Question

Which of the following has the most negative electron gain enthalpy?
A.$Br$
B.$Sn$
C.$Ca$
D.$Mg$

Answer 1

We need to know that during the addition of an electron to an element, there are two possibilities of energies. The two possibilities of energies are either the energy is given by an element or the energy is absorbed by an element. This is referred to as electron gain enthalpy. It is measured in kilo joule per mole $(kJ/mol)$ or electron volts per atom.

Complete answer:
We have to remember that when an electron is added to an atom, if the energy is released by an element, therefore the electron gain enthalpy of that element is negative. If we supply an energy to an element to make the electron go inside, then there will be positive electron gain enthalpy.
Electron gain enthalpy of metals are always a positive value because they are not readily accept electrons and also higher nuclear charge and smaller size will have somewhat less positive electron gain enthalpy.
We must remember that the halogens have highly electron gain enthalpy values, because, if halogens accept an electron they can acquire the nearest noble gas configuration.
The given elements are $Br$, $Sn$,$Ca$ and $Mg$. Among these four elements $Sn$,$Ca$ and $Mg$ are metals and $Br$ is non-metal. If we add an electron to these elements, bromine accepts electron first and the outermost electronic configuration is $4{s^2}{p^5}$, it accepts electron first and acquires noble gas configuration. So the stability will be more. And it will remove maximum energy and the electron gain enthalpy is most negative as compared to $Sn$,$Ca$ and $Mg$.
Therefore, the option (A) is the correct answer.

Note:
We must remember the electron gain enthalpy and electron affinity is not the same. When an electron is added to a gaseous atom or ion the amount of energy is released is defined as electron affinity. If the atom has more positive electron affinity value then the atom has more negative electron gain enthalpy value and vice versa.