Q.7 Physics ⭐ 360° Option Elimination Active

NEET 2026 Physics Question 7 Solution

Topic: Nuclear Physics · Official NTA Booklet Code 50

Question

Consider the following nuclear reaction: ²³⁸U → ²³⁴Th + ⁴He. Take masses of ²³⁸U, ²³⁴Th and ⁴He as 238.050 u, 234.043 u and 4.003 u, respectively. The Q value for the reaction, in keV, is: [Given: 1 u = 931.5 MeV c⁻²]

✓

Verified Against NTA Final Answer Key

Option (1) : Q = 3726 keV

📚 Nuclear Physics Easy

📋 Given Parameters & Problem Setup

Nuclear Reaction ^{238}U -> ^{234}Th + ^4He

Mass of ^{238}U 238.050 u

Mass of ^{234}Th 234.043 u

Mass of ^4He 4.003 u

Mass-Energy Equivalence 1 u = 931.5 MeV/c^2

Target Unit Q value in keV (kiloelectron-volts)

⚡

The 30-Second Exam Hall Shortcut: ⚡ Mass Defect in Milliatomic Units

⏱️ Eliminates unit conversion confusion between MeV and keV.

Compute mass defect in thousandths of u and convert directly to keV

1 Total product mass = 234.043 + 4.003 = 238.046 u.

2 Mass defect Delta m = 238.050 - 238.046 = 0.004 u (4 milli-u).

3 1 u = 931.5 MeV = 931,500 keV.

4 Q = 0.004 * 931,500 keV = 4 * 931.5 = 3726 keV. Done in 20 seconds!

🛡️

Why You Can 100% Trust This Shortcut: Conservation of mass-energy: Q = [m(parent) - sum(m(daughters))] * c^2. Since 1 u = 931.5 MeV, multiplying by 10^3 gives keV directly: 0.004 * 931.5 * 1000 = 3726 keV.

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Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Calculate Total Mass of Decay Products

Sum the masses of the thorium daughter nucleus and the alpha particle:

m_{\text{products}} = m(^{234}\text{Th}) + m(^4\text{He}) = 234.043\text{ u} + 4.003\text{ u} = 238.046\text{ u}

• Carry all decimal places carefully: 234.043 + 4.003 = 238.046 u.

Step 2

Calculate the Mass Defect (Delta m)

Subtract product mass from the parent uranium mass:

\Delta m = m(^{238}\text{U}) - m_{\text{products}} = 238.050\text{ u} - 238.046\text{ u} = 0.004\text{ u}

• Positive mass defect confirms the alpha decay is energetically spontaneous (exothermic).

Step 3

Convert Mass Defect to Energy in keV

Multiply mass defect by energy equivalent per atomic mass unit:

Q = \Delta m \times 931.5\text{ MeV} = 0.004 \times 931.5 = 3.726\text{ MeV}

• Convert MeV to keV: 1 MeV = 1000 keV.
• Q = 3.726 * 1000 keV = 3726 keV.
• Matches Option (1) exactly.

⭐

360° Option Elimination Engine Key Feature

Diagnose every choice: Know exactly why the right option works and why distractors fail.

4-Option Deep Scan

1 Option (1)

✓ 100% Correct

Verdict & Scientific Reason:

Exact mathematical calculation matching Official NTA Final Answer Key.

💡

Why this is airtight: Careful subtraction and accurate conversion from MeV to keV.

Note: Zero rounding error.

2 Option (2)

✗ Incorrect

Verdict & Scientific Reason:

Rounding error.

⚠️

Student Fallacy (Why candidates pick this trap): Rounds 931.5 to 932 or makes an arithmetic addition error.

Note: NTA provides 931.5 to test exact calculation without coarse rounding.

3 Option (3)

✗ Incorrect

Verdict & Scientific Reason:

Calculation drift.

⚠️

Student Fallacy (Why candidates pick this trap): Uses Delta m = 0.00401 u or arithmetic carry-over error.

Note: 238.050 - 238.046 is exactly 0.004.

4 Option (4)

✗ Incorrect

Verdict & Scientific Reason:

Arbitrary distractor.

⚠️

Student Fallacy (Why candidates pick this trap): Rounds 931.5 to 935.

Note: Always use the exact value given in the question header: 931.5 MeV.

⚠️

NTA Trap Alert: The Unit Conversion Oversight (MeV vs keV)

Students who calculate 3.726 often get confused looking for 3.726 in the options, forgetting that the question specifically requests energy in keV.

🛡️ How to avoid the -1 mark: Always circle the requested physical unit in the question stem before starting calculation.

📖

Core NEET Syllabus Standard

NEET UG Physics Core Standards · Nuclei Standard Benchmark

Core Scientific Principle Tested:

"Q = [M_parent - (M_daughter + M_alpha)] * c^2 = Delta m * 931.5 MeV."

Syllabus Focus: Q-Value of Nuclear Reactions and Alpha Decay Energetics

✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗

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NEET 2026 Physics Question 7 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ Mass Defect in Milliatomic Units

Step-by-Step Rigorous Workflow & Mathematical Proof

Calculate Total Mass of Decay Products

Calculate the Mass Defect (Delta m)

Convert Mass Defect to Energy in keV

360° Option Elimination Engine Key Feature

NTA Trap Alert: The Unit Conversion Oversight (MeV vs keV)

Core NEET Syllabus Standard

Ready to attempt under real exam pressure?

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