Atomic Structure and Periodicity
Atomic structure questions on this paper pair quantum-number bookkeeping with the periodic trends that follow from effective nuclear charge (Z_eff). You should be able to write configurations for atoms and ions, justify anomalous ionisation energies, and rank species by radius, ionisation energy or electronegativity.
Quantum numbers and orbital capacity
- n gives the shell; l = 0, 1, 2, 3 labels s, p, d, f; m_l runs from -l to +l; m_s = ±½.
- A subshell holds 2(2l + 1) electrons, so s, p, d, f hold 2, 6, 10 and 14.
- Radial nodes = n - l - 1, so a 3d orbital has none and a 3p orbital has one.
Filling order and ions
The Aufbau order is 1s 2s 2p 3s 3p 4s 3d 4p. Two exceptions are examined repeatedly: Cr = [Ar]3d5 4s1 and Cu = [Ar]3d10 4s1, stabilised by half-filled and filled d subshells. The 4s orbital fills before 3d but empties first on ionisation, so Fe ([Ar]3d6 4s2) gives Fe3+ = [Ar]3d5. For p-block anions, electrons go into the outermost p subshell: O2- = [Ne].
Trends and their origin
Across a period Z_eff rises faster than shielding, so radius falls while first ionisation energy, electronegativity and electron affinity generally rise. Down a group the shell number dominates: radius rises and ionisation energy falls. Explain the anomalies rather than memorising them. IE1(Mg) exceeds IE1(Al) because aluminium's 3p electron is shielded by 3s2 and sits higher in energy. IE1(N) exceeds IE1(O) because oxygen's fourth 2p electron must pair, incurring repulsion. Isoelectronic species shrink as nuclear charge grows: N3- > O2- > F- > Na+ > Mg2+.
Worked example
Rank Na, Mg, Al and Si by first ionisation energy. Na (3s1) is lowest. Mg (3s2) is higher. Al (3s2 3p1) drops below Mg for the shielding reason above. Si (3s2 3p2) exceeds Al because the extra proton outweighs the added 3p electron's repulsion. Order: Na < Al < Mg < Si. This is the period-3 version of the general group 2 to group 13 dip.
Traps that cost marks
Losing 4s first when writing transition-metal ion configurations; treating 4s as universally lower in energy, when that only describes the neutral-atom filling order; forgetting the group 15 to 16 dip exists in every period; and quoting ionisation-energy orders without naming the orbital effect that produces them.