Quick questions on Electron configuration and periodic trends (QCE Chemistry Unit 1)

The capacity of shell n is $2n^2$ electrons. Shell 1: 2 (1s). Shell 2: 8 (2s, 2p). Shell 3: 18 (3s, 3p, 3d).

QCE Chemistry expects you to use the standard aufbau order. For Z up to 36 (krypton), the order is:

For main-group ions, electrons are added or removed from the outermost subshell.

Effective nuclear charge (Z_eff) is the net positive charge experienced by an outer electron, after accounting for shielding (also called screening) by inner electrons.

Atomic radius is the typical distance from the nucleus to the outermost occupied shell.

First ionisation energy (IE_1) is the energy required to remove the most loosely held electron from a gaseous atom.

Electronegativity is the tendency of an atom in a bond to attract bonding electrons toward itself. Pauling scale; fluorine is set at 4.0, the most electronegative element. Caesium is around 0.7.

A single line of reasoning (Z_eff plus shell structure) drives all three. QCAA EA short-response questions often ask you to apply that reasoning to a specific pair, e.g. "Why is the first ionisation energy of Mg higher than that of Na?" or "Why is F more electronegative than Cl?"

Aufbau order has 4s first for the neutral atom. Reverse only when forming cations of transition metals.

Fe^2+ is 3d^6, not 4s^2 3d^4.

Inner-shell shielding is essentially unchanged across a period; only the proton count is changing meaningfully.

They share a single underlying cause (Z_eff and shell structure). State that cause once and apply it.

Electron affinity is the energy released when a neutral gas-phase atom gains an electron (a measurable energy). Electronegativity is a dimensionless tendency in a bond. They correlate but are not the same.