Question

The electronic configuration of gadolinium (atomic number 64) is?
a.) $\left[ {Xe} \right]4{f^3}5{d^5}6{s^2}$
b.) $\left[ {Xe} \right]4{f^7}5{d^2}6{s^1}$
c.) $\left[ {Xe} \right]4{f^7}5{d^1}6{s^2}$
d.) $\left[ {Xe} \right]4{f^8}5{d^6}6{s^2}$

Answer 1

Hint: The problem of electronic configuration is simple, we have to put the electrons in the different orbitals of the atom sequentially. But sometimes some exceptional cases also occur where the original configuration of some atoms are observed differently from the expected configuration. Here in this case of gadolinium also it has exceptional configuration due to the stability factor which is discussed below.

Complete step by step solution:
The filling of electrons in orbitals is done using Hund’s rule of maximum multiplicity and Aufbau principle.
According to Hund’s rule every orbital in a sublevel is first singly occupied before an orbital is doubly occupied and Aufbau states that lower orbitals are filled before higher orbitals.
The atomic number of the gadolinium is 64
The expected electronic configuration of the gadolinium is $\left[ {Xe} \right]4{f^8}6{s^2}$
The observed electronic configuration of the gadolinium is $\left[ {Xe} \right]4{f^7}5{d^1}6{s^2}$
The reason behind this exception is that the half-filled orbital is more stable than partially filled orbital and in case of gadolinium expected configuration f orbital has 8 electrons i.e. is partially filled and with the loss of only one electron the f orbital will become half-filled and become more stable than partially filled. The other thing is that there is not much difference between the energy levels of the 4f and 5d orbital. Therefore the electron from 4f orbital jumps from to 5d orbital to make the compound more stable

Hence, the correct option is C.

Note: In order to solve these types of questions, you need to have a concept of general electronic configurations and energy levels of orbitals. After this the electrons or spins are filled in the sublevels of the orbitals using the Hund’s Rule and Aufbau principle. Also, remember the atomic number of the elements and remember the exceptional electronic configuration of elements.