The electron gain enthalpy values of halogens become more negative in the order F>Cl>Br>I.

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The electron gain enthalpy is defined as the energy released when an electron is added to a neutral gaseous atom to convert it into a negative ion. Large negative values of electron gain enthalpy indicate that it is easier to add an electron to an atom to convert it into an anion, and hence large amount of energy is released. The electron gain enthalpies in halogens decreases in the order

   Cl>F>Br>I

because as we proceed down a group because the size of the atom increases. Therefore the distance of the nucleus from the valence orbital (which accepts the additional electron) increases. Hence the force by which the additional electron is attracted by the nucleus also decreases and the electron gain enthalpies become less negative as we proceed down a group.

However the electron gain enthalpy of F is less negative than of Cl because of its small size. The additional electron in case of F will be added to the 2p orbital while in case of Cl it will go in the 3p orbital. As a 2p orbital occupies a smaller region of space than 3p orbital, it will be more compact because of which electron-electron repulsions would be more. Therefore the incoming electron would be accepted with much less ease in case of F. Hence its electron gain enthalpy is less negative than Cl.

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Hi Javeed.

Since electron gain enthalpy corresponds to the amount of energy liberated when an electron is added to the valence shell of a neutral isolated gaseous atom therefore its value is taken as negative.

In a system whenever there is loss of energy ,the change in energy is taken negative.

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