what is positive nd negative electron gain enthalpy

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 a positive value denotes that energy is absorbed when an electron is gained by an atom. 

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First electron gain enthalpy need not be always negative - but it is mostly negative as the protons in the nucleus exert a powerful charge on the electrons and bind them to the atom; so an addition of an electron lets the proton hold on to another electron and hence the strain on the other electrons is reduced. So in general it is negative. After addition of one electron to the atom, it becomes negatively charged and the second electron is added to a negatively charged ion. The addition of second electron is opposed by the columbic force of repulsion and energy has to be supplied for the addition of the second electron .Consequently, its second electron gain enthalpy is positive.
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