# What is Electron Configuration and Why it’s Important?

The smallest part of an element is called an atom. There are mainly three particles located in this atom. The particles are electrons, protons, and neutrons. Protons and neutrons reside in the center of the atom.

That is, protons and neutrons reside in the nucleus of an atom. But, electrons occupy different orbits at different distances outside the nucleus.

**The position of these electrons in different orbitals of the atom according to certain rules is called electron configuration. In the stable state, the atomic number, number of protons, and number of electrons are the same.**

## How to do electron configuration following different principles?

We already know that an atom has a nucleus at its center and several orbits exist outside the nucleus. These orbits again have sub-orbits. which are called orbital. Electron configuration can be done in two ways.

The first is through orbit and the second is through sub-orbit. Scientists have at different times published different principles for arranging the electrons of atoms.

**Among these, the important principles are:**

- Bohr atomic model
- Aufbau principle
- Hund principle
- Pauli’s exclusion principle

## Electron configuration following the Bohr principle

Renowned Danish physicist Niels Henrik David Bohr proposed this atomic model in 1913. So this model is named the Bohr atomic model after the name of the scientist.

According to scientist Bohr, an atom is composed of two parts. One is the nucleus and the other is the electron. The center of the atom contains all the positive charge and mass. which is called the nucleus.

Outside the nucleus, there are orbits at fixed distances from the nucleus in which the electrons are spinning. Scientists denote these orbits by n. Values of n are 1,2,3… but not 0 respectively.

n is called the principal quantum number of the orbit. Each orbit follows a formula for determining the number of electrons. Each orbital can hold at most 2n^{2} electrons. [n = 1,2,3,4 . . . The serial number of the orbit]

K is the name of the first orbit, L is the second, M is the third, and N is the name of the fourth orbit.

Shell Number (n) | Shell Name | Electrons Holding Capacity (2n^{2}) |

1 | K | 2 |

2 | L | 8 |

3 | M | 18 |

4 | N | 32 |

### Mathematical analysis:

- n = 1 for K orbit.

The maximum electron holding capacity in the K orbit is 2n^{2}= 2 × 1^{2}= 2. - For L orbit, n = 2.

The maximum electron holding capacity in the L orbit is 2n^{2}= 2 × 2^{2}= 8. - n=3 for M orbit.

The maximum electron holding capacity in the M orbit is 2n^{2}= 2 × 3^{2 }= 18. - n=4 for N orbit.

The maximum electron holding capacity in N orbit is 2n^{2}= 2 × 4^{2}= 32.

Therefore, the maximum electron capacity in the first shell is two, in the second shell eight, in the third shell eighteen, and in the fourth shell a maximum of thirty-two.

For example, we know that the atomic number of chlorine is 17. That is, the number of electrons in chlorine is seventeen.

Therefore, the chlorine atom will have two electrons in the first shell, eight in the 2nd orbit, and seven electrons in the 3rd shell. Therefore, the order of the number of electrons in each shell of the chlorine atom is 2, 8, 7. The Bohr atomic model has some limitations.

Electrons can be correctly arranged through the orbit of elements 1 to 18, but the electron configuration of elements with atomic numbers greater than 18 cannot be accurately determined according to the Bohr atomic model. Electron configuration of all elements can be done through the orbital diagram.

## Electron configuration following the Aufbau principle

Atomic energy shells are subdivided into sub-energy levels. These sub-energy levels are also called orbital. The most probable region of electron rotation around the nucleus is called the orbital.

The sub-energy levels depend on the azimuthal quantum number. It is expressed by ‘l’. The value of ‘l’ is from 0 to (n – 1). The sub-energy levels are known as s, p, d, and f.

Orbit Number | Value of ‘l’ | Number of subshells | Number of orbital | Subshell name | Electrons holding capacity | Electron configuration |

1 | 0 | 1 | 1 | 1s | 2 | 1s^{2} |

2 | 0 1 | 2 | 1 3 | 2s 2p | 2 6 | 2s^{2} 2p^{6} |

3 | 0 1 2 | 3 | 1 3 5 | 3s 3p 3d | 2 6 10 | 3s^{2} 3p^{6} 3d^{10} |

4 | 0 1 2 3 | 4 | 1 3 5 7 | 4s 4p 4d 4f | 2 6 10 14 | 4s^{2} 4p^{6} 4d^{10} 4f^{14} |

For example,

- If n = 1,

(n – 1) = (1–1) = 0

Therefore, the value of ‘l’ is 0. So, the sub-energy level is 1s. - If n = 2,

(n – 1) = (2–1) = 1.

Therefore, the value of ‘l’ is 0, 1. So, the sub-energy levels are 2s, and 2p. - If n = 3,

(n – 1) = (3–1) = 2.

Therefore, the value of ‘l’ is 0, 1, 2. So, the sub-energy levels are 3s, 3p, and 3d. - If n = 4,

(n – 1) = (4–1) = 3

Therefore, the value of ‘l’ is 0, 1, 2, 3. So, the sub-energy levels are 4s, 4p, 4d, and 4f. - If n = 5,

(n – 1) = (n – 5) = 4.

Therefore, l = 0,1,2,3,4. The number of sub-shells will be 5 but 4s, 4p, 4d, and 4f in these four subshells it is possible to arrange the electrons of all the elements of the periodic table.

Sub-shell name | Name source | Value of ‘l’ | Value of ‘m’(0 to ± l) | Number of orbital (2l+1) | Electrons holding capacity2(2l+1) |

s | Sharp | 0 | 0 | 1 | 2 |

p | Principal | 1 | −1, 0, +1 | 3 | 6 |

d | Diffuse | 2 | −2, −1, 0, +1, +2 | 5 | 10 |

f | Fundamental | 3 | −3, −2, −1, 0, +1, +2, +3 | 7 | 14 |

The orbital number of the s-subshell is one, three in the p-subshell, five in the d-subshell, and seven in the f-subshell. Each orbital can have a maximum of two electrons.

The sub-energy level ‘s’ can hold a maximum of two electrons, ‘p’ can hold a maximum of six electrons, ‘d’ can hold a maximum of ten electrons, and ‘f’ can hold a maximum of fourteen electrons.

Aufbau is a German word, which means building up. The main proponents of this principle are scientists Niels Bohr and Pauli. The Aufbau method is to do electron configuration through the sub-energy level.

The Aufbau principle is that the electrons present in the atom will first complete the lowest energy orbital and then gradually continue to complete the higher energy orbital.

The energy of an orbital is calculated from the value of the principal quantum number ‘n’ and the azimuthal quantum number ‘l’. The orbital for which the value of (n + l) is lower is the low energy orbital and the electron will enter that orbital first.

Orbital | Orbit (n) | Azimuthal quantum number (l) | Orbital energy (n + l) |

3d | 3 | 2 | 5 |

4s | 4 | 0 | 4 |

Here, the energy of 4s orbital is less than that of 3d. So, the electron will enter the 4s orbital first and enter the 3d orbital when the 4s orbital is full. The method of entering electrons into orbitals through the Aufbau principle is 1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p 7s 5f 6d.

For example, we know the atomic number of sodium is 11. Therefore, a sodium atom has a total of eleven electrons. Now if we want to do the electron configuration of sodium then the first two electrons of sodium will enter the 1s orbital.

Then the next two electrons enter the 2s orbital. The p-orbital can have a maximum of six electrons. So, the next six electrons enter the 2p orbital and the remaining one electron enters the 3s orbital. Therefore, the sodium full electron configuration will be 1s^{2} 2s^{2} 2p^{6} 3s^{1}.

**Note:** The abbreviated electron configuration of sodium is [Ne] 3s^{1}. When writing an electron configuration, you have to write serially.

## Electron configuration following the Hund’s principle

German physicist Friedrich Hund introduced this principle. This principle is named Hund’s principle after him.

Hund’s principle is that when electrons enter orbitals of the same energy, as long as the orbitals remain empty, the electrons will enter the orbitals in odd numbers and the spin of these odd electrons will be unidirectional.

Therefore, as long as multiple orbitals of the same energy remain vacant, new electrons will enter one by one to fill the orbitals. We already know that s-sub orbit has only one orbital.

Hence, the s-sub orbit does not exhibit Hund’s principle when the electron enters. Except for the s sub-orbit, the p, d, and f sub orbits follow Hund’s principle.

For example, we know the atomic number of nitrogen is seven. Therefore, a nitrogen atom has a total of seven electrons. Now, looking at the orbital diagram of nitrogen will give a clear idea about the Hund principle.

1s is the closest and lowest energy orbital to the nucleus. Therefore, the electron will first enter the 1s orbital. According to Hund’s principle, the first electron will enter in the clockwise direction and the next electron will enter the 1s orbital in the anti-clockwise direction.

The 1s orbital is now filled with two electrons. Then the next two electrons will enter the 2s orbital just like the 1s orbital. The 2s orbital is now full.

So the next three electrons will enter the 2p orbital in the clockwise direction. This is clearly shown in the figure of the orbital diagram of nitrogen.

## Electron configuration following Pauli’s exclusion principle

Knowing Pauli’s exclusion principle is very important to know the correct rules of electron configuration. In 1925, American physicist Wolfgang Ernst Pauli proposed a principle based on four quantum numbers. This is now known as Pauli’s exclusion principle.

Pauli’s principle states that the values of the four quantum numbers for any two electrons in the same atom can never be the same. Any one value will be different.

Eg: the Helium atom has two electrons. The quantum number of these two electrons is four. That is, out of four quantum numbers, three quantum numbers will be the same but one will be different.

Electron Number | Principle quantum number (n) | Azimuthal quantum number (l) | Magnetic quantum number (m) | Spin quantum number (s) |

1st | 1 | 0 | 0 | +1/2 |

2nd | 1 | 0 | 0 | -1/2 |

In the case of a helium atom, the first 3 quantum numbers are the same but the spin quantum numbers are different.

## Ground state and excited state electron configuration

The ground state electron configuration is the electron configuration in the stable state. That is, the electron arrangement that is formed when the electrons in the atom are in a stable state is called the ground state electron configuration of the atom.

For example, we know that the atomic number of sulfur is 16. Therefore, an atom of sulfur has a total of sixteen electrons. Now if we do the electron arrangement the electron configuration of sulfur will be 1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{4}.

We already know that the p-subshell has three orbitals. The orbitals are p_{x}, p_{y}, and p_{z} and each orbital can have a maximum of two electrons.

In the sulfur ground-state electron configuration, the four electrons of the 3p orbital are located in the p_{x}(2), p_{y}, and p_{z} orbitals and the spin of the three electrons is the same.

Therefore, the correct electron configuration of sulfur in the ground state will be 1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p_{x}^{2} 3p_{y}^{1} 3p_{z}^{1}.

This electron configuration shows that the last shell of the sulfur atom has two unpaired electrons. Therefore, the valency of sulfur is 2.

Atoms can jump from one orbital to another orbital in an excited state. This is called a quantum jump. The electron arrangement in this excited state of the atom is called excited state electron configuration.

In this condition, the electron’s position changes between different orbitals of the same orbit. However, the electron can jump from a lower energy level to a higher energy level by absorbing higher energy.

For example, when sulfur atoms are excited, then sulfur atoms absorb energy. As a result, an electron in the 3p_{x} orbital jumps to the 3d_{xy} orbital. We already know that the d-subshell has five orbitals.

The orbitals are d_{xy}, d_{yz}, d_{zx}, d_{x2-y2}, and d_{z2} and each orbital can have a maximum of two electrons. Therefore, the electron configuration of sulfur(S*) in an excited state will be 1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p_{x}^{1} 3p_{y}^{1} 3p_{z}^{1} 3d_{xy}^{1}.

The valency of the element is determined by electron configuration in the excited state. Here, sulfur has four unpaired electrons. Therefore, the valency of sulfur is 4.

When sulfur is further excited, then an electron in the 3s orbital jumps to the 3d_{yz} orbital. The second orbit of the sulfur atom is filled with electrons. So the electron of the third orbit jumps and goes to another orbital of the third orbit.

Therefore, the electron configuration of sulfur(S**) in an excited state will be 1s^{2} 2s^{2} 2p^{6} 3s^{1} 3p_{x}^{1} 3p_{y}^{1} 3p_{z}^{1} 3d_{xy}^{1} 3d_{yz}^{1}. This electron configuration shows that the last shell of the sulfur atom has six unpaired electrons.

So the valency of sulfur is 6. From the above information, we can say that sulfur exhibits variable valency. Therefore, the valency of sulfur is 2, 4, 6.

## Importance of electron configuration

Scientist Dmitri Mendeleev first proposed the accepted periodic table in 1869. He divided this periodic table into 12 periods and 8 groups. Later Mendeleev’s periodic table was proved wrong.

Finally, scientist Nelson Bohr created a periodic table based on electron configuration. This is known as the long periodic table or Bohr periodic table.

This periodic table has 18 groups and 7 periods. There are 118 elements in the periodic table. Each of these elements is arranged in the periodic table based on the electron configuration. Eg: The Electron configuration of potassium is 1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{6} 4s^{1}.

An electron exists in the last shell of potassium. Hence, the position of potassium is in group 1 of the periodic table.

Again, its last orbit number is 4. Hence, its period number is 4. Therefore, the electron arrangement plays an important role in determining the element’s position in the periodic table.