The correct order of electron gain enthalpies of Cl, F, Te and Po is

Solution

Chemistry ma’am · 4 steps

Rank by how much energy comes out when the electron goes in: halogens on top, and Cl beats F.

1. What is compared. All four elements release energy when they gain an electron, so every value is negative. The order here compares the magnitude, that is, how much energy is released.
2. Group 17 versus 16. A halogen needs just one electron to reach a noble gas configuration, so it releases far more energy than a group 16 element. Both Cl and F therefore come above Te and Po.

   Element	Position	Δ_(eg)H (approx.)	Reason
   Cl	Group 17, period 3	-349 kJ mol⁻¹	3p shell has room, so the added electron feels little repulsion
   F	Group 17, period 2	-328 kJ mol⁻¹	tiny 2p shell is crowded, so repulsion cuts the energy released
   Te	Group 16, period 5	-190 kJ mol⁻¹	large atom, weaker pull on the added electron
   Po	Group 16, period 6	-174 kJ mol⁻¹	even larger, weakest pull

3. Down group 16. Going from Te to Po the atom gets bigger and the nucleus holds the incoming electron less tightly. So Po releases less energy than Te.
4. So. In increasing magnitude the order is Po < Te < F < Cl, which is option (b).

The idea to keep. Electron gain enthalpy becomes less negative down a group, but the period 2 element is the exception: F, O and N gain less than the element below them because their small shells are crowded.

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