Hund's rule: one electron in each orbital first
The key you need to draw the 3d boxes
1. Electronic configurations and oxidation states
2. Three electrons, three empty orbitals
Nitrogen has 3 electrons in its 2p sublevel, and 2p has 3 empty orbitals. How will the electrons sit?
2p
Pick the right answer
Predict before you see it:
- A. Two together in the first orbital (), the third in the next one
- B. Each electron in its own orbital, all three with the same spin (\ \)
- C. Each electron in its own orbital, but with opposite spins (\ \)
Explanation
By Hund's rule, each of the three 2p electrons takes its own orbital, all with the same (parallel) spin: \ \. Electrons repel each other, so they occupy the orbitals of the sublevel singly before pairing starts.
3. Hund's rule
Remember: an orbital holds 2 electrons at most, and the two have opposite spins ().
Hund's rule
Electrons occupy the orbitals of the same sublevel singly first, with parallel spins, before pairing starts.
↑↑↑
2pWhy? Electrons are negative and repel each other. When they stay apart in separate orbitals, the energy is lower.
4. Fill the d sublevel yourself
The d sublevel has 5 orbitals. Add electrons one at a time and watch the number of unpaired electrons.
Electrons in the d sublevel 1
↑
dunpaired electrons 1
Reveal
- From d¹ to d⁵: each new electron goes into an empty orbital, so the number of unpaired electrons goes up.
- From d⁶ to d¹⁰: each new electron makes a pair, so the number of unpaired electrons goes down to zero.
5. Count the unpaired electrons
Pick the right answer
A sublevel has 6 electrons (d⁶). How many unpaired electrons does it have?
- A. 0
- B. 4
- C. 6
- D. 2
Explanation
d⁶: [][][][][], so 4 unpaired electrons.
Pick the right answer
Which diagram is correct for d⁸?
- A. [][][][][\ \ ]
- B. [][][][][]
- C. [][][][][]
Explanation
8 = 5 singly + 3 that make pairs. The 2 unpaired electrons have parallel spins.
Need a hint?
- Start by putting one electron in each box.
- After the first 5, each extra electron pairs with the first box that has only one arrow.
- Count the boxes that have only one arrow.
6. Summary
- Hund's rule: singly and with parallel spins first, then pairing.
- The reason: electrons repel each other, so spreading out gives a lower energy.
- Quick count: d⁵ = 5 unpaired, d⁶ = 4, d⁸ = 2, d¹⁰ = 0.
↑↑↑↑↑
d, 5 electrons↑↓↑↑↑↑
d, 6 electrons↑↓↑↓↑↓↑↑
d, 8 electronsCan you explain in one sentence why nitrogen has 3 unpaired electrons, not just 1?