# Why does 5d1 fill before 4f in lanthanum z=57?

sanjeetmanna | Certified Educator

We have to follow three principles while writing electronic configuration:
1) Aufbau principle,
2) Pauli exclusion principle,
3) Hund's rule

• Aufbau principle states that filling of orbitals should take place by starting from lowest energy level.
• Pauli exclusion principle states that no two electron in a orbital will have same quantum numbers.
• Hund's rule states that all sub orbitals in a orbitals are first filled with one electron and then pairing of electron takes place.

lanthanum z=57

Electronic configuration = `[Xe]5d^1 6s^2`

This is the exception which do not follow the rule.

The expected configuration is [Xe]6s^2 4f^1

The 4f sub shell and 5d sub shell has almost same energy level so it is tought to tell the diference.

Lanthanides has a special feature known as Lanthanide contraction.

This Lanthanide contraction explains why 5s and 5p orbitals Penetrate the 4f sub-shell and filling of electron takes place in 5d sub shells instead of in 4f subshell.