Question

Which halogen has the highest value of negative electron gain enthalpy?
A.Fluorine
B.Chlorine
C.Bromine
D.Iodine

Answer 1

Hint: The amount of energy released when an electron is added to a gaseous isolated atom is known as its electron gain enthalpy. This released energy can be either negative or positive depending whether the energy is released or absorbed by the atom.

Complete step by step answer:
As we all know, the energy released when an electron is added to an isolated gaseous atom is known as electron gain enthalpy. The trend of electron gain enthalpy can be easily recognized on the basis of position of element in periodic table.
When we move from left to right in a period the electron gain enthalpy increases gradually because along the period from left to right the number of electrons increases but in the same shell so the nuclear charge increases which results in the increase of electron gain enthalpy.
When we move down along a group we can see that the number of electrons are increasing along with the increase in number of shells which results in increase of atomic size, hence we can see the decrease of electron gain enthalpy along a group.
So the order of electron gain enthalpy of $F,Cl,Br$ and $I$ should be \[F > Cl > Br > I\] .
But this is not the correct answer because due to the small size of fluorine its electron gain enthalpy becomes less than chlorine so the correct order of electron gain enthalpy is $Cl > F > Br > I$.
Hence chlorine has the highest value of negative electron gain enthalpy among all of these.

Note:
 The point of confusion in this problem can be the order of electron gain enthalpy of fluorine and chlorine because due to small size of fluorine its electron gain enthalpy decreases and chlorine comes at the first place in halogens.