Atomic Structure and Periodicity
What this domain tests
Atomic structure and periodicity items on the UEE ask you to move fluently between electron configuration, quantum numbers and the periodic trends that follow from them: atomic and ionic radius, ionisation energy, electron affinity, electronegativity and metallic character. Marks are usually lost on the reason, not the direction of the trend.
Core rules
- Effective nuclear charge: Z_eff = Z − S. Slater's rules: for an ns/np electron, every electron in shell n−1 or lower shields 1.00; every other electron in the same shell shields 0.35; electrons in higher shells shield nothing.
- Fill by the aufbau (n + l) order, Pauli exclusion and Hund's rule. Cr is [Ar]3d⁵4s¹ and Cu is [Ar]3d¹⁰4s¹ because half-filled and filled d subshells gain exchange stability.
- 4s fills before 3d, but 3d electrons are removed first on ionisation: Fe²⁺ is [Ar]3d⁶, not [Ar]3d⁴4s².
- Across a period Z_eff rises, so radius falls while ionisation energy, electron affinity and electronegativity rise. Down a group n rises and Z_eff barely changes, so radius rises and ionisation energy falls.
- Successive ionisation energies jump sharply when a new shell is breached; this locates the group number.
Worked example
Estimate Z_eff for a 3s electron of sodium. Na has 11 electrons: 1s²2s²2p⁶3s¹. The ten electrons in shells 1 and 2 shield 1.00 each, so S = 10 and Z_eff = 11 − 10 = 1. A valence electron of neon (same core, one proton more) has Z_eff = 2, consistent with sodium's much lower first ionisation energy (496 vs 2081 kJ mol⁻¹).
Traps that cost marks
- Ionisation energy dips at Group 13 (ns² → np¹, the p electron is higher in energy and well shielded) and at Group 16 (np³ → np⁴, added pairing repulsion).
- For an isoelectronic series, radius falls as nuclear charge rises: N³⁻ > O²⁻ > F⁻ > Na⁺ > Mg²⁺.
- Electron affinity of nitrogen is close to zero (positive in some data) because the half-filled 2p³ subshell is already stable.
- Do not write [Ar]4s²3d⁸ for Ni²⁺; write [Ar]3d⁸.