why the second electron gain enthalpy of an atom is always positive?

1.	why the second electron gain enthalpy of an atom is always positive?
Answer» When an electron is added to an isolated oxygen atom, it becomes uninegative ion. Now if one more electron has to be added, then it will experience a repulsive force or columbic force of repulsion. Hence more energy has to be supplied for the addition of an electron to this uninegative ion. Therefore , the second electron gain enthalpy is positive for oxygen.

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