Question

The electron affinity of Bromine is a negative quantity while it is a positive quantity for Krypton, using the electron configurations of the two elements, how would you explain the difference?

Answer 1

Electron affinity if the energy required to add an electron in the outermost orbital of an atom, in its isolated gaseous state. The electron affinity is also known as the electron gain enthalpy represented by the short-form E.A.

Complete answer:
First, we have to understand what a negative electron affinity is. Consider any halogen ‘X’. The ionization of X can be shown as:
$X + {e^ - } \to {X^ - } + energy$
When an electron is added in the outermost orbital of the halogen X energy is released. The reaction is exothermic When energy is released, the quantity is said to be negative, the same as the exothermic reaction having a negative value of $\Delta H$. When the electron affinity is said to be negative, it means that adding an electron releases energy.
So, if a reaction releases energy, we can say that the electron affinity is a positive value i.e. ${E_a} = - \Delta E$
Now, if the value of electron affinity is negative, it means that the reaction absorbs energy from the surrounding or it requires energy.
According to the sign convection, the electron affinity of bromine is actually positive as energy is released on adding an electron to the outermost shell. On the other hand, adding an electron to Krypton requires supply of energy, hence the E.A is negative. This can be related to the electronic configuration of both the elements.
$Br:[Ar]3{d^{10}}4{s^2}4{p^5}$
$Kr:[Ar]3{d^{10}}4{s^2}4{p^6}$
The outermost shell of both the elements is in the fourth shell. This shell is completely occupied in Krypton. In simple words, we can say that Kr has a complete octet configuration, which is the stable electronic configuration. But, Br on the other hand incompletely filled 4p orbital. It is short of one electron to gain the stable electronic configuration. Hence Br will immediately accept one electron and acquire the stable configuration. For this process, a large amount of energy is released to accept the incoming electron.
In Krypton’s case, it will be difficult to let go of the stable configuration and accept the incoming electron. Hence, a large amount of energy has to be supplied in order to add the electron in the next shell 5s, making the atom extremely unstable.

Note:
The electron affinity of Bromine was found to be negative because of the energy being absorbed. It was found to be -96 kJ/mol. Which is a huge amount. Remember that hindering the stable electronic configuration in any atom will require a high supply of energy.