Question

The electronic configuration of ${}_{59}Pr$(Praseodymium) is-
A.$\left[{}_{54}\text{Xe}\right]4f^{2}5d^{1}6s^2$
B.$\left[{}_{54}\text{Xe}\right]4f^{1}5d^{2}6s^2$
C.$\left[{}_{54}\text{Xe}\right]4f^{3}6s^2$
D.$\left[{}_{54}\text{Xe}\right]4f^{3}5d^2$

Answer 1

Hint: The element Praseodymium has atomic number $59$ .It is an f-block element and comes under lanthanides. The maximum number of electrons that can be filled in s-orbital is $2$, p-orbital is $6$, d-orbital is $10$ and f-orbital is $14$. The general electronic configuration is-$\left(n-2\right)f^{1-14}\left(n-1\right)d^{0-1}n{s}^2$.

Complete step by step answer:
We have to find the electronic configuration of Praseodymium. Its atomic number is $59$.
According to the Aufbau principle,
The electrons first occupy the orbitals whose energy is lowest. The electrons enter higher energy orbitals only when lower energy orbitals are completely filled.
The order of increase in energy of orbitals is determined by the (n+l) rule where n is the principal quantum number and l is the azimuthal quantum number.
If two orbitals have equal (n+l) value then electrons will first enter the orbital having a lower n value.
The order of filling of electrons in orbitals is-$1s,2s,2p,3s,3p,4s,3d,4p,5s,4d,5p,6s,4f,5d,6p$ and so on.
We know the nearest noble gas configuration closest to Praseodymium is Xenon with atomic number $54$. So we will use its abbreviation to indicate the electronic configuration till number $54$. Now we know the general configuration of f-block elements is-$\left(n-2\right)f^{1-14}\left(n-1\right)d^{0-1}n{s}^2$.
According to this, On putting n=$6$ we get the ideal configuration should be-$\left[{}_{54}\text{Xe}\right]4f^{2}5d^{1}6s^2$ . But the observed electronic configuration is $\left[{}_{54}\text{Xe}\right]4f^{3}6s^2$ . It is because the energies of $4f$ and $5d$ are very close so electrons can exchange their positions.

The correct answer is option C.

Note:
1.Pr is silvery soft metal which tarnishes in air rapidly. Its ions are coloured. Praseodymium is used as following-
2.Its oxides are used to make lenses of goggles for glass makers as it filters out the yellow light which is produced in glass bowing.
3.It forms alloy with magnesium which is used in aircraft engines.