As per  Hund’s first rule, the lowest energy atomic state is the one which maximises the overall spin quantum number for the electrons in the open subshell. Before double occupation, the orbitals of the subshell are each occupied singly with electrons of parallel spin. (This is sometimes referred to as the “bus seat rule,” because it is similar to the behaviour of bus passengers, who tend to fill all double seats individually before double occupancy arises.)

Hund’s Rule 

• According to Hund’s rule, an atom with a higher total spin state is occasionally more stable. For determining the state of an excited electron configuration, this rule is fairly trustworthy (with a few exceptions). Friedrich Hund found it in 1925. Hund’s rule is as follows: Before any orbital in a sublevel is double occupied, each orbital is individually occupied. Each electron in a separately occupied orbital has the same spin (to maximise total spin). The Rule of Maximum Multiplicity is another name for Hund’s rule.
• Electrons must always enter an empty orbital before partnering up, according to the first rule. Instead of sharing an orbital with another electron, electrons will occupy their own to reduce repulsion. Electrons in singly occupied orbitals are less efficiently protected from the nucleus, according to quantum-mechanical simulations.
• The second rule states that electrons in unpaired singly occupied orbitals have the same number of spins. A sublevel’s first electron can either “spin-up” or “spin-down.” Once the spin of the first electron in a sublevel is determined, the spins of all other electrons in that sublevel are determined by that first spin.

Explanation of  Hund’s Rule

• An atom is made up of a nucleus around which electrons orbit in distinct energy orbitals. The Aufbau principle states that electrons first fill the lowest energy level before moving on to the higher ones. As a result, electrons are located in separate atomic orbitals, forming the electron configuration. The filling of orbitals, on the other hand, according to a set of rules known as Hund’s rule. Because paired and unmated electrons have different properties, Hund’s Rule can assist in anticipating atom properties (specifically with interactions with magnetic fields).
• When atoms come into contact with one another, their exterior electrons, or valence shells, interact first. When an associated atom’s valence shell is not full, it is the least stable (and thus the most reactive). The chemical behaviour of an associate element is primarily determined by its valence electrons. Chemical characteristics are usually similar amongst parts with an equivalent range of valence electrons. When all of an associate atom’s orbitals are filled with electrons, it is the most stable (and thus unreactive). The noble gases, which are exceedingly stable and do not generally react with one another, have these configurations.

Hund’s rule notes

The filling of electrons in the subshell of atoms is dealt with by Hund’s maximum multiplicity rule. It aids in the determination of the overall electronic configuration by reducing anomalies in subshells with the same energy orbitals. Hund’s rule states that electrons populating the orbitals of the same subshell follow the same criteria. Hund’s rule, which determines the ground state multiple of electrons at the atomic level, is one of the cornerstones of correlation physics. When electrons hop between atoms in actual systems, they gain itinerancy, which is commonly represented by the band theory. The goal of correlation theories is to provide a trustworthy approach to characterise the condition in between the two extremes.

Hund’s Rule And Electronic Configuration

• The Aufbau principle, the Hund’s rule, and the Pauli Exclusion principle are three essential rules that regulate the overall electronic arrangement of atoms in the periodic table.
• The Aufbau rule dictates that the lowest-energy orbitals be filled first.
• According to the Pauli exclusion rule, no two electrons in an orbital can have the same spin.
• Hund’s rule is concerned with the electrical arrangement of orbitals belonging to the same subshell, called degenerate orbitals.

Applications of Hund’s Rule

• In quantum chemistry, Hund’s rule is extremely important. It shows the stability of the same subshell’s degenerate orbitals.
• In spectroscopy, Hund’s rule is used to construct the atomic spectra of elements.
• Hund’s rule ensures maximum spin multiplicity, which aids in determining a stable electronic arrangement.

Conclusion

The maximum multiplicity rule of Hund gives a mechanism for organising electrons in subshell orbitals. The Aufbau rule is based on the energy levels of the subshells, and it decides which subshell should be filled first. The number of orbitals in each subshell (s,p,d,f) is different. Hund’s rule states that electrons filling the orbitals of the same subshell follow the criteria. If two or more degenerate (i.e., identical energy) orbitals are available, one electron enters into each until they are all half full before pairing up, according to Hund’s Rule. No two electrons may be identified by the same set of quantum numbers, according to the Pauli Exclusion Principle.