Pauli exclusion principle and Hund's rule
Pauli: no two electrons in an atom can have all four quantum numbers the same, so an orbital holds at most two, with opposite spins. Hund: in equal-energy orbitals, electrons spread out singly before pairing up.
Last updated
Analogy
Like people on a bus: each takes an empty double seat alone first, and only shares when every seat is taken.
Example
↑ _ _ is wrong↑ ↑ ↑ is rightThree 2p orbitals, one electron each,all spinning the same way
In practice
Hund's rule explains why oxygen has two unpaired electrons and is attracted to a magnet when cooled to a liquid.
Practice
Flashcards
- State the Pauli exclusion principle.No two electrons in an atom share all four quantum numbers, so an orbital holds at most two, with opposite spins.
Interview questions
- Write the electron configuration of Fe and Fe³⁺ and explain which electrons are lost first.
- Fe is [Ar] 4s² 3d⁶
- 4s electrons are lost before 3d
- Fe³⁺ is [Ar] 3d⁵
- Half-filled d subshell is stable
- Why does the first ionization energy of oxygen fall slightly below that of nitrogen?
- Nitrogen has three unpaired 2p electrons (half-full)
- Oxygen has one paired 2p orbital
- Repulsion between paired electrons makes one easier to remove
Questions people ask
Why do electrons avoid pairing in the same orbital?
Two electrons in one orbital sit close together and repel each other strongly. Placing them in separate equal-energy orbitals keeps them further apart, so the atom has lower energy. They only pair up when no empty orbital of that energy is left.
What does unpaired electron mean for magnetism?
Each electron acts like a tiny magnet. Paired electrons have opposite spins, so their magnetic effects cancel. Atoms with unpaired electrons are paramagnetic, which means they are pulled into a magnetic field. Atoms with all electrons paired are diamagnetic and are weakly pushed away.