The process that is NOT endothermic in nature is :

Solution

Chemistry ma’am · 2 steps

Ask one thing of each process: does the particle actually want this electron change, or must you force it?

1. What decides it. A process releases energy (exothermic) only when the product is more stable than the starting particle. Removing an electron always needs energy. Adding one releases energy only if the atom has room for it and nothing pushes it away.

   Process	What happens	Energy change
   (a) Ar(g) + e⁻ → Ar-(g)	Ar has a full octet, so the electron must go into a new, higher shell	about +96 kJ mol⁻¹, endothermic
   (b) H(g) + e⁻ → H-(g)	The electron fills the 1s orbital and gives the stable 1s² configuration	about -73 kJ mol⁻¹, exothermic
   (c) Na(g) → Na⁺(g) + e⁻	Ionisation: the nucleus still holds the 3s electron, so pulling it off costs energy	about +496 kJ mol⁻¹, endothermic
   (d) O-(g) + e⁻ → O²⁻(g)	The incoming electron is repelled by the negative charge on O⁻	about +780 kJ mol⁻¹, endothermic

2. So. Three of the processes absorb energy. Only (b), adding an electron to a hydrogen atom, releases energy, so it is the one that is not endothermic.

The idea to keep. Ionisation is always endothermic. Adding an electron is endothermic for noble gases and for any anion, and exothermic for most neutral atoms with a vacancy, like H or the halogens.

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