Quick questions on Atomic structure and electronic configuration: Singapore A-Level H2 Chemistry

Iron ($Z = 26$): $1s^2\,2s^2\,2p^6\,3s^2\,3p^6\,3d^6\,4s^2$, often written $[\text{Ar}]\,3d^6\,4s^2$.

The first ionisation energy is the energy needed to remove one mole of electrons from one mole of gaseous atoms:

Successive ionisation energies always increase, because each electron is pulled from an increasingly positive ion. A large jump appears whenever the next electron must come from a shell closer to the nucleus. Counting electrons removed before the big jump gives the number of valence electrons, hence the group.

Across Period 3, first ionisation energy rises overall (nuclear charge increases, same shell), but there are two dips:

Write the full electronic configuration of (a) sulfur and (b) the $\text{Fe}^{3+}$ ion. [2 marks]

Explain why the first ionisation energy of sulfur is lower than that of phosphorus. [3 marks]

The successive ionisation energies of element Q (kJ per mole) are 578, 1817, 2745, 11577, 14842. (a) Deduce the group of Q. (b) Write its electronic configuration.