The mass number of argon is 40. Which one of the following statements is correct?

Q: What is the answer to NDA (I) 2022 GAT Q62?

C. Correct — C, the number of electrons in argon is 18. Argon has atomic number 18, so a neutral argon atom has 18 protons and 18 electrons. With a mass number of 40, the number of neutrons is 40 − 18 = 22. Therefore the statement that argon has 18 electrons is the correct one.

(a)The number of protons in argon is 22.
(b)The number of neutrons in argon is 18.
(c)The number of electrons in argon is 18.
(d)The sum of numbers of protons and electrons in argon is 40.

Answer

Why

Correct — C, the number of electrons in argon is 18. Argon has atomic number 18, so a neutral argon atom has 18 protons and 18 electrons. With a mass number of 40, the number of neutrons is 40 − 18 = 22. Therefore the statement that argon has 18 electrons is the correct one.

Why the others are wrong

(a)
The number of protons in argon is 22. — The number of protons equals the atomic number, which is 18 for argon; 22 is actually the neutron count.
(b)
The number of neutrons in argon is 18. — Neutrons = mass number − atomic number = 40 − 18 = 22, not 18 (18 is the number of protons or electrons).
(d)
The sum of numbers of protons and electrons in argon is 40. — Protons (18) + electrons (18) = 36, not 40; the value 40 is the mass number, which is protons + neutrons.

Concept

The atomic number of an element equals its number of protons, and in a neutral atom the number of electrons is the same. The mass number equals protons plus neutrons, so the number of neutrons is the mass number minus the atomic number. Argon's atomic number is 18.

Argon (Z = 18) has 18 protons and 18 electrons; with mass number 40 it has 40 − 18 = 22 neutrons. Testing each statement against these values leaves only the electron count (18) correct.

Key facts

Atomic number = number of protons = number of electrons in a neutral atom; for argon it is 18.
Mass number = protons + neutrons; for this argon atom it is 40.
Number of neutrons = mass number − atomic number = 40 − 18 = 22.
Argon is a noble gas with a stable, fully filled outer shell (configuration 2, 8, 8).

Sub-atomic particle counts for argon (mass number 40)

Atomic number (protons = electrons)→18

Mass number (protons + neutrons)→40

Neutrons→40 − 18 = 22

A neutral argon atom has 18 electrons — option (c).

Study next

Atomic number and mass numberProtons, neutrons and electronsIsotopesElectronic configuration and the noble gasesThe modern periodic table

Common traps

Swapping the proton count (18) with the neutron count (22).
Thinking the mass number equals protons + electrons instead of protons + neutrons.

Compute the proton, neutron and electron counts from the atomic and mass numbers, then test each statement for the true one.

Related PYQs

NDA_GAT_2024_II_Q1182024Open & attempt →

For Aluminium (Al) (atomic number: 13, mass number: 27), which one among the following statements is NOT correct?

(a) Number of electrons present in Al is 13
(b) Number of protons present in Al is 13
(c) Number of neutrons present in Al is 14
(d) The valency of Al is 2

Answer(d) The valency of Al is 2

Same concept — reading proton, neutron and electron counts from the atomic and mass numbers. That NDA item does it for aluminium (Z = 13, A = 27, so 14 neutrons); this one does it for argon (Z = 18, A = 40).

Practice

practice — not a real PYQ
An atom has 11 protons and a mass number of 23. The number of neutrons in it is
- (a)11
- (b)12
- (c)23
- (d)34
Answer(b) 12 — neutrons = mass number − protons = 23 − 11.
practice — not a real PYQ
In a neutral atom, the number of electrons is equal to the number of
- (a)neutrons
- (b)protons
- (c)nucleons
- (d)isotopes
Answer(b) protons.