about ionization enthalpy down the group

雖然A-level Chemistry中，有些題目（尤其是Inorganic Chemistry）是由題目決定答案的，但這題不是，是有原因的。

The magnitude of first ionization enthalpy of an element depends mainly on the factors below :
(1) The greater the nuclear charges (the number of protons), the greater the ionization enthalpy.
(2) The greater the screening effect, the lower the ionization enthalpy.
(3) The longer the distance between the nucleus and the outermost electron (the electron to be removed), the lower the ionization enthalpy.
(4) If a structure half-filled subshell is obtained when an electron is removed from the atom, less energy is needed.
(5) When an electron is removed from an atom with a structure of half-filled outermost subshell, more energy is needed.
(6) The greater the repulsion between electrons, the lower the ionization enthalpy. However, this factor is usually negligible in comparing with the above factors.

Compare the ionizations of O(g) and N(g) :
O (1s2 2s2 2p4) → O+ (1s2 2s2 2p3) + e-
N (1s2 2s2 2p6 3s‑2 3p6) → N (1s2 2s2 2p6 3s‑2 3p3) + e-

Firstly, as usual, neglect factor (6).

Secondly, a half-filled np subshell is obtained in each case. Thus, omit factors (4) and (5).

Thirdly, as considering factor (1), N has 8 more nuclear charges than O, but also has 8 more inner electrons in screening effect. The two effects are approximately (not exactly) 1 : 1. Therefore, factors (1) and (2) approximately cancel each other.

Now, you have to consider only factor (3). This will lead the conclusion : The first ionization enthalpy of O is greater than that of S.

I understand your problems.

1 因為sulphur多個electron shell,increase in distance of outermost electron shell,smaller attractive force of outermost electron from nucleus

2 但係sulphur 有多d proton, effective nuclear charge大d,and outweight replusion of electrons
so greater attraction of electrons in the outermost shell from nucleus

Reasons one and two exist together. I would say that the effect of reason 1 outweighs that of reason 2.

So, when answering A-level chemistry, you need to use "reasons favor the answer".

Chemistry is such kind of subject in A-level. 自圓其說