Question

How do electron configurations in the same group compare?

Answer 1

To determine the answer to this question we should know what group and electronic configuration is, how we write the electronic configuration. The arrangement of electrons in orbitals in increasing order gives the electronic configuration. We will take some examples of electronic configuration of the elements of a group then we will compare the configuration.

Complete solution:
The electronic configuration of group-I, alkali metals are as follows:
Li = ${\text{1}}{{\text{s}}^{\text{2}}}{\text{2}}{{\text{s}}^{\text{1}}}$
Na = ${\text{1}}{{\text{s}}^{\text{2}}}{\text{2}}{{\text{s}}^{\text{2}}}{\text{2}}{{\text{p}}^{\text{6}}}{\text{3}}{{\text{s}}^{\text{1}}}$
K = ${\text{1}}{{\text{s}}^{\text{2}}}{\text{2}}{{\text{s}}^{\text{2}}}{\text{2}}{{\text{p}}^{\text{6}}}{\text{3}}{{\text{s}}^{\text{2}}}{\text{3}}{{\text{p}}^6}{\text{4}}{{\text{s}}^1}$
The above given metals are of the same group. If we compare the outermost shell electronic configuration of the above elements of the same group we find that on going downward, the principal quantum number of the outermost shell increasing but the number of electrons in the outermost shell remains the same one.
Let’s, take the electronic configuration of group-$18$ as follows:
He = ${\text{1}}{{\text{s}}^{\text{2}}}$
Ne = ${\text{1}}{{\text{s}}^{\text{2}}}{\text{2}}{{\text{s}}^{\text{2}}}{\text{2}}{{\text{p}}^{\text{6}}}$
Ar = ${\text{1}}{{\text{s}}^{\text{2}}}{\text{2}}{{\text{s}}^{\text{2}}}{\text{2}}{{\text{p}}^{\text{6}}}{\text{3}}{{\text{s}}^{\text{2}}}{\text{3}}{{\text{p}}^{\text{6}}}$
Kr = ${\text{1}}{{\text{s}}^{\text{2}}}{\text{2}}{{\text{s}}^{\text{2}}}{\text{2}}{{\text{p}}^{\text{6}}}{\text{3}}{{\text{s}}^{\text{2}}}{\text{3}}{{\text{p}}^{\text{6}}}{\text{3}}{{\text{d}}^{10}}\,{\text{4}}{{\text{s}}^2}{\text{4}}{{\text{p}}^{\text{6}}}$
On comparing the outermost shell electronic configuration of the above elements of the same group we find that on going downward, the principal quantum number of the outermost shell increases but the number of electrons in the outermost shell remains the same six.
Therefore, electron configurations in the same group have the same number of electrons in the outermost shell.

Note: The outermost shell is known as valence shell. As we go down in a group, the principal quantum number of the outermost shell increases. Due to the same valence electrons the elements of a group show various similar chemical properties. Even the helium does not have ${{\text{p}}^{\text{6}}}$ outermost electronic configuration but helium has a fully-filled electronic configuration like neon, so it is placed in the group of neon.