Why does Platinum has Exceptional Electronic Configuration?

Platinum’s unusual electron arrangement is due to Stability. When the electrons fill the f orbitals and starts moving to the next level orbitals, two main factors come into play –

1. Energy of higher adjacent d and s orbitals becomes almost equal.

2. The attraction of the nucleus over the outer shell electrons becomes weaker due to the shielding effect. That is the electrons in the inner shell are more attracted by the nucleus and can’t pull the outside ones as much. As a result, outer shell electrons in orbitals have more chance of repelling themselves. Which reduces stability. Hence electrons in the outer shell give more electronic stability when they are single (similar to Hund’s principle).

The d orbitals are dxy, dyz, dzx, dx²-y², dz². A total of five orbitals. The energies of 5dz2 and 6s are almost equal in the case of platinum. 6s can be called the part of 5d in terms of energy.

If the electronic configuration is 5d¹⁰ 6s⁰ then: 5dxy², 5dyz², 5dzx², 5d(x²-y²)², 5d(z²)², 6s⁰. In this case the 6s orbital of nearly equal energy of dz² becomes empty which has more electrons to repel each other.

But if the electronic configuration is 5d⁹ 6s¹ then: 5dxy², 5dyz², 5dzx², 5d(x²-y²)², 5d(z²)¹, 6s¹. In this case the 6s orbital of nearly equal energy of 5dz² will be half filled which will stabilize the platinum because it has less electrons to repel each other.

In this case of platinum, half filled orbitals give more stability than empty orbitals. That is why the electron arrangement of platinum is abnormal.