Question

Hund's rule states that:

• A. Matter and radiation have a dual nature
• B. In the orbitals of equivalent energy, pairing begins only after all the orbitals have been singly occupied.
• C. The position and velocity of an electron cannot be exactly measured at the same time.
• D. No two electrons in an atom can have the same set of four quantum numbers.

Answer 1

The correct option is B In the orbitals of equivalent energy, pairing begins only after all the orbitals have been singly occupied.

Hund's rule states that:
Every orbital in a sublevel is singly occupied before any orbital is doubly occupied.
All of the electrons in singly occupied orbitals have the same spin (to maximize total spin).
Hence, the correct option is (b) i.e. In the orbitals of equivalent energy, pairing begins only after all the orbitals have been singly occupied
Example:
Oxygen electronic configuration:
$1s^{2}2s^{2}2p^4$
Correct configuration:


Incorrect configuration: ( because pairing started before all the orbitals were singly occupied)