Which element has the highest negative electron gain enthalpy value?

Hint: Electron gain enthalpy defines the amount of released energy on adding an electron to a gaseous isolated atom. On the basis of energy absorbed or released, the value of electron gain enthalpy can be positive or negative.

Complete Step by Step Solution:
In the periodic table, an element's position decides its electron gain enthalpy. On moving across a period, the count of electrons increases but the count of shells remains the same. So, there is an increase in nuclear size. This results in the increase of electron gain enthalpy across a period.

 Along a group, on moving down we find the increase in the number of electrons because of the increase in the count of shells that ultimately results in the atomic size increase. Therefore, we see a decrease of electron gain enthalpy.

So, the 17^th group elements have the higher electron gain enthalpy among all the elements. Let's find out which halogen has the highest negative electron gain enthalpy. From the above discussion, the order of electron gain enthalpy is, Fluorine > Chlorine > Bromine > Iodine.

But, the above order is not correct because of the smaller size of fluorine. Therefore, its electron gain enthalpy is less than chlorine. So, the order of halogens is Cl>F>Br>I
Hence, chlorine has the highest electron gain enthalpy.

Note: It is to be noted that students might get confused between fluorine and chlorine while deciding the highest electron gain enthalpy. Fluorine due to its small size possesses less electron gain enthalpy than chlorine.