Question

why electron gain enthalpy of noble gases are zero

Answer 1

Electron gain enthalpy can be defined as the enthalpy change (or energy change) accompanying the addition of an electron to the valence shell of an isolated gaseous atom and formation of the corresponding anion. ... Noble gases already have a very stable fully filled electronic configuration.

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Answer 2

Electron gain enthalpy of noble gases is 0 because there outer shell has complete octet. So, no electron is gained.

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