# Why does while writing the electronic configuration for elements with atomic number more than 20,3d orbital is filled first than 4s orbital?Is it correct to write electronic configuration of...

Why does while writing the electronic configuration for elements with atomic number more than 20,3d orbital is filled first than 4s orbital?

Is it correct to write electronic configuration of scandium as [AR]4s23d1 or as[AR]3d14s2

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.

In case of scandium

Atomic number = 21

Electronic configuration =

1S2 2S2 2P6 3S2 3p6 4S2 3d1 [or]

[Ar] 4S2 3d1

This is correct way of writing the electronic configuration.

The filling of electron will take place first in 4S followed by 3d since 4S has low energy than 3d.

Filling of electrons start will low energy level orbitals followed by higher energy orbitals.

NOTE :- Please go  through the link to see the patern of filling of orbitals.

kavya--kammana | Student

( some important points are given below )

1. Elements b/w s block and p block are called d block elements.
• they found a transition b/w s block of metals to p block of non metals.
• so they are called transition elements
• in transition elements electrons are first filled in in outer s subshell and then penultimate d subshell iis filled.
• thats why outer s subshell has same no. of electrons
• these elements exhibit horizontal similarity.

eg. Sc. (21) 1s2  2s2 2p6 3s2 3p6 4s2 3d1

Mn (25) 1s2  2s2  2p6 3s2 3p6 4s2 3d5

Fe  (26) 1s2  2s2  2p6  3s2 3p6 4s2 3d6

subshell electronic configuration of elements atomic number  more than 20 only have  d subshell

eg   K ( 19) 1s2 2s2 2p6 3s2 3p6 4s1