How would you account for the irregular variation of ionisationenthalpies (first and second) in the first series

The irregular variation of ionisation enthalpies in the first series can be accounted for by considering the following factors:

1. Atomic Size: The size of an atom is a crucial factor in determining its ionisation enthalpy. Smaller atoms have higher ionisation enthalpies because the electrons are closer to the nucleus and thus more strongly attracted to it. In the first series, the atomic size generally decreases from left to right, leading to an increase in ionisation enthalpy.

2. Nuclear Charge: The number of protons in an atom's nucleus (its nuclear charge) also affects its ionisation enthalpy. Atoms with more protons have a stronger pull on their electrons, making it harder to remove an electron and thus increasing the ionisation enthalpy. In the first series, the nuclear charge increases from left to right, which also contributes to the increase in ionisation enthalpy.

3. Electron Configuration: The arrangement of electrons in an atom can also affect its ionisation enthalpy. Atoms with fully filled or half-filled electron shells have extra stability, which makes it harder to remove an electron and thus increases the ionisation enthalpy. This can lead to irregularities in the trend of ionisation enthalpies in the first series.

4. Electron Shielding: The inner electrons of an atom can shield the outer electrons from the pull of the nucleus, which can decrease the ionisation enthalpy. However, in the first series, the shielding effect is generally constant, so it does not contribute significantly to the irregular variation of ionisation enthalpies.

5. Penetration Effect: The ability of an electron to penetrate towards the nucleus is known as the penetration effect. Electrons in s-orbital can penetrate more towards the nucleus than p, d, and f orbitals. Hence, the ionisation enthalpy of s-block elements is more than that of p, d, and f block elements.

In summary, the irregular variation of ionisation enthalpies in the first series can be explained by a combination of atomic size, nuclear charge, electron configuration, and the penetration effect.