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Next exam on November 14 – 15, 2026:
All concepts

Atomic Structure

原子结构

How proton, neutron and electron counts relate, the rules governing electron arrangement and how it is written down, and the ideas of isotope and nuclide.

11 real-exam questions · 4 sittings15 in mock papers24 practice

Formulas

  • Mass number and atomic composition

    Definition
    A=Z+NA=Z+N

    Applies when：A is the mass number, Z the proton number, N the neutron number; a neutral atom has Z electrons, an ion differs by its charge

How the real exam tests it

Questions from real CSCA papers that test this concept. Work each one out before you open the answer.

  1. CSCA Chemistry April 2026 Past Paper
    Which of the following pairs are isotopes? ( )
    • A. Ethane and propane
    • B. n-butane and isobutane
    • C. 16O{}^{16}\mathrm{O} and 18O{}^{18}\mathrm{O}
    • D. Red phosphorus and white phosphorus
    Show answer & solution

    Answer: C

    Answer: C 16O{}^{16}\mathrm{O} and 18O{}^{18}\mathrm{O} have the same number of protons but different numbers of neutrons; they are isotopes.
    Take the full CSCA Chemistry April 2026 Past Paper — timed & graded
  2. CSCA Chemistry March 2026 Past Paper
    Which pair consists of isotopes?
    • A．H2\mathrm { H } _ { 2 } and D2\mathrm { D } _ { 2 }
    • B．16O18O{ } ^ { 16 } \mathrm { O } ^ { 18 } \mathrm { O } and O2\mathrm { O } _ { 2 }
    • C．35Cl{ } ^ { 35 } \mathrm { Cl } and 37Cl{ } ^ { 37 } \mathrm { Cl }
    • D．Diamond and graphite
    Show answer & solution

    Answer: C

    35Cl{ } ^ { 35 } \mathrm { Cl } and 37Cl{ } ^ { 37 } \mathrm { Cl } : same protons (17), different neutrons = isotopes. A/B: molecules. D: allotropes. Answer: C.
    Take the full CSCA Chemistry March 2026 Past Paper — timed & graded

Real-exam evidence

Recorded past-paper appearances are listed below; online verification is labelled separately.

11 questions across 4 sittings1 of 4 sittings available to verify free online
  • CSCA Chemistry April 2026 Past Paper4 questionsreading page not yet public
  • CSCA Chemistry March 2026 Past Paper2 questionsreading page not yet public
  • CSCA Chemistry January 2026 Past Paper4 questionsreading page not yet public
  • CSCA Chemistry December 2025 Past Paper1 questionView question 4 and answer free (full paper)

15 questions across 5 mock papers — our own papers, so a coverage signal rather than real-exam evidence

24 related practice questions availablePractise free: Atomic Structure and Periodic Law

Sitting evidence comes from the CSCA past papers on record; questions whose original paper is missing are excluded, and sprint papers are deliberately not counted.

Related concepts

The Periodic Table and Periodic Law