Which one of the following statements is correct?

Q: What is the answer to NDA (I) 2025 GAT Q72?

B. Correct — B, the electron's mass. An electron's mass — about 1/1836 of a proton's — is roughly 1/2000 that of a proton, so it is nearly negligible next to the nucleons. Statement (b) is therefore the one that is correct. (Provisional — pending the official UPSC key.)

(a)A neutron is formed by combination of an electron and a proton. Therefore, it is neutral
(b)The mass of an electron is about 1/2000 times that of a proton
(c)An isotope of cobalt is used in the treatment of goiter
(d)J. J. Thomson proposed that the nucleus of an atom contains only neutrons

Answer

Why

Correct — B, the electron's mass. An electron's mass — about 1/1836 of a proton's — is roughly 1/2000 that of a proton, so it is nearly negligible next to the nucleons. Statement (b) is therefore the one that is correct.

Why the others are wrong

(a)
A neutron is formed by combination of an electron and a proton. Therefore, it is neutral — A neutron is a fundamental nucleon, not literally an electron stuck onto a proton; describing it that way is incorrect, even though its net charge is zero.
(c)
An isotope of cobalt is used in the treatment of goiter — Cobalt-60 is used to treat cancer by radiotherapy, not goitre; goitre and thyroid disorders are treated with Iodine-131.
(d)
J. J. Thomson proposed that the nucleus of an atom contains only neutrons — Thomson proposed the plum-pudding model — electrons embedded in a positive sphere; he never said the nucleus holds only neutrons, and the neutron was found only later, by Chadwick in 1932.

Concept

An atom has a tiny dense nucleus of protons and neutrons, with electrons around it. A proton and a neutron each weigh about one atomic mass unit, while the electron is roughly 1836 times lighter.

Each wrong option hides a familiar mistake — a false model of the neutron, a swapped medical isotope, or a mis-attributed atomic model. Only the electron-mass figure survives scrutiny.

Key facts

An electron's mass is about 1/1836 (roughly 1/2000) that of a proton.
A proton and a neutron each have a mass of about one atomic mass unit.
Cobalt-60 is used in cancer radiotherapy; Iodine-131 is used for the thyroid and goitre.
J. J. Thomson discovered the electron in 1897 and gave the plum-pudding model.

Portrait photograph of physicist J. J. Thomson, discoverer of the electron, in spectacles and moustache — Thomson discovered the electron in 1897 and proposed the plum-pudding model; an electron's mass is about 1/2000 that of a proton.Image: Bain News Service · Wikimedia Commons (Public domain)

Study next

Sub-atomic particles — electron, proton and neutron, and their masses and chargesAtomic models — Thomson, Rutherford and BohrRadioisotopes in medicine — Cobalt-60 and Iodine-131Isotopes and relative atomic massDiscovery of the electron by Thomson and the neutron by Chadwick

Common traps

Confusing Cobalt-60 (cancer) with Iodine-131 (thyroid and goitre).
Treating the neutron as a bound electron-proton pair.

A 'which statement is correct' item mixing particle masses, medical isotopes and atomic-model attributions.

Related PYQs

NDA_GAT_2024_I_Q642024Open & attempt →

The relative atomic mass of boron (which exists in two isotopic forms 10B and 11B) is 10·81. What will be the abundance of 10B and 11B, respectively (consider a sample of 100 atoms) ?

(a) 19% and 81%
(b) 81% and 19%
(c) 38% and 62%
(d) 62% and 38%

Answer(a) 19% and 81%

Same atomic-structure family — isotopes and relative atomic mass. It connects to statement (c) here about a cobalt isotope, and both rest on understanding masses of atoms and their particles.

Practice

practice — not a real PYQ
Which radioisotope is commonly used in the treatment of goitre and other thyroid disorders?
- (a)Cobalt-60
- (b)Iodine-131
- (c)Carbon-14
- (d)Uranium-235
Answer(b) Iodine-131 — it is taken up by the thyroid gland.
practice — not a real PYQ
The mass of a proton is approximately how many times that of an electron?
- (a)200
- (b)1836
- (c)92
- (d)6
Answer(b) About 1836 times.