Ionisation energy — common mistakes

When explaining trends in ionisation energy, always refer to the three key factors: nuclear charge, atomic radius, and shielding. For anomalies, you must be specific about subshells (e.g., 'p-subshell is higher in energy than the s-subshell') or electron repulsion (e.g., 'spin-pair repulsion in a doubly-occupied p-orbital'). Avoid vague statements like 'it is more stable'.

No, they are related but distinct concepts. Shielding refers to the repulsion from inner-shell electrons that 'screens' or 'shields' the outer electrons from the full pull of the nucleus. Effective nuclear charge is the net positive charge experienced by an outer electron after accounting for shielding. You can think of it as: Effective Nuclear Charge ≈ Nuclear Charge - Shielding Effect.

Ionisation energy is a measure of the energy needed to overcome the attraction between the nucleus and an electron within a single, isolated atom. Using the gaseous state ensures that the atoms are far apart and we are not also measuring the energy required to break intermolecular forces (like in a solid or liquid), which would give an inaccurate value for the ionisation energy itself.