Question

What is Hund's Rule, Pauli Exclusion Principle, and the Aufbau Principle?

Answer 1

Hund's rule:

1. According to Hund's Rule, if there are two or more degenerate orbitals (orbitals with the same energy) available, one electron must enter each one until they are all half full before pairing up.
2. Because every partially filled electron in an orbital of a subshell has the same spin, the orbital posses maximum stability
3. The lone pair of electrons in an atom and the valency can be determined using Hund's rule.

Pauli exclusion principle:

1. According to the Pauli exclusion principle, no two electrons in a single atom will have the same set of quantum numbers (n, l, ml, and ms). Simply said, each electron should have or exist in its own unique state (singlet state).
2. The two electrons in the same orbital must have opposing spins or be in an antiparallel configuration.
3. A large number of physical behaviors, including the structure of atoms' electron shells and how they share electrons, are explained by the Pauli exclusion principle.

Aufbau principle:

1. According to this, electrons are placed into atomic orbitals in ascending sequence of orbital energy. The Aufbau principle states that atomic orbitals that are available and have the lowest energy levels get filled before those that have higher energy levels.
2. The sequence of energy levels is as follows $1\mathrm{s},2\mathrm{s},2\mathrm{p},3\mathrm{s},3\mathrm{p},4\mathrm{s},3\mathrm{d},4\mathrm{p},5\mathrm{s},4\mathrm{d},5\mathrm{p},4\mathrm{f},5\mathrm{d},6\mathrm{p},7\mathrm{s}$.. and so on.