Q.Under certain circumstances, a nucleus can decay by emitting a particle more massive than an α-particle. Consider the following decay processes:
[!FORMULA]
88223Ra→82209Pb+614C
[!FORMULA]
88223Ra→86219Rn+24He
Calculate the Q-values for these decays and determine that both are energetically allowed.
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🔒 Start your 14-day free trial to unlock the full solution →Concept understanding — Nuclear Reaction Balancing
Nuclear Reaction Balancing: The Intuition
Think of a nuclear reaction like a game of atomic Lego. You start with a certain set of blocks (the reactants), and after the reaction, you end up with a different set of blocks (the products). The fundamental rule is: you cannot lose or gain any Lego pieces. You can rearrange them, break some apart, or fuse them together, but the total number of each type of piece must stay the same.
In the atomic world, the "pieces" are:
- Protons (positive charge, found in the nucleus)
- Neutrons (neutral charge, also in the nucleus)
- Energy (which can appear or disappear, but that's a separate story)
The nucleus of an atom is made of protons and neutrons. When a nuclear reaction happens, the nuclei change. But the total number of protons and the total number of neutrons must be conserved — they cannot be created or destroyed.
This is different from chemical reactions, where atoms themselves are conserved. In nuclear reactions, atoms can change into different elements, but the nucleons (protons + neutrons) are conserved.
The Precise Statement
A nuclear reaction is balanced when two quantities are equal on both sides of the reaction arrow:
- Mass number (A) — the total number of nucleons (protons + neutrons). This is the superscript number.
- Atomic number (Z) — the total number of protons. This is the subscript number.
For any nuclear reaction:
Reactant1+Reactant2→Product1+Product2+…
The balancing conditions are:
∑Areactants=∑Aproducts
∑Zreactants=∑Zproducts
Nuclear Reaction Balancing Rules
Total mass number (A) on left=Total mass number (A) on right
Total atomic number (Z) on left=Total atomic number (Z) on right
How to Write a Nuclear Equation
Every nuclear particle is written as:
ZAX
Where:
- X = chemical symbol of the element
- A = mass number (top left)
- Z = atomic number (bottom left)
Common particles you'll encounter:
| Particle | Symbol | A | Z |
|---|---|---|---|
| Alpha particle | α or 24He | 4 | 2 |
| Beta particle | β− or −10e | 0 | -1 |
| Gamma ray | γ or 00γ | 0 | 0 |
| Neutron | n or 01n | 1 | 0 |
| Proton | p or 11p | 1 | 1 |
| Positron | β+ or +10e | 0 | +1 |
A common mistake: forgetting that beta particles have Z=−1 (for β−) or Z=+1 (for β+). This is because a neutron turns into a proton (or vice versa), and the beta particle carries away the "missing" charge.
Worked Example
Problem: Balance the following alpha decay reaction:
92238U→90234Th+?
Step 1: Identify what's missing. We have an unknown particle on the right.
Step 2: Balance mass numbers (A).
Left: A=238
Right: A=234+Aunknown
So 238=234+Aunknown⟹Aunknown=4
Step 3: Balance atomic numbers (Z).
Left: Z=92
Right: Z=90+Zunknown …
Why this formula?
Why Nuclear Reaction Balancing Works
Nuclear reaction balancing rests on a single, non-negotiable principle: conservation laws are absolute. In every nuclear reaction — whether natural decay, artificial transmutation, or fission/fusion — two quantities never change:
- Total mass number (A) — the sum of protons + neutrons
- Total atomic number (Z) — the sum of protons
These aren't arbitrary rules. They follow from deeper physics: baryon number conservation (protons and neutrons are baryons, and their total count is fixed) and charge conservation (electric charge cannot be created or destroyed). A nuclear reaction is just a rearrangement of nucleons; the number of nucleons stays constant, and the total charge stays constant.
For a reaction Z1A1X+Z2A2Y→Z3A3W+Z4A4Z:
A1+A2=A3+A4
Z1+Z2=Z3+Z4
The Reasoning Behind Each Conservation Law
Mass number conservation (A conserved):
A nucleon (proton or neutron) can change identity — a neutron can beta-decay into a proton, or a proton can capture an electron and become a neutron — but it cannot vanish or appear from nothing. The total count of nucleons before the reaction equals the total count after. This is why, for example, in alpha decay:
92238U→90234Th+24He
The left side has A=238; the right side has 234+4=238. The alpha particle carries away exactly 4 nucleons.
Atomic number conservation (Z conserved):
Charge is strictly conserved. The total positive charge (proton count) before equals the total after. In the same alpha decay, Z goes from 92 to 90+2=92. If charge weren't conserved, atoms would spontaneously change their chemical identity — which never happens in a closed system.
A common mistake is to think mass number conservation means mass is conserved. It does not. Mass-energy is conserved, but the rest mass can change (and usually does, releasing energy). The mass number A is a count of nucleons, not a measure of mass in kilograms.
How to Apply It: A Worked Example
Suppose you see: 92235U+01n→56141Ba+??Kr+301n
You know the total A on the left: 235+1=236.
On the right, you have 141+AKr+3(1)=144+AKr. …
Q for each decay is the mass difference between parent and products converted to energy; the real NCERT exercise does not restate these isotope masses in its own text (it expects the standard atomic-mass appendix table), so standard nuclear-data values are used here. …
Using standard atomic mass values for Ra-223, Pb-209, C-14 and Rn-219 (this exercise's real textbook printing does not restate these masses inline — it relies on the book's own Appendix mass table, unlike most other exercises in this chapter), Q≈31.8 MeV for carbon-14 emission and Q≈5.98 MeV for ordinary alpha emission — both positive, confirming both channels are allowed, though the far larger Q for alpha decay is why it dominates in practice.
Masses used (standard nuclear data, since not given in the exercise's own printed text):
m(223Ra)≈223.018502 u,m(209Pb)≈208.981091 u
m(14C)≈14.003242 u,m(219Rn)≈219.009480 u,m(4He)=4.002603 u (given)
Channel 1: 88223Ra→82209Pb+614C
Q1=[m(223Ra)−m(209Pb)−m(14C)]×931.5
=[223.018502−208.981091−14.003242]×931.5
=0.034169×931.5=31.83 MeV
Channel 2: 88223Ra→86219Rn+24He
Q2=[m(223Ra)−m(219Rn)−m(4He)]×931.5
=[223.018502−219.009480−4.002603]×931.5 …
Showing the 12 most recent of 13 on this concept.
- TG EAPCET 2026Set eng-2026-05-10-AN1 markMCQQ.If the masses of proton and neutron are mp and mn respectively, the experimental masses of 2He4 and 8O16 nuclei are M1 and M2 respectively, then (A) M2=8(mp+mn) (B) M2<8(mp+mn) (C) M1=2(mp+mn) (D) M1>2(mp+mn)
›Reveal solutionSolution
The mass of a stable nucleus is always less than the sum of the masses of its individual protons and neutrons due to the mass defect from binding energy. For both helium-4 and oxygen-16, the experimental mass is less than the sum of the constituent nucleon masses, so the correct choice is (B).
The key concept here is mass defect and nuclear binding energy. When protons and neutrons bind together to form a nucleus, a small amount of mass is converted into energy (the binding energy) that holds the nucleus together. This means the actual mass of a stable nucleus is always less than the total mass of its separate nucleons. The question tests whether you remember this fundamental principle — not just for one nucleus, but for both.
Let’s examine each option step by step.
-
Understand the composition of each nucleus
- 2He4 has 2 protons and 2 neutrons. If there were no mass defect, its mass would be exactly 2mp+2mn=2(mp+mn).
- 8O16 has 8 protons and 8 neutrons. Without mass defect, its mass would be 8mp+8mn=8(mp+mn).
-
Apply the mass defect principle
For any stable nucleus, the experimental mass is less than the sum of the masses of its free nucleons. This is because the binding energy released during formation corresponds to a loss of mass via E=Δmc2.
- Therefore, for helium-4: M1<2(mp+mn).
- For oxygen-16: M2<8(mp+mn).
-
Evaluate the given options
- (A) M2=8(mp+mn) — This would mean no binding energy, which is false for a real nucleus.
- (B) M2<8(mp+mn) — This matches the mass defect for oxygen-16.
- (C) M1=2(mp+mn) — Again, this would imply no binding energy, false.
- (D) M1>2(mp+mn) — This would mean the nucleus has more mass than its parts, which is impossible for a stable nucleus (it would be energetically unfavorable and would decay). …
-
- TG EAPCET 2026Set ap-2026-05-04-AN1 markMCQQ.If E is the kinetic energy of the alpha particle emitted in the decay of a radioactive nucleus of mass number A, then the disintegration energy released in the process is (Parent nucleus is at rest) (A) A(A+4)E (B) A−4AE (C) A(A−4)E (D) A−44E
›Reveal solutionSolution
In alpha decay, the disintegration energy (Q-value) is shared between the alpha particle and the recoiling daughter nucleus as kinetic energy. Using conservation of momentum and energy, the Q-value is found to be A−4AE, which corresponds to option (B).
The key idea here is that when a parent nucleus at rest emits an alpha particle, the daughter nucleus recoils to conserve momentum. The kinetic energy you measure for the alpha particle is only part of the total energy released — the rest goes into the recoil of the daughter. So the disintegration energy (often called the Q-value) is the sum of both kinetic energies.
Let’s work through why this leads to the formula in option (B).
-
Set up the decay process.
A parent nucleus of mass number A decays by emitting an alpha particle (mass number 4). The daughter nucleus therefore has mass number A−4. The parent is initially at rest, so total momentum is zero.
-
Apply conservation of momentum.
Let mα be the mass of the alpha particle and md the mass of the daughter. Let vα and vd be their speeds after decay. Since initial momentum is zero:
mαvα=mdvd
This tells us the daughter recoils in the opposite direction with a speed inversely proportional to its mass.
- Relate kinetic energies. The kinetic energy of the alpha particle is E=21mαvα2. The kinetic energy of the daughter is Ed=21mdvd2. From momentum conservation, vd=mdmαvα, so:
Ed=21md(mdmαvα)2=21mdmα2vα2=mdmα⋅21mαvα2=mdmαE
- Express masses in terms of mass numbers. For nuclei, mass is approximately proportional to mass number (since nucleon masses are nearly equal). So mα∝4 and md∝A−4. Thus: …
-
- TG EAPCET 2025Set eng-2025-05-03-AN1 markMCQQ.The range of weak nuclear force is of the order of (A) 1016 m (B) 10−10 m (C) 1010 m (D) 10−16 m
›Reveal solutionSolution
The weak nuclear force has an extremely short range, roughly 10−18 m, which is closest to 10−16 m among the given options. The correct choice is (D).
The weak nuclear force is one of the four fundamental forces of nature. Unlike gravity or electromagnetism, which have infinite range, the weak force acts only over subatomic distances. This is because it is mediated by very massive particles — the W and Z bosons — whose mass limits how far the force can be felt.
Why this approach works: The range of a force mediated by a massive particle is inversely proportional to the particle’s mass, given by the uncertainty principle. For the weak force, the mediator mass is about 80–90 GeV/c², which yields a range on the order of 10−18 m. Comparing this to the options, only 10−16 m is in the right ballpark (the others are astronomically larger or atomic-scale).
-
Recall the relation between range and mediator mass
In quantum field theory, the range R of a force is roughly R≈mcℏ, where m is the mass of the force-carrying particle. This comes from the Heisenberg uncertainty principle: a virtual particle of mass m can only exist for a time Δt∼ℏ/(mc2), and thus travels at most a distance cΔt∼ℏ/(mc).
-
Plug in the weak boson mass
The W and Z bosons have masses around 80–90 GeV/c². Using ℏc≈197 MeV·fm (where 1 fm = 10−15 m), we get:
R≈80000 MeV197 MeV⋅fm≈0.0025 fm=2.5×10−18 m.
- Compare with the options
- (A) 1016 m — larger than the solar system; impossible. …
-
- TG EAPCET 2025Set eng-2025-05-03-FN1 markMCQQ.The range of weak nuclear force is of the order of (A) 1016 m (B) 10−10 m (C) 1010 m (D) 10−16 m
›Reveal solutionSolution
The weak nuclear force has an extremely short range, roughly 10−18 m, which is much smaller than the nuclear scale; the closest option given is 10−16 m, so the answer is (D).
The weak nuclear force is one of the four fundamental forces of nature. Unlike gravity or electromagnetism, which have infinite range, the weak force acts only over subatomic distances. Why? Because it is mediated by very massive particles — the W and Z bosons — whose mass limits how far they can travel before being absorbed. The range of a force mediated by a massive particle is roughly given by the Compton wavelength of that particle:
R≈mcℏ
where m is the mass of the mediator. For the W and Z bosons, m≈80–90 GeV/c2, which gives a range of about 10−18 m. That’s about a thousandth the size of a proton’s diameter (∼10−15 m). So the weak force is indeed very short-ranged.
Now let’s match this to the options:
-
Option (A): 1016 m — This is a light-year scale, far larger than the solar system. No fundamental force except gravity has such a range, and even gravity doesn’t “act” weakly at that scale in the sense of particle physics. Clearly wrong.
-
Option (B): 10−10 m — This is the size of an atom (the Bohr radius). The electromagnetic force dominates here; the weak force is negligible at this distance. Wrong.
-
Option (C): 1010 m — This is about the distance from Earth to the Sun. Again, not relevant for a subnuclear force. Wrong. …
-
- TG EAPCET 2025Set eng-2025-05-04-AN1 markMCQQ.When an element X90232X2902232Th decays into X82208X2822208Pb, the number of α and β− particles emitted respectively are (A) 4, 8 (B) 8, 2 (C) 6, 2 (D) 6, 4
›Reveal solutionSolution
The key is to track changes in mass number (A) and atomic number (Z) separately: each α reduces A by 4 and Z by 2; each β⁻ increases Z by 1 without changing A. Solving gives 6 α and 4 β⁻, so the answer is (D).
Concept & Intuition
When a heavy nucleus decays to a lighter one, it can emit alpha particles (⁴₂He, which lowers both mass and charge) and beta-minus particles (an electron, which raises the atomic number by converting a neutron to a proton). The mass number only changes via α emission, while the atomic number changes via both α and β⁻. So we can first find the number of α particles from the mass difference, then use the atomic number difference to find the number of β⁻.
- Find the number of α particles from the change in mass number. Thorium-232 has mass number A=232; lead-208 has A=208. Each α particle reduces the mass number by 4. Let x = number of α particles.
232−4x=208⇒4x=24⇒x=6.
So 6 α particles are emitted.
- Find the net change in atomic number from the α emissions alone. Thorium has Z=90; each α reduces Z by 2. After 6 α emissions, the atomic number would be:
90−6×2=90−12=78.
But lead has Z=82. So we need to increase the atomic number from 78 to 82 — a gain of 4.
- Determine the number of β⁻ particles. …
- TG EAPCET 2025Set ap-2025-04-29-AN1 markMCQQ.In a nuclear fusion reaction, if the mass defect is 0.25%, then the energy released in the fusion of 400 μg mass of a substance is (A) 9×107 J (B) 9×1010 J (C) 4.5×107 J (D) 4.5×1010 J
›Reveal solutionSolution
Only 0.25% of the 400μg is converted to energy: Δm=1μg=10−9 kg. Then E=Δmc2=9×107 J (option A).
Concept. In fusion the mass defect Δm is the mass that disappears and reappears as energy, E=Δmc2. Here the mass defect is 0.25% of the reacting mass.
Step 1 - Mass converted.
Δm=1000.25×400μg=1μg.
In SI units, 1μg=10−6 g=10−9 kg.
Step 2 - Energy released.
With c=3×108 m/s: …
- TG EAPCET 2024Set ap-2024-05-08-FN1 markMCQQ.If the nucleus X240, initially at rest, splits into two daughter nuclei Y100 and Z140, then the ratio of the kinetic energies of Y and Z is (A) 5:7 (B) 7:5 (C) 1:1 (D) 49:25
›Reveal solutionSolution
When a nucleus at rest splits into two fragments, their momenta are equal in magnitude and opposite in direction due to conservation of momentum. This leads to their kinetic energies being inversely proportional to their masses, so the ratio of kinetic energies of Y and Z is 7:5.
When a heavy nucleus undergoes fission, it splits into two or more lighter nuclei. A fundamental principle governing such processes, especially when the initial system is isolated and at rest, is the conservation of linear momentum. This principle dictates how the resulting fragments will move.
Here's the core idea: If the parent nucleus is initially at rest, its total momentum is zero. After it splits, the total momentum of the daughter nuclei must still be zero. This means the two daughter nuclei must move in opposite directions with momenta of equal magnitude.
The relationship between kinetic energy (K), momentum (p), and mass (m) is given by K=2mp2. Since the magnitudes of the momenta of the two daughter nuclei are equal, their kinetic energies will be inversely proportional to their masses. This inverse relationship is key to solving the problem.
-
Apply the Principle of Conservation of Linear Momentum:
The nucleus X240 is initially at rest. This means its initial momentum is zero.
The total initial momentum Pinitial=0.
When it splits into two daughter nuclei, Y100 and Z140, let their momenta be pY and pZ respectively. According to the conservation of linear momentum, the total momentum after the split must also be zero.
pY+pZ=Pinitial=0
pY=−pZ
This equation tells us two things: * The two daughter nuclei move in opposite directions. * The magnitudes of their momenta are equal: $|p_Y| = |p_Z|$. Let's denote this common magnitude as $p$.2. Relate Kinetic Energy to Momentum and Mass:
The kinetic energy K of a particle with mass m and momentum p is given by:
K=21mv2
Since momentum $p = mv$, we can express velocity as $v = p/m$. Substituting this into the kinetic energy formula:K=21m(mp)2=21mm2p2
> [!FORMULA] > $$ K = \frac{p^2}{2m} $$ This form is particularly useful when dealing with problems involving momentum conservation. … -
- TG EAPCET 2023Set ap-2023-05-10-FN1 markMCQQ.Which of the following statements regarding nuclear forces is true? (A) The coulomb force between two charges is much stronger than the nuclear force (B) The nuclear force between two nucleons falls rapidly to zero with the increase of the distance between them (C) The nuclear force between neutron-neutron is much stronger than the nuclear force between proton-proton (D) The nuclear force between two charged particles depends on the charges of the particles
›Reveal solutionSolution
Nuclear forces are short-range, charge-independent, and vastly stronger than the Coulomb force at nuclear distances — so only the statement about rapid fall-off with distance is correct.
The key to this question is understanding what nuclear forces actually are. They are the forces that hold protons and neutrons (collectively called nucleons) together inside the nucleus. Without them, the positively charged protons would fly apart due to electrostatic repulsion. So right away, you can sense that nuclear forces must be much stronger than the Coulomb force at those tiny distances — otherwise, no nucleus could exist.
Let’s examine each statement one by one.
-
Statement (A): The Coulomb force between two charges is much stronger than the nuclear force.
This is false. At the scale of the nucleus (about 10−15 m), the nuclear force is about 100 times stronger than the electromagnetic repulsion between two protons. That’s why the nucleus stays bound despite the repulsion. The Coulomb force only becomes dominant at larger distances, where the nuclear force has already died out.
-
Statement (B): The nuclear force between two nucleons falls rapidly to zero with the increase of the distance between them.
This is true. Nuclear forces have an extremely short range — roughly 1 to 2 femtometers (10−15 m). Beyond that, the force drops off so steeply that it is effectively zero. This is why nuclei have a finite size and why nucleons only interact with their nearest neighbours inside the nucleus.
-
Statement (C): The nuclear force between neutron-neutron is much stronger than the nuclear force between proton-proton. …
-
- TG EAPCET 2023Set eng-2023-05-12-AN1 markMCQQ.Nuclear fission and fusion can be explained on the basis of (A) Einstein’s theory of relativity (B) Einstein specific heat equation (C) Einstein mass-energy equation (D) Einstein photoelectric equation
›Reveal solutionSolution
The key idea is that both nuclear fission and fusion involve a change in mass, which is converted into energy according to Einstein’s mass-energy equivalence. The correct option is (C).
Nuclear fission and fusion are processes where the nucleus of an atom changes, releasing enormous amounts of energy. To understand why option (C) is correct, we need to recall what each of Einstein’s equations describes and which one directly explains the energy release in nuclear reactions.
-
Identify the core phenomenon
In both fission (splitting a heavy nucleus) and fusion (combining light nuclei), the total mass of the products is slightly less than the total mass of the reactants. This “missing” mass is called the mass defect. The energy released comes from this mass defect.
-
Match the phenomenon to the correct equation
Einstein’s mass-energy equation is E=mc2. It states that mass and energy are interchangeable; a small amount of mass can be converted into a huge amount of energy (because c2 is enormous). This directly explains why the mass defect in nuclear reactions yields the observed energy output.
-
Eliminate the other options
- (A) Einstein’s theory of relativity is a broader framework (including special and general relativity). While E=mc2 is part of it, the question asks for the specific equation that explains the energy release, not the entire theory.
- (B) Einstein’s specific heat equation deals with how the heat capacity of solids varies with temperature (quantum theory of solids). It has nothing to do with nuclear reactions.
- (D) Einstein’s photoelectric equation explains the emission of electrons from a metal surface when light shines on it (E=hf−ϕ). It is about light and electrons, not nuclear mass-energy conversion.
-
Confirm with a classic example …
-
- TG EAPCET 2023Set eng-2023-05-12-FN1 markMCQQ.If S is the surface area of a nucleus of mass number A, then (A) S∝A (B) S∝A1/3 (C) S∝A2 (D) S∝A2/3
›Reveal solutionSolution
The surface area of a nucleus scales with the two-thirds power of its mass number because nuclear volume is proportional to A and the radius scales as A^{1/3}, making surface area proportional to (radius)² ∝ A^{2/3}.
The key idea is that a nucleus is roughly spherical, and its volume is proportional to the number of nucleons (mass number A). Since volume scales as the cube of the radius, the radius scales as A^{1/3}. Surface area, being proportional to the square of the radius, therefore scales as (A^{1/3})² = A^{2/3}.
Let’s walk through the reasoning step by step.
- Nuclear volume is proportional to mass number. A nucleus contains A nucleons (protons and neutrons). Since nucleons are nearly identical in size and the nucleus is densely packed, the total volume V is directly proportional to A:
V∝A.
- For a sphere, volume relates to radius as V ∝ R³. Assuming the nucleus is spherical (a very good approximation), the volume is
V=34πR3⇒R3∝V.
Combining with step 1 gives
R3∝A⇒R∝A1/3.
- Surface area of a sphere is S = 4πR². Since S ∝ R², substituting the relation from step 2 yields
S∝(A1/3)2=A2/3.
- Check the options.
- (A) S ∝ A would mean surface area grows linearly with volume — impossible for a sphere (that would require a constant radius). …
- TG EAPCET 2023Set eng-2023-05-14-FN1 markMCQQ.The ratio of the relative strengths of strong and weak nuclear forces is (A) 1013 (B) 1026 (C) 1039 (D) 1011
›Reveal solutionSolution
The relative strengths of the strong and weak nuclear forces are compared by their coupling constants at typical energy scales; the strong force is about 1013 times stronger than the weak force, so the correct answer is (A).
The key idea here is that fundamental forces have intrinsic "strengths" given by dimensionless coupling constants. For the strong nuclear force, the coupling constant αs≈1 at low energies, while for the weak nuclear force, the effective coupling constant is αw≈10−5 (or, more precisely, the Fermi constant gives a strength ratio of about 1013). This huge difference explains why the strong force dominates inside the nucleus, while the weak force only manifests in rare processes like beta decay.
Let’s work through the reasoning step by step:
-
Understand what "relative strength" means.
In particle physics, the strength of a force is quantified by its coupling constant — a dimensionless number that determines the probability of interaction. For the strong force, the coupling αs is roughly 1 at the scale of a few femtometers (the size of a nucleus). For the weak force, the effective coupling at low energies is given by the Fermi constant GF times the energy squared, but a more direct comparison uses the weak coupling constant αw≈1/30 at the mass of the W boson. However, because the weak force is mediated by very heavy particles (W and Z bosons), its effective strength at nuclear distances is much smaller.
-
Compare the forces at typical nuclear energies.
The strong force’s coupling is αs≈1. The weak force’s effective coupling at low energies is about 10−5 (this comes from αw≈1/30 multiplied by a factor (mp/mW)2≈(1/80)2≈1.6×10−4, giving roughly 10−5). The ratio of strong to weak is then 1/10−5=105. But wait — this is not the number in the options. The standard textbook comparison uses a different convention: the weak force’s strength is often quoted as 10−13 relative to the strong force, meaning the strong force is 1013 times stronger.
-
Why 1013 and not 105? …
-
- TG EAPCET 2022Set ap-2022-07-31-FN1 markMCQQ.The ratio of the radii of the nuclei 13Al27 and 52Te125 is (A) 3:5 (B) 27:125 (C) 13:52 (D) 14:73
›Reveal solutionSolution
The nuclear radius depends on the mass number as R=R0A1/3, so the ratio of radii is the cube root of the ratio of mass numbers. For 27Al and 125Te, this gives 3:5, which is option (A).
The key concept here is the nuclear radius formula. Unlike atomic radii, which vary in a complicated way across the periodic table, nuclear radii follow a remarkably simple rule: the volume of a nucleus is proportional to the number of nucleons (protons + neutrons). Since the nucleus is roughly spherical, its radius R is proportional to A1/3, where A is the mass number. This is one of the earliest and most important empirical results in nuclear physics.
Why does this work? Because nucleons are packed together at nearly constant density — like marbles in a bag. Double the number of marbles, you roughly double the volume, so the radius increases by the cube root of 2. So when comparing two nuclei, we don't compare their mass numbers directly; we compare the cube roots of those numbers.
Let’s work through it step by step.
-
Write the nuclear radius formula.
For any nucleus, R=R0A1/3, where R0 is a constant (about 1.2×10−15 m). The constant cancels out when taking a ratio.
-
Identify the mass numbers.
For 13Al27, the mass number A1=27.
For 52Te125, the mass number A2=125.
-
Set up the ratio of radii.
R2R1=R0(125)1/3R0(27)1/3=(12527)1/3 …
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