UPSC Prelims 2001 · General Studies

Question 119: Science and Technology

In an atom, the order of filling up of the orbitals is governed by

aAufbau principle

bHeisenberg’s uncertainty principle

cHund’s rule

dPauli’s exclusion principle

AnswerThe answer is (a).

The Aufbau principle says electrons fill orbitals in order of increasing energy, lowest first (1s, 2s, 2p, 3s, 3p, 4s, 3d and so on). Hund's rule governs how electrons are placed within orbitals of equal energy, Pauli's exclusion principle limits an orbital to two electrons of opposite spin, and Heisenberg's principle concerns uncertainty in position and momentum. Remember that Aufbau means building up.

Why each option is right or wrong

(a) Right: Aufbau principle.
(b) Heisenberg's principle is about uncertainty in position and momentum.
(c) Hund's rule is about filling orbitals of equal energy singly first.
(d) Pauli's principle limits each orbital to two electrons with opposite spins.

Explanation: written by UPSC Grade, not by UPSC.

Practise this paper timedPrevious question Next question

Source: UPSC, Civil Services (Preliminary) Examination 2001, General Studies. UPSC's own paper isn't online; the words are from a published compilation. The paper on upsc.gov.in.