Electron Shell

What are Shells?

According to Bohr’s Atomic model electrons revolve around the nucleus in a specific circular path known as orbit or called a shell. Shells have stationary energy levels, the energy of each shell is constant.

Each stationary orbit or shell is associated with a definite amount of energy. The greater the distance of the orbit from the nucleus, the more shall be the energy associated with it. These shells are called energy levels. It is numbered as 1, 2, 3, 4, ……. or K, L, M, N, ….. from the nucleus outwards.

An electron shell may contain only a fixed number of electrons, each shell is associated with a particular range of electron energy, and thus each shell must fill completely before electrons can be added to an outer shell.

Table of Contents

• Characteristic of Shells
• Definition of Subshell
• Number of electrons present in the shell
• The order of energy of the subshell
• Arrangement of electrons in the subshell
• FAQs

Characteristic of Shells

• The principal quantum number is denoted by ‘n’. It represents the name, size and energy of the shell to which the electron belongs. The value of n lies between 1 to ∞.

• Higher the value of ‘n’ the greater the distance of the shell from the nucleus.

r₁ < r₂ < r₃ < r₄ <…….

r = (n²/Z) x 0.529 Å

• Greater the value of ‘n’ higher the magnitude of energy.

E₁ < E₂ < E₃ < E₄ < E₅ ……

• Energy separation between two shells decreases on moving away from the nucleus.

(E₂ – E₁) > (E₃ – E₂) > (E₄ – E₃) > ……..

• The maximum number of electrons present in the shell on the 2n² rule.

• The angular momentum of each shell can be calculated by the formula.

mvr = (nh/2π)

Definition of Subshell

All the electrons having the same shall do not have the same energy. Based on the energy of the electrons, the shells are divided into sublevels or subshells. Each shell is composed of one or more subshells, which are themselves composed of atomic orbitals.

• The subshells are described with the help of Azimuthal quantum numbers (l).
• The value of ‘l’ depends upon the value of the shell (n) with which it is associated.
• These subshells have been designated as s (l = 0), p (l =1), d (l = 2) and f (l =3) .
• The energies of the various subshells in the same shell are in the order s < p < d <f.
• The subshell having equal ‘l’ value but with different n values have similar shape but their size increases as the value of n increases. 2s- subshell is greater in size than 1s- subshell.
• The maximum electrons present in a subshell = 2(2l +1).

Number of electrons present in the shell

• Each shell contains one or more subshells.
• K shell contains only one subshell – 1s
• L shell contains two subshells – 2s, 2p
• M shell contains three subshells – 3s, 3p, 3d
• N shell contains four subshells – 4s, 4p, 4d, 4f

The electrons are arranged in the shell in the following manner:

The order of energy of the subshell

The filling of the shells and subshells with electrons proceeds from subshells of lower energy to subshells of higher energy. This follows the Aufbau principle.

According to this subshells with a lower n + ℓ value are filled before those with higher n + ℓ values. In the case of equal n + ℓ values, the subshell with a lower n value is filled first.

A subshell that has a lower n + ℓ means lower energy. Among 4s and 3d;

4s ; (n + l) = (4 + 0) = 4

3d ; (n + l) = (3 + 2) = 5

4s < 3d (energy)

The energy order of the subshells is given below

Arrangement of electrons in the shell and subshell

The first 20 element electrons are arranged in shell and subshell according to their energy level in the below table.

In order to know more about the electronic configuration of an element, you may visit our website.

Electronic configuration.