SSC CGL 2024 Tier-I · 18 September 2024, Shift 3 · General Awareness · Question 1 of 98

Text as printed on a candidate's response sheet for this shift; answer per SSC's tentative key as marked on that sheet. Solution last reviewed 10 September 2026.

Where this paper and its answers come from

SSC does not publish CGL question papers or a public answer key. The questions and options here are reproduced as printed on a candidate's own response sheet for this shift — the Commission's text, unchanged. Each answer is SSC's tentative key as marked on that sheet; SSC allowed challenges to it and never publishes a final key. The solutions are written by UnlockIAS.

Identify a monoatomic molecule.

(a)Helium
(b)carbon monoxide
(c)Chlorine
(d)Oxygen

Answer

Why

Correct — A. Helium is a noble gas, and noble gases go about as single atoms — the outermost shell is already complete, so there is nothing to be gained by pairing up.

One helium atom is therefore one molecule of helium: its atomicity is 1, which is what monoatomic means.

The other three go about as two-atom units — Cl2, O2, and the compound CO — so none of them can be the monoatomic entry, and option (a) is the one that fits.

Why the others are wrong

(b)
carbon monoxide — Carbon monoxide is diatomic — one carbon atom bonded to one oxygen atom. The mon- in monoxide counts the oxygen atoms in the formula, not the atoms in the molecule.
(c)
Chlorine — Chlorine is a diatomic element, Cl2. A single chlorine atom has seven electrons in its outer shell and pairs up at once to complete the octet.
(d)
Oxygen — Oxygen circulates as O2, two atoms sharing a double bond. Its three-atom form, ozone (O3), is triatomic, so neither version of oxygen is a single atom.

Concept

Atomicity is the number of atoms in one molecule of a substance.

The noble gases — helium, neon, argon, krypton, xenon, radon — are monoatomic, because a filled outermost shell leaves them with no tendency to bond.

Most non-metal elements are diatomic: H2, N2, O2, F2, Cl2, Br2, I2. Phosphorus goes about as P4 and sulphur as S8.

A question that says molecule does not rule out a single atom. For a noble gas, the atom is the molecule.

SSC's phrase, monoatomic molecule, unsettles candidates who expect a molecule to have at least two atoms.

In chemistry a molecule is the smallest particle of a substance that can exist independently, and for a noble gas that particle is one atom.

Key facts

Atomicity is the number of atoms in one molecule of an element: helium 1, oxygen 2, ozone 3, phosphorus 4, sulphur 8.
All the noble gases are monoatomic because their outermost shells are already complete.
Carbon monoxide is a compound of two different atoms, so it is diatomic, not monoatomic.
Chlorine and oxygen occur naturally as Cl2 and O2.

Study next

Atomicity of the common elements and of the noble gasesWhy noble gases are inert: the octet and duplet rulesA molecule of an element against a molecule of a compound

Common traps

Reading molecule as requiring two or more atoms, and rejecting helium on that ground.
Letting the mono in monoxide suggest that carbon monoxide is monoatomic.

SSC returns to the noble gases through their uses and properties rather than their names — argon in welding at 25 Sep 2024, 16:00, GA Q.10, and the red glow of neon in a discharge tube at 12 Sep 2024, 12:30, GA Q.21.

Related PYQs

No directly related past PYQ was found.