Question

Which of the following is true about electrons in cobalt?
(A) The last orbital to gain electrons as well as the first orbital to lose electrons is 4s
(B) The last orbital to gain electrons as well as the first orbital to lose electrons is 3d
(C) The last orbital to gain electrons is 3d while the first orbital to lose electrons is 4s
(D) The last orbital to gain electrons is 4s while the first orbital to lose electrons is 3d

Answer 1

Hint: In case of transition elements due to the presence of electrons at d orbitals, which is closer to the outermost shell of the metal. They show a variable oxidation state. By increasing the number of electrons of the d orbitals (up to 5 electrons).

Complete step by step answer:
In the case of cobalt, the valance sells electronic configuration is, \[\left[ {{\text{Ar}}} \right]{\text{3}}{{\text{d}}^7}{\text{4}}{{\text{s}}^{\text{2}}}\].
Now according to the Aufbau principle electrons occupy the lower energy level before occupying the higher energy level. For example, the 2s shell is filled before the 2p shell. According to the Aufbau principle the energy of the orbitals depends upon the (L+S) value. Higher the value of (L+S) higher will be the energy of the orbital and vice-versa.
As the (L+S) value of 3d is greater than 4s, the energy of 3d is greater than 4s. Due to his reason, 3d is the last orbital to gain electrons. And as 4s is the farthest orbital from the nucleus that is why the first orbital to lose electrons is 4s.

So, the correct option is A.

Note: Transition elements show a magnetic moment due to the presence of unpaired electrons to their d orbitals. With increasing the number of unpaired electrons, the spin magnetic moment value increases and vice-versa. . If there is no unpaired electron then it is diamagnetic.