement 2, 8. | Fewer electrons than protons means a positive ion; the difference is the charge. |
| 10 | Neutrons equal nucleon number minus proton number, so the two match only for hydrogen-1. | Always note both the proton number and the nucleon number before subtracting. |
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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