Question

Atomic number of an element is 42. Find out the number of electrons present in the valence shell.
A.1
B.3
C.2
D.4

Answer 1

Hint: Write the electronic configuration of the element according to aufbau principle in mind. After writing the configuration identify the number of electrons in the outermost energy level. That will give us the number of valence electrons.

Complete answer:
In order to identify the number of valence electron, first of all we will write the electronic configuration of the element .The expected electronic configuration of the element will be either of the two-
$
  1{s^2}2{s^2}2{p^6}3{s^2}3{p^6}3{d^{10}}4{s^2}4{p^6}4{d^6} \\
  1{s^2}2{s^2}2{p^6}3{s^2}3{p^6}3{d^{10}}4{s^2}4{p^6}4{d^5}5{s^1} \\
 $
In the second configuration the $d$ orbital is half –filled. The half-filled and completely filled orbitals possess comparatively lower energy i.e. they are more stable. This is because , half-filled and completely filled orbitals are symmetrical . And it is well known that symmetry leads to stability . Electrons in the same orbitals possess equal energies but different spatial distributions . Their shielding effect is decreased and they are tightly bound by the nucleus. In order to achieve this symmetrical configuration one or more electrons shift from lower energy orbitals to higher energy orbitals . This brings stability to the element.
And hence , the correct electronic configuration is –
$1{s^2}2{s^2}2{p^6}3{s^2}3{p^6}3{d^{10}}4{s^2}4{p^6}4{d^5}5{s^1}$ .
The element is Molybdenum, with the configuration : $[Kr]4{d^5}5{s^1}$
Now , we will count the number of electrons in the highest energy level. The highest energy level here is that of $5s$ subshell and it has one valence electron.

So, The correct answer is (A).

Note: The electronic configuration of each element plays an important role in determining its physical and chemical properties. It also gives us the idea of bonding ability , valency, magnetism and various other important properties. In a periodic table ,elements are divided on the basis of their electronic configuration.