Hund's Rule of Maximum Multiplicity
Electronic Configuration | Structure of Atom | Class 11
1. Statement of Hund's Rule
Hund's Rule deals with the filling of electrons in orbitals belonging to the same subshell (i.e., orbitals having the same energy, known as degenerate orbitals).
"Pairing of electrons in the orbitals belonging to the same subshell ($p, d,$ or $f$) does not take place until each orbital belonging to that subshell has got one electron each, i.e., it is singly occupied with parallel spins."
2. What are Degenerate Orbitals?
Orbitals that have exactly the same energy are called degenerate orbitals. For example:
- The three $p$ orbitals ($p_x, p_y, p_z$) of a given principal quantum number (like $2p$) have the same energy.
- The five $d$ orbitals of a given level (like $3d$) are degenerate.
- The seven $f$ orbitals are degenerate.
Note: The $s$-subshell only has one orbital, so Hund's Rule of pairing does not apply to it. Pairing in an $s$-orbital starts immediately with the second electron.
3. Why Does This Happen? (The Science Behind It)
Why do electrons prefer to sit in separate orbitals with parallel spins before pairing up? There are two main reasons:
A. Minimizing Electron Repulsion
Electrons are negatively charged and repel each other. Placing them in different orbitals keeps them further apart in space, which minimizes the inter-electronic repulsion and lowers the energy of the atom.
B. Maximizing Exchange Energy
Electrons with parallel spins (spins pointing in the same direction, usually represented as $\uparrow$) can exchange their positions. The more exchanges possible, the more energy is released (called Exchange Energy). Keeping spins parallel before pairing maximizes this exchange energy, leading to greater stability.
4. Examples of Hund's Rule
Nitrogen ($N$, Atomic Number = 7)
Total electrons = 7. Electronic configuration is $1s^2 \ 2s^2 \ 2p^3$.
According to Hund's rule, the three electrons in the $2p$ subshell will occupy the $p_x$, $p_y$, and $p_z$ orbitals singly with parallel spins:
It will not be $2p_x^2 \ 2p_y^1 \ 2p_z^0$, as that violates Hund's Rule.
Oxygen ($O$, Atomic Number = 8)
Total electrons = 8. Electronic configuration is $1s^2 \ 2s^2 \ 2p^4$.
First, three electrons occupy the $p_x, p_y,$ and $p_z$ orbitals singly. The fourth electron is then forced to pair up in the $p_x$ orbital (with opposite spin, due to Pauli's exclusion principle):
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