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

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.

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?"

Write the full electron configuration of $\text{Cl}^-$ and identify the noble gas it is isoelectronic with. [2 marks]

First ionisation energies (kJ mol$^{-1}$) across period 3 are: Na $496$, Mg $738$, Al $577$, Si $786$. Explain the dip from Mg to Al. [3 marks]

Consider $\text{F}$ and $\text{I}$. (a) Compare atomic radius. (b) Compare electronegativity.