Q.Elements of group 16 generally show lower values of first ionisation enthalpy compared to the elements of corresponding period of group 15. Why ?

Group 15 elements have an exactly half-filled np$^3$ valence subshell -- an extra-stable arrangement that resists losing an electron -- while group 16 elements (np$^4$) lack this extra stability.

Step 1. Valence configurations: group 15 = ns$^2$np$^3$ (each p-orbital singly occupied), group 16 = ns$^2$np$^4$ (one p-orbital doubly occupied).

Step 2. An exactly half-filled p-subshell has extra stability (symmetrical distribution and maximum exchange energy), so removing an electron from a group 15 atom must break this stable arrangement and costs more energy.

Step 3. In a group 16 atom the fourth p-electron is paired in one orbital; inter-electronic repulsion within that pair makes it comparatively easier to remove, and its removal actually PRODUCES the stable half-filled np$^3$ configuration.

Comparing the valence configurations ns2np3 (group 15) and ns2np4 (group 16): an exactly half-filled p-subshell is extra stable, so ionising a group 15 atom costs more energy.

• Expecting ionisation enthalpy to simply rise left-to-right and concluding group 16 must be HIGHER than group 15 -- the half-filled-subshell stability of np3 reverses this one step.
• Crediting the difference to atomic size: within one period the size difference between neighbouring groups is small; the dominant factor is the np3 extra stability.
• Writing the configurations as np3/np4 without noting WHICH one is half-filled.