Hund’s rule of maximum multiplicity

Hund’s greatest multiplicity rule is a spectral observation rule used to estimate the ground state of an atom or molecule having one or more open electronic shells. The rule asserts that the lowest energy term for a given electron configuration is the one with the highest spin multiplicity value. If two or more orbitals of equal energy are available, electrons will fill them individually before filling them in pairs. Friedrich Hund discovered the rule in 1925, and it is widely used in atomic chemistry, spectroscopy, and quantum chemistry. It is sometimes reduced to Hund’s rule, disregarding Hund’s other two principles.

What is Hund’s Rule of Maximum Multiplicity? 

Hund’s Rule of Maximum Multiplicity asserts that the term with the highest multiplicity has the lowest energy for a given electron configuration. According to this rule, electron pairing in the p, d, and f orbitals cannot happen until each subshell’s orbital has one electron or is singly occupied.

1. According to Hund’s rule, every orbital in a sublevel is singly occupied before it can be doubly occupied.

2. In singly occupied orbitals, all electrons have the same spin (to maximise total spin).

Before coupling with another electron, it tries to fill all orbitals with the same energy (also known as degenerate orbitals) before pairing with another electron in a half-filled orbital. Atoms in their ground states have the most unpaired electrons feasible. 

3. To understand it better, consider how electrons behave like the same poles of a magnet if they were to come in contact with each other; when negatively charged electrons fill orbitals, they initially strive to go as far away from each other as possible before interacting.

Hund’s Rule Explanation

Before partnering up with each other, the electrons enter an empty orbital. Electrons repel each other because they are negatively charged. The electrons do not share orbitals to reduce repulsion. The spins of unpaired electrons in singly occupied orbitals are the same when the second rule is applied. The spin of the sub-earliest level’s electrons determines the spin of the remaining electrons. The electrons do not share orbitals in order to decrease repulsion.

Things to keep in mind based on Hund’s Maximum Multiplicity Rule

• According to Hund’s Rule, every orbital in the sub-level is singly occupied before any orbital is doubly occupied.
• To optimise total spin, all electrons in a single-occupancy orbital have the same spin.
• Because electrons are negatively charged, they repel each other. Therefore, the electrons do not share orbitals in order to reduce repulsion.
• The position of electrons as well as their stability may be determined via electronic configuration.

Atoms of Nitrogen (Example 1)

Take a look at the right electron configuration of the nitrogen atom (Z = 7):  1s2 2s2 2p3

There are three electrons and three p orbitals, which are half-filled. This is due to the fact that the three electrons in the 2p subshell will first fill all the vacant orbitals before combining with electrons in them.

Oxygen Atoms (Example 2)

The atom of oxygen (Z = 8) is the second element in the same period after nitrogen; its electron configuration is: 1s2 2s2 2p4 

Carbon Atoms (Example 3)

Next, consider carbon’s electron configuration: Two electrons will be paired in the 1s orbital, two electrons in the 2s orbital, and the two remaining electrons in the 2p orbitals. Following Hund’s Rule, the accurate orbital diagram will show the two 2p electrons to be stranded in two of the three accessible orbitals, both with “spin-up.” Because electrons usually occupy an empty orbital before filling it, drawing the two 2p electrons in the same orbital would be erroneous, leaving vacant orbitals unfilled.

In conclusion:- 

The Aufbau Principle imposes limits on the way atomic orbitals are filled in the ground state as a result of Hund’s rule. Other orbitals in the same subshell must each contain one electron before any two electrons may occupy an orbital in that subshell. Furthermore, before opposing spin electrons begin to fill a subshell, electrons that are filling it will have parallel spin (after the first orbital gains a second electron). As a result, the highest amount of unpaired electrons (and hence the largest total spin state) is guaranteed while filling up atomic orbitals.