The process that is NOT endothermic in nature is:

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Strategy:
 Endothermic processes require energy input (). Exothermic processes release energy (). Neutral atoms gaining one electron is usually exothermic (except for noble gases and stable subshells).

Step 1: Analyze each process
- (1) : Adding an electron to a stable noble gas core requires significant energy. (Endothermic)
- (2) : Hydrogen () gains an electron to form a stable  hydride ion, releasing energy (). (Exothermic)
- (3) : Gaining a second electron always requires energy to overcome

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