how to determine when will electron gain enthalpy gets negative or positive for example for first electron gain enthalpy of oxygen is either negative or positive and for second electron gain enthalpy of oxygen is either positive or negative?

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Solution

Electron Gain Enthalpy or Electron Affinity is the amount of energy released when an isolated, neutral, gaseous atom takes up an extra electron to form uninegative gaseous ions.

X(g) + e⁻ ----→ X⁻ ; Δ H = Δ _eg.

Negative value of electron gain enthalpy denotes that energy is given out and and the atom readily accepts the electron.
A positive value on the other hand denotes that energy is absorbed when an electron is gained by an atom.

Usually the second electron gain enthalpy is positive as the atom has already accepted an electron (i.e. formed and anion) and is negatively charged leading to repulsion between the anion and electron leading to higher energy requirement and resulting into positive electron gain enthalpy.

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