Summary of "Afbau , Pauli and Hunds rules | Atomic structure - Bsc 1st year inorganic chemistry"

Summary of the Video: "Afbau, Pauli and Hund's Rules | Atomic structure - BSc 1st year inorganic chemistry"

This lecture covers three fundamental principles/rules related to the electronic structure of atoms: the Aufbau Principle, Pauli Exclusion Principle, and Hund’s Rule of maximum multiplicity. These rules explain how electrons fill atomic orbitals in the ground state and how electron spins and Quantum Numbers govern their arrangement.

1. Aufbau Principle (Building-up Principle)

Meaning: "Aufbau" is a German word meaning "building up" or "construction," analogous to building a wall step-by-step.
Concept: Electrons fill atomic orbitals starting from the lowest energy orbital to higher energy orbitals in the ground state.
Filling Order:
- Orbitals fill in order of increasing energy.
- The order is determined by the n + l rule (also called the Bahar-Burris rule):
  - Orbitals with lower values of (n + l) fill first.
  - If two orbitals have the same (n + l) value, the orbital with the lower principal quantum number (n) fills first.
Example:
- 2p (n=2, l=1; n+l=3) fills before 3s (n=3, l=0; n+l=3) because 2 < 3.
Energy order of subshells: 1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p < 5s < 4d < 5p < 6s < 4f < 5d < 6p < 7s < 5f < 6d < 7p < 8s
Exceptions:
- Lanthanides and Actinides show exceptions to the n + l rule.
- For example, 5d fills before 4f in some cases, and 6d fills before 5f, despite what the n + l rule predicts.

2. Pauli Exclusion Principle

Proposed by: Wolfgang Pauli in 1924.
Statement: No two electrons in an atom can have the same set of all four Quantum Numbers (n, l, m, s).
Implications:
- Each orbital can accommodate a maximum of two electrons.
- These two electrons must have opposite spins (spin quantum number s = +1/2 and -1/2).
Example:
- In the 1s orbital (1s²), both electrons have the same n=1, l=0, m=0 but opposite spins (+1/2 and -1/2).
Summary: Electrons in the same orbital differ in their spin quantum number to satisfy the Pauli Exclusion Principle.

3. Hund’s Rule of Maximum Multiplicity

Statement: Electrons occupy degenerate orbitals (orbitals with the same energy within a subshell) singly first, with parallel spins, before pairing begins.
Explanation:
- Electron pairing starts only after all orbitals in the subshell are singly occupied.
- This arrangement minimizes electron-electron repulsions and leads to greater stability.
Example:
- For three electrons in a p subshell (p³), the electrons occupy three different p orbitals with parallel spins.
Alternative phrasing: The most stable arrangement is the one with the maximum number of electrons having parallel spins.
Implication:
- Before pairing, all electrons in a subshell have parallel spins.
- For example:
  - p³ has three electrons with parallel spins.
  - d⁵ has five electrons singly occupying five d orbitals with parallel spins.
  - Pairing starts only after all orbitals are singly occupied.

Summary of All Three Rules Together:

Aufbau Principle: Electrons fill orbitals from lower to higher energy.
Pauli Exclusion Principle: No two electrons have the same four Quantum Numbers; orbitals hold max two electrons with opposite spins.
Hund’s Rule: Electrons fill degenerate orbitals singly with parallel spins before pairing.

Speakers/Sources Featured:

The lecturer (unnamed) presenting the concepts and explaining with examples.
Reference to Wolfgang Pauli (historical figure who proposed the Pauli Exclusion Principle).