Atomic Structure and Periodicity
What this domain tests
Atomic structure and periodicity questions ask you to move between electron configuration, quantum numbers and measurable periodic trends - ionisation energy, atomic and ionic radius, electronegativity and melting point.
Core rules
- Fill orbitals in Aufbau order but remember the 4s/3d crossover: 4s fills first, yet 3d electrons are lost first when a transition metal ionises. Fe is [Ar]3d6 4s2 but Fe2+ is [Ar]3d6.
- Cr and Cu are exceptions: [Ar]3d5 4s1 and [Ar]3d10 4s1, because half-filled and full d subshells are more stable.
- First ionisation energy depends on nuclear charge, shielding, distance, and subshell penetration (s > p > d).
- Two characteristic dips: Al (3p1 is higher in energy and better shielded than 3s) and S (3p4 has one paired electron, giving extra repulsion).
- Across a period, radius falls and electronegativity rises; down a group the reverse. In an isoelectronic series (Na+, Mg2+, Al3+, F−, O2−) radius falls as nuclear charge rises.
Worked example
Successive ionisation energies of element X are 590, 1150, 4940 and 6480 kJ mol−1. The large jump between the second and third values means the third electron comes from an inner shell, so X has two valence electrons and sits in Group 2 (the values fit calcium). Within one shell the effective nuclear charge rises only gradually; once a shell is emptied, the next electron is much closer to the nucleus.
Traps
Writing 3d before 4s in ion configurations; claiming radius increases across a period; confusing electronegativity with electron affinity; forgetting that d-electron shielding makes Ga smaller than a simple trend predicts.