What is Hund's Rule? | Simple Explanation for Class 12

S7-SA5-0030

What is Hund's Rule of Maximum Multiplicity?

Grade Level:

Class 12

AI/ML, Physics, Biotechnology, FinTech, EVs, Space Technology, Climate Science, Blockchain, Medicine, Engineering, Law, Economics

Definition

What is it?

Hund's Rule of Maximum Multiplicity tells us how electrons fill up orbitals within an atom. It says that electrons prefer to occupy separate orbitals within a subshell with parallel spins before pairing up in the same orbital. This makes the atom more stable, like people preferring their own seat on a bus before sharing.

Simple Example

Quick Example

Imagine a school bus with three empty seats in a row, all facing the same way. If three students get on, Hund's Rule says each student will take a separate empty seat first, rather than two students sharing one seat and leaving another empty. Only when all seats are taken will new students start sharing seats.

Worked Example

Step-by-Step

Let's fill electrons in the p-subshell, which has three orbitals (like three rooms in a flat).
---STEP 1: We have 3 electrons to place in the 2p subshell. The 2p subshell has three orbitals: 2px, 2py, 2pz.
---STEP 2: According to Hund's Rule, the first electron goes into 2px (spin up).
---STEP 3: The second electron goes into 2py (spin up), not pairing with the first electron.
---STEP 4: The third electron goes into 2pz (spin up), again not pairing up.
---STEP 5: Now, if we have a fourth electron, it will start pairing up. The fourth electron would go into 2px (spin down), pairing with the first electron.
---ANSWER: For 3 electrons, each orbital (2px, 2py, 2pz) gets one electron with parallel spin (all 'spin up').

Why It Matters

Common Mistakes

MISTAKE: Students often fill all electrons in one orbital before moving to the next. | CORRECTION: Remember to fill one electron into each orbital of a subshell first, all with parallel spins, before starting to pair them up.

MISTAKE: Students pair up electrons with opposite spins immediately, even if empty orbitals are available. | CORRECTION: Electrons will only pair up with opposite spins in an orbital AFTER all orbitals in that subshell have at least one electron with parallel spin.

MISTAKE: Students forget that the 'spin' (represented by an arrow up or down) must be parallel when filling separate orbitals. | CORRECTION: When placing one electron in each orbital of a subshell, ensure all these electrons have the same spin direction (e.g., all 'up' arrows).

Practice Questions

Try It Yourself

QUESTION: How many unpaired electrons are there in a nitrogen atom (atomic number 7) according to Hund's Rule? | ANSWER: 3 unpaired electrons

QUESTION: Draw the orbital diagram for an oxygen atom (atomic number 8) showing how electrons fill the 2p subshell based on Hund's Rule. | ANSWER: The 2p subshell will have two orbitals with one electron each (parallel spin) and one orbital with two electrons (paired, opposite spins). So, 2 unpaired electrons.

QUESTION: A d-subshell has 5 orbitals. If you have 6 electrons to place in a d-subshell, how many unpaired electrons will there be, and how will they be arranged? | ANSWER: There will be 4 unpaired electrons. Each of the 5 orbitals will get one electron with parallel spin, and the 6th electron will pair up in one of the orbitals with an opposite spin.

MCQ

Quick Quiz

Which of the following statements correctly describes Hund's Rule?

Electrons fill lower energy orbitals first.

Each orbital can hold a maximum of two electrons with opposite spins.

Electrons prefer to occupy separate orbitals within a subshell with parallel spins before pairing up.

No two electrons in an atom can have the same set of four quantum numbers.

The Correct Answer Is:

Option C directly states the core principle of Hund's Rule. Options A, B, and D refer to the Aufbau principle, Pauli's exclusion principle, and Pauli's exclusion principle respectively, which are different rules.

Real World Connection

In the Real World

Hund's Rule helps scientists understand why certain materials are magnetic or not. For instance, in making powerful magnets used in electric vehicles (EVs) or MRI machines in hospitals, engineers need to know how electrons are arranged to predict and control the magnetic properties of elements like iron or neodymium. It's all about how those tiny electrons spin!

Key Vocabulary

Key Terms

ORBITAL: A region around the nucleus where an electron is most likely to be found | SUBSHELL: A group of orbitals within an electron shell (e.g., s, p, d, f) | SPIN: A property of an electron, like a tiny magnet, represented as 'spin up' or 'spin down' | PARALLEL SPINS: Electrons having the same spin direction (e.g., both 'up') | UNPAIRED ELECTRONS: Electrons occupying an orbital alone, not paired with another electron.

What's Next

What to Learn Next

Next, you should explore the 'Aufbau Principle' and 'Pauli's Exclusion Principle'. These rules, along with Hund's Rule, form the complete set for understanding how electrons are arranged in atoms, which is fundamental to all of chemistry!