Rules for Filling of Electron in Orbitals for JEE

In atomic physics and quantum chemistry, electron placement is the distribution of electrons in an atom or molecule (or other physical structure) in the orbit of the atom or molecule. For example, the electron configuration of a neon atom is 1s² 2s² 2p⁶. This means that the 1s, 2s, and 2p subshells are occupied by 2, 2, and 6 electrons, respectively. Moving diagonally through the periodic table, the elements show certain similarities, but these are far less pronounced than the similarities within the group. The diagonal relationship is especially noticeable in the elements of the 2^nd and 3^rd periods of the periodic table.

The electron configuration of the element represents how the electrons are distributed in their atomic orbitals. The electron configuration of an atom follows the standard notation in which an atomic subshell containing all electrons (including the number of electrons in the superscript) is arranged in sequence. For example, the electron configuration of sodium is 1s² 2s² 2p⁶ 3s¹. We shall find out the rules for filling electrons in orbitals ahead.

Electronic Configuration of Elements

• Shell

The maximum number of electrons that can be accommodated in a shell depends on the principal quantum number (n). This is expressed by equation 2n². Here, "n" is the shell number. The shell, the value of n, and the total number of electrons that can be accommodated are shown below.

The table below shows the maximum number of electrons that a shell can have, and shell and 'n' value shell.

Number of Electrons in a Shell

• Subshell

The subshell to which the electrons are distributed is based on the orbital angular momentum (indicated by "l").

This quantum number depends on the value of the principal quantum number n. Therefore, if the value of n is 4, there are 4 different subshells possible. When n = 4, the subshells correspond to l = 0, l = 1, l = 2, and l = 3, and are called s, p, d, and f subshells, respectively. The maximum number of electrons that a subshell can contain is given by Equation 2 * (2l + 1).

Azimuthal Quantum Numbers 

Therefore, the s, p, d, and f subshells can hold up to 2, 6, 10, and 14 electrons, respectively. Above are all possible subshells with values ​​from n to 4; main quantum number value orbital angular momentum value subshell resulting from electron configuration.

Therefore, it is understood that the 1p, 2d, and 3f orbitals do not exist because the orbital angular momentum value is always smaller than the main quantum number.

• Notation

The electron configuration of atoms is described using subshell labels. These labels contain the shell number (indicated by the principal quantum number), the subshell name (indicated by the orbital angular momentum), and the total number of electrons in the superscripted subshell.

For example, if two electrons are filled in the "s" subshell of the first shell, the resulting notation would be "1s²".

Aufbau Principle Example

• To generate the ground-state electron configuration of any element, we need to know how the atomic sublevels are organised in order of increasing energy.

• Electrons are added to atomic orbitals in order from low energy to high energy  according to the Aufbau principle. 

• The lowest energy sublevel is always the 1s sublevel and consists of one orbit.

• The only electron in a hydrogen atom occupies a 1s orbital when the atom is in the ground state. 

• When you move to an atom with multiple electrons, those electrons are added to the next lower sublevel (2s, 2p, 3s, etc.). 

• After the 3p sublevel, it seems natural that the 3d sublevel has the next lowest energy. 

• However, the 4s sublevel has slightly lower energy than the 3d sublevel. So it fills up first. This is due to the “n+l” value rule, which states that higher the value of n+l, higher will be the energy.

• After filling the 3d sublevel, 4p, 5s, 4d follow. 

• Note that the 4f sublevel is only filled immediately after the 6s sublevel.

Now the next question is “How to fill electrons in orbitals”

Rules for Filling of Electrons in Orbitals

The three rules for filling the orbit are:

Rule 1:

The lowest energy orbit must be filled first. The filling pattern is as follows:

1s, 2s, 2p, 3s, 3p, 4s, 3d and so on.

• The orbit of the subshell is degenerate.

• That is, the entire subshell must be filled before filling the next orbit. 

• This depends on the energy of the lower shell.

Rule 2:

• According to the Pauli exclusion principle, the number of electrons allowed per orbit is 2, and they must have opposite spins. 

• If one electron in the orbit is spinning clockwise, the other electron in the orbit must be spinning counterclockwise. 

• Such electrons are called unpaired electrons.

Rule 3:

• According to Hund's rule, the subshell provides maximum stability when the number of unpaired electrons is maximum, that is, when the spin directions are the same. 

• This is due to the degeneracy of the orbit. 

• Therefore, all orbits in the same subshell have the same energy. A pair of electrons occurs when all orbitals are filled individually.

These are also known as the order of filling orbitals.

Summary

The structural principles establish some basic criteria for filling the orbits of atoms. Ground state atoms have the lowest energy and are the most stable. The structural principle regulates the filling of orbitals in the ground state of an atom. These principles are based on Pauli exclusion principle, Hund's rule of maximum multiplicity, and orbital relative energy. According to the principle, electrons are introduced into different orbits as the order of energy increases. The electrons first enter the lowest available energy orbit and then move to the higher energy orbit when the low energy orbit is filled.