Question

The number of unpaired electrons in the $F{e^{3 + }}$ ion (Atomic no. = 26) is
A.5
B.6
C.2
D.8

Answer 1

Hint: To answer this question, you should recall the concept of laws which govern the filling of atomic orbitals. The laws which dictate the filling of electrons in atomic orbitals are Aufbau principle, Hund's Rule of Maximum Multiplicity and Pauli’s Exclusion Principle. The last few electrons of iron are filled in the 3d orbital.

Complete step by step answer:
The electronic configuration of elements is based on majorly 3 rules:
- According to the Pauli’s Exclusion Principle in an atom, no two electrons will have an identical set or the same quantum numbers. There salient rules of Pauli Exclusion Principle are that only two electrons can occupy the same orbital and the two electrons that are present in the same orbital should be having opposite spins.
- According to Hund’s Rule of Maximum Multiplicity rule for a given electronic configuration of an atom, the electron with maximum multiplicity falls lowest in energy.
- According to the Aufbau principle, the electrons will start occupying the orbitals with lower energies before occupying higher energy orbitals. Exceptions of Aufbau Rule. The electron configuration of chromium is \[\left[ {Ar} \right]3{d^5}4{s^1}\] and not \[\left[ {Ar} \right]3{d^4}4{s^2}\] (as suggested by the Aufbau principle). This exception is attributed to several factors such as the increased stability provided by half-filled subshells and the relatively low energy gap between the 3d and the 4s subshells.
Therefore, the electronic configuration \[F{e^{3 +}}:\left[ {Ar} \right]3{d^5}\]
Hence, it has 5 unpaired electrons.

Therefore, we can conclude that the correct answer to this question is option A.

Note:
The main reason for filling of the electron in \[3d\] orbital rather than \[4s\] is due to increased stability of half-filled and fully orbitals are:
1. Symmetrical distribution: Nature loves symmetry as it leads to increased stability and less energy.
2. Exchange energy: The electrons when present in their degenerate orbitals i.e. orbitals with the same energy with parallel spin have shown to exchange their position. The energy released by these exchanges is known as exchange energy. More the exchange of energy and more stability.