Question

What is Madelung’s rule?

Answer 1

Hint: This rule tells us the order of the electrons being filled in the increasing order of the energy and if any two atomic orbitals have the same n+l number, then it also helps to find the one which will have higher energy.

Complete answer:
All the elements in nature are differentiated by the number of electrons in them. All the elements are differentiated into blocks in the periodic table according to the orbital in which the last electron enters. So, there is an order of the orbitals in which the electrons are filled.
So, Madelung’s gave the rule as:
The increasing energy of the orbitals is based on the increasing number of n+l and if two orbitals have the same value for n+l value then the one with a lower value of n will have lower energy and it will be filled first, where n is the principal quantum number and l is the Azimuthal quantum number.
For example, the n+l value for 2p and 3s will be:
2p = 2 + 1 = 3
3s = 3 + 0 = 3
Both of them have the same value, so the energy of 2p will be lower because n is lower for 2p.
The n+l value for 3d and 4s will be:
3d = 3 + 2 = 5
4s = 4 + 0 = 4
So, the energy of 3d will be higher than the energy of 4s.
Therefore, according to the energy, the series of orbitals are:
1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p 7s 5f 6d 7p……..

Note:
For finding the value of n+l, the value of n will be given and the value of l is taken 0 for s-orbital, 1 for p-orbital, 2 for d-orbital, and 3 for f-orbital, and so on. We can also call Madelung’s rule as (n+l) rule.