UPSC Prelims 2001 Q119: Science & Technology – Chemistry
In an atom, the order of filling up of the orbitals is governed by
- (a) Aufbau principle
- (b) Heisenberg’s uncertainty principle
- (c) Hund’s rule
- (d) Pauli’s exclusion principle
Answer: (a) Aufbau principle
Explanation
Principle that governs the order of filling of orbitals
| Principle | What it says |
|---|---|
| Aufbau principle | Orbitals fill in order of increasing energy: 1s, 2s, 2p, 3s, 3p, 4s, 3d… |
| Pauli exclusion principle | No two electrons have the same four quantum numbers; at most 2 per orbital |
| Hund's rule | In orbitals of equal energy, electrons stay single before pairing |
| Heisenberg's uncertainty principle | Position and momentum cannot both be known exactly |
The order in which orbitals fill is given by the Aufbau ('building up') principle: lowest energy first, using the (n + l) rule. Pauli and Hund's rules decide how electrons sit within orbitals, not the order of filling.
Answer: (a) Aufbau principle.
Why each option is right or wrong
- (a) CORRECT. The Aufbau principle gives the order of filling orbitals.
- (b) Incorrect. The uncertainty principle is about measurement, not filling.
- (c) Incorrect. Hund's rule decides pairing within a subshell.
- (d) Incorrect. Pauli's principle limits each orbital to two electrons.
Key point
(n + l) rule: lower n + l fills first; if equal, lower n fills first — so 4s fills before 3d.
Remember
Order of filling orbitals = Aufbau principle.
Common mistakes
- Choosing Hund's rule because it is about electrons in orbitals.
- Choosing Pauli because it is the best known.
- Mixing order of filling with pairing.
Exam tip
Order = Aufbau; capacity = Pauli; pairing = Hund.
Sources
- NCERT Class XI — Chemistry Part I, chapter 'Structure of Atom'
- J. D. Lee — Concise Inorganic Chemistry, chapter on atomic structure