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Playbook

Atomic Structure and Periodic Classification

35 q · 9% HARD. Periodic trends (atomic radius, ionisation energy, electronegativity — across vs down), valency from group number, atomic-number/mass-number/subatomic-particle arithmetic, atomic models (Dalton → Thomson → Rutherford → Bohr — what each PROVED, not just what they said), isotopes vs isobars vs isoelectronic.

questions in the bank
35
tagged HARD
9%
subtopic(s)
5
worked examples
2

When you’ll see it

A periodic-trend ordering (atomic radius / IE / EN / metallic character), an atomic-number↔subatomic-particle count, an atomic-model history (Dalton/Thomson/Rutherford/Bohr), or an isotope vs isobar vs isoelectronic classification.

How this chapter is tested

35 q in 10 years · 9% HARD. Five subtopics. Periodic Trends (12 q) is the largest — memorise the 4 master rules and the chapter falls open. ACROSS a period (left → right): nuclear charge ↑, atomic radius ↓, ionisation energy ↑, electronegativity ↑, metallic character ↓. DOWN a group: atomic radius ↑ (more shells), IE ↓, EN ↓, metallic character ↑. Valency = group number (1–4); for groups 5–8, valency = 8 − group. Order of valency: noble gases (group 18) = 0; Mg (group 2) = 2; N (group 15) = 3; Si (group 14) = 4. Predict any 'order of X' question from these.

Atomic Models history (6 q · 17% HARD): Dalton — solid indivisible spheres (1808). Thomson — plum pudding (1897, after discovering e⁻). Rutherford — small dense nucleus + mostly empty space (1909, gold-foil α-scattering). Bohr — electrons in quantised orbits (1913). Rutherford did NOT propose quantised orbits — that was Bohr (common trap). What Rutherford couldn't explain: why orbiting electrons don't spiral into nucleus (Bohr's quantum postulate fixed this).

Subatomic particles (7 q): proton (Goldstein 1886, +1, mass 1 amu), electron (J.J. Thomson 1897, −1, mass ≈ 1/1836 amu), neutron (Chadwick 1932, 0, mass 1 amu). Mass number A = p + n. Atomic number Z = p (= e in neutral atom). For ³²₁₆S²⁻: 16 protons, 16 neutrons (32−16), 18 electrons (16+2 from charge). Isotopes — same Z, different A (¹H, ²H, ³H). Isobars — same A, different Z (⁴⁰Ar, ⁴⁰K, ⁴⁰Ca). Isoelectronic — same e⁻ count (Na⁺, F⁻, Ne all have 10 e⁻).

The sub-skills

The rules and habits that decide whether you get a question right.

  • Periodic-trend master rules

    Across period →: atomic radius ↓, IE ↑, EN ↑, metallic character ↓, electron affinity ↑. Down group ↓: atomic radius ↑, IE ↓, EN ↓, metallic character ↑. Reactivity: metals MOST reactive bottom-left (Fr, Cs); non-metals MOST reactive top-right (F, O).

  • Valency from group number

    Groups 1–2 (alkali, alkaline earth): valency = group number (Na=1, Mg=2). Groups 13–14: valency = group number − 10 (Al=3, C/Si=4). Groups 15–17: valency = 18 − group (N=3, O=2, F=1). Group 18 (noble gases): valency = 0.

  • Atomic-model history

    Dalton (1808): indivisible solid sphere. Thomson (1897): plum pudding (positive sphere with embedded e⁻). Rutherford (1909): nucleus + mostly empty space (gold-foil). Bohr (1913): quantised orbits (fixed Rutherford's stability problem). Quantum mechanics (1920s): probability clouds.

  • Subatomic particle counting

    For ᴬ_Z X^q: protons = Z, neutrons = A − Z, electrons = Z − q (charge q subtracts e⁻). Mass number A = sum of nucleons. Atomic number Z fixes the element. Charge q shifts e⁻ count only.

  • Isotope / isobar / isoelectronic classification

    Isotopes: same Z (same element), different A (¹H/²H/³H). Isobars: same A, different Z (⁴⁰Ar/⁴⁰K/⁴⁰Ca). Isotones: same N = A − Z, different Z (¹⁴C/¹⁵N/¹⁶O have N=8). Isoelectronic: same total e⁻ count (Na⁺/Mg²⁺/F⁻/Ne all 10 e⁻).

2 worked examples from the bank

Real past-year questions illustrating the playbook. Click to reveal options + solution.

Example 1Atomic Structure and Periodic ClassificationHARD
As proposed by Dalton, which of the following symbol represents phosphorus?

[Q116 · Sep · 2023]

Example 2Atomic Structure and Periodic ClassificationHARD
Which one of the following is the correct order of the valencies of elements Ne, Si, N and Mg?

[Q63 · Apr · 2022]

Traps to expect

Distractor shapes specific to this chapter. The page-wide Traps section covers the bank-level patterns.

  • Rutherford 'proposed quantised orbits'

    Rutherford proposed the NUCLEAR model (small dense nucleus). He did NOT propose quantised orbits — that was Bohr (1913). The trap option attributes Bohr's contribution to Rutherford.

  • Atomic radius increases across period

    Atomic radius DECREASES across a period (left → right, increasing nuclear charge pulls e⁻ closer). Wrong option says 'increases.' Down a group, radius DOES increase (more shells).

  • Isotope vs isobar swap

    Isotopes = SAME element, different mass. Isobars = SAME mass, different elements. Wrong option swaps definitions or treats them as synonymous.

Drill every atomic structure and periodic classification question

35 questions from the bank, scoped to 5 bundled subtopics.

Drill the 35 questions

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