Assertion (A) : Nuclear forces are always attractive. Reason (R) : The nuclear force between protons and neutr

Question

Q. Assertion (A) : Nuclear forces are always attractive. Reason (R) : The nuclear force between protons and neutrons in a nucleus is a weak force. (A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A). (B) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A). (C) Assertion (A) is true, but Reason (R) is false. (D) Both Assertion (A) and Reason (R) are false.

CBSECBSE Class XII Board 2026MCQ· 1mImportance★★★★★

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The assertion that nuclear forces are always attractive is false (they are repulsive at very short range), and the reason that the nuclear force is weak is also false (it is the strongest known force). Therefore both statements are false, and the correct option is (D).

Why this question tests two separate ideas

This is a classic "Assertion–Reason" problem from Indian board exams. It checks two things:

Whether you know the true nature of the nuclear force (range, sign, strength).
Whether you can spot a wrong reason even if the assertion were true.

Let’s break it down cleanly.

1. Is the assertion true? — "Nuclear forces are always attractive."

No. The nuclear force (strong force) between nucleons has a very short range (~1–2 fm). At distances around 0.8–1.5 fm, it is strongly attractive — that’s what binds protons and neutrons together. But if two nucleons get too close (below ~0.5 fm), the force becomes strongly repulsive. This repulsive core prevents the nucleus from collapsing.

Watch out

A common mistake is to think "nuclear force = always attractive" because we only see its binding effect. In reality, it has a repulsive core — like a spring that pushes back when compressed too much.

So Assertion (A) is false.

2. Is the reason true? — "The nuclear force between protons and neutrons is a weak force."

Absolutely false. The nuclear force is the strongest fundamental force in nature — about 100 times stronger than electromagnetism at nuclear scales. The "weak force" is a completely different interaction (responsible for beta decay), with a strength about $10^{-6}$ times that of the strong force.

Relative strengths of fundamental forces (approximate): Force Relative strength Strong nuclear 1

Force	Relative strength
Strong nuclear	1

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Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

CBSE 2026Set 55/3/11 markMCQ
$Q. Assertion (A) : Nuclear forces are always attractive. Reason (R) : The nuclear force between protons and neutrons in a nucleus is a weak force. (A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A). (B) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A). (C) Assertion (A) is true, but Reason (R) is false. (D) Both Assertion (A) and Reason (R) are false.$
›Reveal solutionSolution
The assertion that nuclear forces are always attractive is false (they are repulsive at very short range), and the reason that the nuclear force is weak is also false (it is the strongest known force). Therefore both statements are false, and the correct option is (D).

Why this question tests two separate ideas

This is a classic "Assertion–Reason" problem from Indian board exams. It checks two things:

Whether you know the true nature of the nuclear force (range, sign, strength).

Whether you can spot a wrong reason even if the assertion were true.

Let’s break it down cleanly.

1. Is the assertion true? — "Nuclear forces are always attractive."

No. The nuclear force (strong force) between nucleons has a very short range (~1–2 fm). At distances around 0.8–1.5 fm, it is strongly attractive — that’s what binds protons and neutrons together. But if two nucleons get too close (below ~0.5 fm), the force becomes strongly repulsive. This repulsive core prevents the nucleus from collapsing.

Watch out

A common mistake is to think "nuclear force = always attractive" because we only see its binding effect. In reality, it has a repulsive core — like a spring that pushes back when compressed too much.

So Assertion (A) is false.

2. Is the reason true? — "The nuclear force between protons and neutrons is a weak force."

Absolutely false. The nuclear force is the strongest fundamental force in nature — about 100 times stronger than electromagnetism at nuclear scales. The "weak force" is a completely different interaction (responsible for beta decay), with a strength about $10^{-6}$ times that of the strong force.
$Relative strengths of fundamental forces (approximate): Force Relative strength Strong nuclear 1$
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CBSE 2025Set 55/6/11 markMCQ
$F_{pp}$
›Reveal solutionSolution
The strong nuclear force is charge-independent: it acts equally between any pair of nucleons (proton or neutron) at the same separation, so $F_{pp} = F_{pn} = F_{nn}$ .

Why the nuclear force doesn't care about charge

The strong nuclear force—the interaction that binds protons and neutrons together in the nucleus—has a remarkable property discovered through decades of scattering experiments and nuclear structure studies: it is charge-independent. This means the force between two nucleons depends only on their separation and spin alignment, not on whether they happen to be protons or neutrons.

This might seem counterintuitive at first. After all, two protons also experience electromagnetic repulsion because both carry positive charge, while a neutron-neutron pair feels no such repulsion. But the nuclear force itself—the residual strong interaction mediated by pion exchange at the quark level—treats all nucleons identically.

Think of it this way: protons and neutrons are not fundamentally different particles to the strong force. They are two states (isospin states) of the same entity, the nucleon. The strong force is "blind" to this distinction.

Step-by-step reasoning

The nature of the strong force

The nuclear force arises from the exchange of mesons (primarily pions) between nucleons. At a deeper level, it's a residual effect of the color force binding quarks inside each nucleon. This interaction depends on the distance between nucleons and their quantum numbers (spin, isospin), but crucially, not on electric charge.

Experimental evidence

Scattering experiments comparing proton-proton, proton-neutron, and neutron-neutron interactions (after accounting for electromagnetic effects in the $pp$ case) show that the strong force component is the same in all three. The binding energies of mirror nuclei—pairs that differ by swapping protons for neutrons—confirm this charge independence.

What about the Coulomb repulsion?

Two protons do experience electromagnetic repulsion in addition to the nuclear attraction. However, the question asks specifically about the nuclear force $F_{pp}$ , $F_{pn}$ , $F_{nn}$ —not the total force. The nuclear component alone is identical in all three cases.

Isospin symmetry …
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CBSE 2025Set ANNUAL1 markMCQ
$Q. Heavy stable nuclei have more neutrons than protons. This because of the fact that:$
(a) neutrons are heavier than protons
(b) electrostatic force between protons are repulsive
(c) neutrons decay into protons through beta decay
(d) nuclear forces between neutrons are weaker than that between protons.
›Reveal solutionSolution

Extra neutrons add to the short-range attractive nuclear force without adding to the long-range repulsive Coulomb force between protons, helping keep heavy nuclei bound.

The strong nuclear force is attractive between all nucleons (n-n, p-p, n-p) and is short-ranged, while the electrostatic (Coulomb) force between protons is repulsive and long-ranged, so it accumulates faster as the number of protons Z grows. In heavy nuclei, this growing Coulomb repulsion tends to destabilise the nucleus. Adding extra neutrons increases the total attractive nuclear force (more nucleon pairs bound by the strong force) without adding any extra Coulomb repulsion, since neutrons are uncharged. This is why heavy stabl …

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CBSE 2024Set A11 markMCQ
$Q. The nuclear force is$
(a) attractive for distance $r = 0.5\ fm$
(b) attractive for distance $r < 0.8\ fm$
(c) repulsive for distance $r > 0.8\ fm$
(d) repulsive for distance $r < 0.8\ fm$
›Reveal solutionSolution

(d) repulsive for distance $r < 0.8\ fm$ . …

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CBSE 2024Set ANNUAL1 markMCQ
$Q. Assertion (A): Nuclear binding force dominates over the Coulomb repulsive force between the protons inside the nucleus. Reason (R): Nuclear forces are much stronger than Coulomb forces.$
(a) Both A and R are correct and R is the correct explanation of A.
(b) Both A and R are correct but R is not the correct explanation of A.
(c) A is correct but R is incorrect.
(d) Both A and R are incorrect.
›Reveal solutionSolution

Both statements are true, and the greater strength of the nuclear force over the short range is exactly why it can overpower Coulomb repulsion.

Assertion (A) is correct: inside a stable nucleus, the attractive nuclear (strong) binding force between nucleons dominates over the electrostatic Coulomb repulsion between protons -- this is why nuclei with more than one proton remain bound. Reason (R) is also correct: nuclear forces are indeed much stronger (roughly 100 times stronger at s …

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CBSE 2024Set ANNUAL1 markMCQ
$Q. The nature of nuclear force is$
(a) electric charge dependent
(b) spin-dependent
(c) long range
(d) unsaturated
›Reveal solutionSolution
The nuclear force is short-range, charge-independent, saturating, and spin-dependent — of the given options only spin-dependence is a genuine property.

The strong nuclear force that binds protons and neutrons inside a nucleus has several well-established characteristics:

It is the strongest known force in nature at nuclear distances (about 100 times the electromagnetic force).

It is SHORT-RANGE: attractive for separations of about 1–2 fm, falling to zero beyond about 10 fm — it is NOT long-range like gravity or the electrostatic force.

It is CHARGE-INDEPENDENT: the force between p–p, n–n, and p–n is (nearly) the same, so it does not depend on electric charge.

It is SATURATED: a nucleon interacts strongly with only its nearest neighbours, not with every other nucleon in the nucleus — this is why the binding energy per nucleon becomes roughly constant for larger nuclei rather than growing indefinitely. …
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CBSE 2023Set ANNUAL1 markMCQ
$Q. The nature of strong nuclear force is$
(a) repulsive and long-range
(b) repulsive and short-range
(c) attractive and long-range
(d) attractive and short-range
›Reveal solutionSolution
The strong nuclear force that binds nucleons together is attractive in nature and has an extremely short range, of the order of a few femtometres.

The nuclear force holding protons and neutrons together in the nucleus is the strongest of the fundamental forces. Its key features are:

It is attractive for distances greater than about 0.8 fm (it becomes strongly repulsive at very short distances, preventing nucleon overlap, but overall it must be attractive to bind the nucleus). …
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CBSE 2022Set ANNUAL1 markMCQ
$Q. The nuclear force between the two nucleons is the result of the exchange of:$
(a) neutrino
(b) π-meson
(c) photon
(d) μ-meson
›Reveal solutionSolution

Yukawa's meson theory explains the short-range, strong nuclear force between nucleons as arising from the exchange of π-mesons (pions).

The strong nuclear force that binds protons and neutrons together in a nucleus is fundamentally different from the electromagnetic and gravitational forces. Hideki Yukawa proposed that this force results from nucleons continuously exchanging virtual particles called π-mesons (pions), analogous to how the electromagnetic force between charges can be described in terms of photon exchange.

…

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Why this question tests two separate ideas

1. Is the assertion true? — "Nuclear forces are always attractive."

2. Is the reason true? — "The nuclear force between protons and neutrons is a weak force."

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