Why 2nd electron gain enthalpy of oxygen is positive.if it is positive means we need to give energy to oxygen to put 2nd electron in it .but if we are providing energy the electron inside the oxygen should move to exited state means higher energy state. how we can say that it is positive?
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.