Hund’s principles and explanations
Hund’s Rule:
The rule for the distribution of electrons in degenerate orbitals (orbitals having the same energy level) based on the magnetic energy values of electrons was proposed by the physicist Friedrich Hund.
“In degenerate orbitals, electrons initially occupy the available orbitals singly with parallel spins (maintaining maximum unpaired or single states). Subsequently, if more electrons are available, pairing occurs with opposite spins.”
In other words, electrons prefer to remain unpaired rather than paired in degenerate orbitals as long as possible. This is because electrons with parallel spins repel each other, and an atom attains maximum stability when its electrons are in unpaired configurations.
1. Explanation of Hund’s Rule for Nitrogen (7N) Atom:
The outermost p-subshell of a Nitrogen (N) atom contains 3 electrons. The p-subshell is further divided into three degenerate orbitals: 2px, 2py, and 2pz. According to Hund’s rule, the 3 valence electrons will initially occupy these orbitals singly with parallel spins as 2px1, 2py1, and 2pz1 respectively.
2. Explanation of Hund’s Rule for Oxygen (8O) Atom:
On the other hand, in the case of an Oxygen (O) atom, its outermost p-subshell contains 4 electrons. The p-subshell is split into three degenerate orbitals: 2px, 2py, and 2pz. According to Hund’s rule, among the 4 valence electrons, 3 will initially enter these orbitals singly with parallel spins. Following this, the remaining 4th electron will pair up with an opposite spin in the first orbital (resulting in 2px2).
Hund’s Rule — Questions and Answers
Question: State Hund’s Rule.
Answer: While entering degenerate orbitals, electrons will occupy the available empty orbitals singly or unpaired as long as possible, and these unpaired electrons will have parallel spins (spins in the same direction).
Question 1: For which subshells is Hund’s rule not applicable?
Answer: Hund’s rule is not applicable to the s subshell or s orbital (because the s subshell does not have any degenerate sub-orbitals or divisions).
Question 2: What are degenerate orbitals?
Answer: Orbitals belonging to the same subshell and having equal energy are called degenerate orbitals; for example, the three orbitals of the p subshell: 2px, 2py, 2pz.
Question 1: How does Hund’s rule operate in the electronic configuration of Oxygen (8O)? Explain.
According to Hund’s rule, among the 4 available electrons, 3 will initially enter each orbital singly with parallel spins. Subsequently, the remaining 4th electron will pair up in the first orbital with an opposite spin.
Therefore, the correct electronic configuration of Oxygen according to Hund’s rule is: 1s2 2s2 2px2 2py1 2pz1
Question 2: Explain the electronic configuration of Carbon (6C) according to Hund’s rule.
Consequently, the correct electronic configuration of Carbon will be: C → 1s2 2s2 2px1 2py1 2pz0
Thus, a carbon atom contains two unpaired electrons.
Question 3: Explain the electronic configuration of Nitrogen (7N) in light of Hund’s rule.
Hence, the correct configuration will be: N → 1s2 2s2 2px1 2py1 2pz1
Question 4: Which of the following configurations is correct and why? (a) 2px2 2py1 2pz0 (b) 2px1 2py1 2pz1
According to Hund’s rule, electrons prefer to maintain the maximum number of unpaired states in degenerate orbitals. In configuration (a), pairing occurs in one orbital while leaving another orbital completely empty, which violates Hund’s rule. On the other hand, in configuration (b), the electrons correctly occupy the degenerate orbitals singly first, which perfectly adheres to the rule.
