ZIMSEC A Level · Chemistry · Form 5
Sub-shells and orbitals
Electrons occupy s, p, d and f sub-shells. An orbital holds up to two electrons of opposite spin. Order of filling: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p.
- s orbitals are spherical; p orbitals are dumb-bell shaped.
- Exceptions: Cr is [Ar]3d⁵4s¹ and Cu is [Ar]3d¹⁰4s¹. Transition metal ions lose 4s electrons before 3d (Fe²⁺ = [Ar]3d⁶).
Worked example: Write the full electron configuration of iron (Z = 26) and Fe³⁺.
Fe: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶ 4s²; Fe³⁺: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁵
Ionisation energy
The first ionisation energy is the energy needed to remove one mole of electrons from one mole of gaseous atoms: X(g) → X⁺(g) + e⁻.
Factors: nuclear charge, distance of the electron from the nucleus, shielding by inner electrons, and spin-pair repulsion.
- It generally increases across a period (more protons, same shielding) and decreases down a group (more shells).
- Dips: from Be to B (2p electron is higher in energy) and from N to O (paired electrons repel in the 2p sub-shell).
- Large jumps in successive ionisation energies show a new, inner shell – they reveal the group number.
Mass spectrometry
Gives isotopic masses and abundances, from which relative atomic mass is calculated.
Exam tip: Write ionisation equations with state symbols – (g) is essential.
Check yourself
- Write the electron configuration of Cu⁺ (Z = 29).
- Why is the first ionisation energy of oxygen lower than that of nitrogen?
- An element has successive ionisation energies with a large jump between the 3rd and 4th. Which group is it in?
Show answers
- 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰
- Oxygen's fourth 2p electron is paired, and repulsion between paired electrons makes it easier to remove.
- Group 13 (Group III).
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