Atom Structure and Periodic Law
The Atom Structure and Periodic Law domain asks you to build electron configurations from quantum numbers and then use them to predict periodic trends instead of memorising lists of numbers.
Rules that carry the marks
- Aufbau order follows n + l: 1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p. Fill 4s before 3d, but list 3d before 4s in the final configuration.
- Pauli: two electrons per orbital, opposite spins. Hund: singly occupy degenerate orbitals before pairing.
- Period = highest principal quantum number. For main-group elements the group number equals the valence-electron count; for d-block elements valence = (n−1)d + ns electrons.
- Half-filled and filled subshells are stabilised: Cr is [Ar]3d⁵4s¹, Cu is [Ar]3d¹⁰4s¹.
- Across a period Z_eff rises, so atomic radius falls and first ionisation energy generally rises; down a group radius rises and ionisation energy falls.
Worked example
Arrange Na, Mg and Al in order of increasing first ionisation energy. Sodium's single 3s electron is the easiest to remove because the effective nuclear charge felt by it is lowest. Aluminium's outermost electron is 3p, higher in energy and shielded by the filled 3s pair, so it leaves more easily than magnesium's 3s electron. Order: Na < Al < Mg.
Traps
- Ionising transition metals: 4s electrons leave first. Fe²⁺ is [Ar]3d⁶, never [Ar]3d⁴4s².
- Reciting 'ionisation energy always increases across a period' and missing the group 2→13 and 15→16 dips.
- Mixing the two radius trends: radius falls across a period but rises down a group.