Atomic Structure and the Periodic Law
What you must know
Candidates should be fluent with the nuclear model: protons and neutrons in the nucleus, electrons in orbitals described by the quantum numbers n, l, m and spin. Know the order of filling (Aufbau), the n+l rule, Hund's rule and the Pauli exclusion principle, and be able to write electron configurations including the anomalous cases Cr and Cu. Understand periodic law as the recurrence of properties with increasing atomic number, and connect position to configuration: group number from valence electrons, period from the highest principal quantum number.
Traps
- Confusing mass number with relative atomic mass (weighted average of isotopes).
- Writing the 4s orbital after 3d when ionising: 4s electrons are removed first in transition-metal cations.
- Forgetting that ionisation energy generally increases across a period but dips at group 13 (s–p switch) and group 16 (pairing repulsion).
- Treating atomic radius as increasing across a period (it decreases).
Worked example
Configuration of Fe (Z = 26): fill 1s²2s²2p⁶3s²3p⁶4s²3d⁶. For Fe²⁺, remove the two 4s electrons first, giving 3d⁶ - not 4s²3d⁴. Rule of thumb: build up in n+l order, but empty the outermost shell first when forming cations.