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Chemistry · Practice book · Senior 4 · Unit 2 of 17

Electron configurations of atoms and ions

Senior 4 Chemistry · 14 questions with answers. Try each question on paper, then tap it to see the answer.

Unit 2 Electron configurations of atoms and ions

Key unit competence: Interpret Bohr's model of the atom, the hydrogen spectrum, electronic configurations and ionisation energies.

Key ideas

  • Bohr model: electrons move in fixed energy levels (n = 1, 2, 3 ...). An electron absorbs energy to jump to a higher level and emits light of a definite frequency when it falls back: ΔE = hf. This explains line spectra.
  • Hydrogen emission spectrum: Lyman series (transitions to n = 1, ultraviolet), Balmer series (to n = 2, visible), Paschen series (to n = 3, infrared). Lines converge at higher frequency; the convergence limit gives the ionisation energy.
  • Quantum numbers: principal (n), azimuthal (l: s, p, d, f), magnetic (m) and spin (+½, −½). Orbitals: s (spherical, 1 orbital, 2 electrons), p (dumbbell, 3 orbitals, 6 electrons), d (5 orbitals, 10 electrons), f (7 orbitals, 14 electrons).
  • Filling rules: Aufbau (lowest energy first: 1s 2s 2p 3s 3p 4s 3d 4p ...), Pauli (max 2 electrons per orbital with opposite spins), Hund (fill orbitals of equal energy singly first). Exceptions: Cr [Ar]3d5 4s1 and Cu [Ar]3d10 4s1.
  • Ions: remove electrons from the highest n first; for transition metals, 4s electrons are lost before 3d, e.g. Fe2+ [Ar]3d6, Fe3+ [Ar]3d5.
  • First ionisation energy is the energy needed to remove one mole of electrons from one mole of gaseous atoms: X(g) → X+(g) + e^−. It depends on nuclear charge, distance from the nucleus and shielding. Successive ionisation energies increase; large jumps show a change of shell.

Common misconceptions

  • "Electrons can have any energy in an atom." No: In the Bohr model electrons occupy only definite energy levels.
  • "When iron forms Fe2+, it loses 3d electrons first." No: The 4s electrons are lost first.
  • "Ionisation energy always increases across a period without exception." No: There are dips: from group 2 to 13 (e.g. Be to B) and from group 15 to 16 (e.g. N to O).

Worked examples

Example 1. Write the full electronic configurations of S, S2− and Fe, Fe2+. (S Z = 16, Fe Z = 26)

Solution

S: 1s2 2s2 2p6 3s2 3p4.

S2−: 1s2 2s2 2p6 3s2 3p6 (gains 2 electrons).

Fe: 1s2 2s2 2p6 3s2 3p6 3d6 4s2.

Fe2+: 1s2 2s2 2p6 3s2 3p6 3d6 (4s electrons lost first).

Example 2. Explain how the Bohr model accounts for the line spectrum of hydrogen.

Solution

Electrons exist only in definite energy levels.

When excited, an electron jumps to a higher level; when it falls back, it emits a photon with energy ΔE = hf equal to the gap between levels.

Only certain gaps exist, so only certain frequencies appear: a line spectrum.

Different series end on different levels (n = 1 Lyman, n = 2 Balmer, n = 3 Paschen).

Practice questions

  1. State the Aufbau principle.

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    Electrons fill orbitals in order of increasing energy.

    Common mistake: Confusing it with Hund's rule.

  2. Write the configuration of phosphorus (Z = 15).

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    1s2 2s2 2p6 3s2 3p3.

    Common mistake: Writing 3p5.

  3. How many electrons can an f sub-level hold?

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    14.

    Common mistake: Saying 10.

  4. Define first ionisation energy, with an equation for sodium.

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    Energy to remove 1 mol of electrons from 1 mol of gaseous atoms: Na(g) → Na+(g) + e^−.

    Common mistake: Leaving out the (g) state symbols.

  5. Give the region of the electromagnetic spectrum for the Paschen series.

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    Infrared.

    Common mistake: Saying ultraviolet.

  6. Write the configuration of Cu2+ (Cu Z = 29).

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    [Ar]3d9.

    Common mistake: Writing [Ar]3d7 4s2.

  7. Explain why ionisation energy decreases down group 1.

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    Outer electron further from nucleus and more shielded, so less attraction.

    Common mistake: Saying nuclear charge decreases.

  8. How many unpaired electrons in nitrogen (Z = 7)?

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    3.

    Common mistake: Saying 1.

  9. What does a large jump between the 2nd and 3rd ionisation energies show?

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    The element has 2 outer electrons (group 2); the 3rd comes from an inner shell.

    Common mistake: Saying it has 3 outer electrons.

  10. Why do chromium and copper have unusual configurations?

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    Half-filled (3d5) and full (3d10) d sub-levels are extra stable.

    Common mistake: Saying they lose 4s electrons as atoms.

Matching and drag-and-drop

  1. Matching

    Match each item in column A with the correct answer in column B. Write the letter next to each number.

    Column AColumn B
    1. Which principle states that an orbital holds at most two electrons of opposite spin?a. 3d
    2. Light is emitted when an electron:b. Pauli exclusion principle
    3. Which sub-level fills immediately after 4s?c. increase
    4. Successive ionisation energies of an element always:d. falls from a higher to a lower energy level
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    1 → b (Pauli exclusion principle) · 2 → d (falls from a higher to a lower energy level) · 3 → a (3d) · 4 → c (increase)

    • No two electrons have the same four quantum numbers.
    • The energy difference is released as a photon.
    • 3d is slightly higher in energy than 4s.
    • Each electron is removed from a more positive ion.

  2. Drag and drop

    Choose the correct word or phrase from the box for each gap. One word in the box is not needed.

    [Ar]3d10 4s1  ·  6  ·  [Ar]3d5 4s1  ·  [Ar]4s2 4p4
    1. The electronic configuration of chromium (Z = 24) is .
    2. The maximum number of electrons in a p sub-level is .
    3. Copper (Z = 29) has the configuration .
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    1: [Ar]3d5 4s1 · 2: 6 · 3: [Ar]3d10 4s1

    • A half-filled d sub-level is extra stable.
    • Three p orbitals × 2 electrons.
    • A full d sub-level is extra stable.

  3. Matching

    Match each item in column A with the correct answer in column B. Write the letter next to each number.

    Column AColumn B
    1. Which element has the highest first ionisation energy?a. 1s2 2s2 2p6 3s2 3p6
    2. The electronic configuration of sodium (Z = 11) is:b. Helium
    3. The spectral lines of hydrogen get closer together at higher frequency because:c. energy levels get closer together at higher n
    4. The configuration of the Cl− ion (Cl Z = 17) is:d. 1s2 2s2 2p6 3s1
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    1 → b (Helium) · 2 → d (1s2 2s2 2p6 3s1) · 3 → c (energy levels get closer together at higher n) · 4 → a (1s2 2s2 2p6 3s2 3p6)

    • Small atom with a full shell.
    • 2 + 2 + 6 + 1 = 11.
    • The gaps between levels decrease.
    • 18 electrons, like argon.

  4. Drag and drop

    Choose the correct word or phrase from the box for each gap. One word in the box is not needed.

    fixed energy levels  ·  5  ·  straight lines  ·  [Ar]3d5
    1. The number of orbitals in a d sub-level is .
    2. The configuration of Fe3+ (Fe Z = 26) is .
    3. In the Bohr model, electrons move in .
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    1: 5 · 2: [Ar]3d5 · 3: fixed energy levels

    • Five d orbitals hold 10 electrons.
    • Remove 4s2 then one 3d electron.
    • Each level has a definite energy.

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