The element having the highest first ionization enthalpy is

Solution

Chemistry ma’am · 3 steps

Ionisation enthalpy rises up a group and across a period, and nitrogen is highest and furthest right here.

1. The two trends. Across a period, nuclear charge rises while the size shrinks, so the outer electron is held more tightly. Down a group, the outer electron sits farther from the nucleus and is easier to remove.
2. Place the elements. C and N are in period 2. Al and Si are in period 3, one row lower, so they start with lower values. Of the period 2 pair, N is one place to the right of C.

   Element	Position	Outer configuration	First IE (kJ mol⁻¹)
   Al	period 3, group 13	3s² 3p¹	577
   Si	period 3, group 14	3s² 3p²	786
   C	period 2, group 14	2s² 2p²	1086
   N	period 2, group 15	2s² 2p³, half-filled	1402

3. So. Nitrogen is small, has the highest nuclear charge among these small atoms, and has an extra-stable half-filled 2p³ subshell. It has the highest first ionisation enthalpy, option (d).

The idea to keep. For ionisation enthalpy, look up and to the right of the periodic table first. Then check for a half-filled or fully filled subshell, which adds extra stability.

Answer from the NTA final answer key. Solution by PrepWiser.