# Hund's rule: one electron in each orbital first

> The key you need to draw the 3d boxes


## Objectives

- Draw the electrons of any p or d sublevel in boxes using Hund's rule: one electron in each orbital with parallel spins, then pairing.
- Count the unpaired electrons in any p or d sublevel (for example, $d^6$ = 4 and $d^8$ = 2).

## 1. Electronic configurations and oxidation states




## 2. Three electrons, three empty orbitals

Predict before you see it:

- Each electron in its own orbital, all three with the same spin ($\uparrow\ \uparrow\ \uparrow$) ✓
- Two together in the first orbital ($\uparrow\downarrow$), the third in the next one
- Each electron in its own orbital, but with opposite spins ($\uparrow\ \downarrow\ \uparrow$)

By Hund's rule, each of the three 2p electrons takes its own orbital, all with the same (parallel) spin: $\uparrow\ \uparrow\ \uparrow$. Electrons repel each other, so they occupy the orbitals of the sublevel singly before pairing starts.

[ar-EG] حسب الـ Hund's rule، كل electron من التلاتة اللي في الـ 2p بياخد orbital لوحده، وكلهم بنفس الـ spin (parallel): $\uparrow\ \uparrow\ \uparrow$. الـ electrons بتتنافر مع بعض، فبتملا orbitals الـ sublevel فرادى قبل ما الأزواج تبتدي.
## 3. Hund's rule

Remember: an orbital holds 2 electrons at most, and the two have opposite spins ($\uparrow\downarrow$). Hund's rule Electrons occupy the orbitals of the same sublevel **singly** first, with **parallel spins**, before **pairing** starts. $\ce{_7N}$: $2p^3$ = 3 unpaired electrons Why? 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 $d^{0}$ $d^{1}$ $d^{2}$ $d^{3}$ $d^{4}$ $d^{5}$ $d^{6}$ $d^{7}$ $d^{8}$ $d^{9}$ $d^{10}$ unpaired electrons From $d^1$ to $d^5$: each new electron goes into an empty orbital, so the number of unpaired electrons goes up. From $d^6$ to $d^{10}$: each new electron makes a pair, so the number of unpaired electrons goes down to zero.


## 5. Count the unpaired electrons

A sublevel has 6 electrons ($d^6$). How many unpaired electrons does it have? Which diagram is correct for $d^8$?


## 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: $d^5$ = 5 unpaired, $d^6$ = 4, $d^8$ = 2, $d^{10}$ = 0. $d^5$ = 5 unpaired, $d^6$ = 4, $d^8$ = 2 **Hund's rule**: electrons occupy the orbitals of the same sublevel **singly**, with parallel spins, before **pairing** starts. To fill the 3d sublevel in the first transition series. Can you explain in one sentence why nitrogen has 3 unpaired electrons, not just 1?



Chapter: Transition Elements (https://yaani.ai/en/learn/chemistry/chem-s3-en.ch1)
Source: https://yaani.ai/en/lesson/chem-s3-en.lesson-atom.hund-rule