Nuclear Size and Mass Defect NEET 2026: Option D Explained

What is the correct option for nuclear size and mass defect NEET 2026?

Answer option D is correct: statements B and D are true, while statements A and C are false. The nuclear size and mass defect NEET 2026 question belongs to Physics, chapter Atoms and Nuclei. The question bank tags it medium and sets an expected solving time of 60 seconds, not a measured student average.

Here, A means mass number, the total number of protons and neutrons in a nucleus. Evaluate these four claims and choose the matching answer option:

• Statement A: Nuclear volume varies as the cube root of mass number.
• Statement B: Nuclear volume varies directly with mass number.
• Statement C: Mass defect means the mass difference between an atom and its nucleus.
• Statement D: Mass defect means the mass difference between a nucleus and its separate constituents.

Each answer option also declares the unselected statements false:

• Answer option A: Statements A and C are true; B and D are false.
• Answer option B: Statements B and C are true; A and D are false.
• Answer option C: Statements A and D are true; B and C are false.
• Answer option D: Statements B and D are true; A and C are false.

How do you derive nuclear volume from the radius formula?

Start with the nuclear-radius relation: $r=r_0{A}^{1/3}$

The radius constant is treated as independent of mass number. Volume is proportional to mass number, not its cube root, because calculating volume requires cubing the radius.

Cube both sides, including the exponent on mass number:

Using the spherical approximation, nuclear volume is:

Substitute the expression for the cubed radius:

The entire coefficient multiplying mass number is independent of it:

Therefore: $V\propto A$

Statement A is false. Statement B is true. Radius follows the cube-root dependence, whereas volume follows a direct dependence.

For a numerical check, take an illustrative mass-number ratio of 8. This is a chosen ratio, not an isotope measurement:

The radius doubles, but the volume becomes eight times as large. Keeping the same factor for both misses the cubing step.

Which masses define mass defect, and why is answer option D correct?

Nuclear mass defect compares separate, free nucleons with the bound nucleus. The constituents are protons and neutrons, not the electrons surrounding the nucleus. Under the positive mass-defect convention, subtract the actual nucleus’s mass from the sum of its free nucleon masses:

The symbols mean:

First, add the masses of all the free nucleons:

Then subtract the measured mass of the bound nucleus:

A bound nucleus has less mass than those free nucleons taken together. That mass difference corresponds to its binding energy.

Comparing a neutral atom with its nucleus instead involves the electrons’ masses and their electronic binding contribution. It does not give the nuclear mass defect.

Thus, statement C is false and statement D is true. The complete truth-value check is:

• Statement A: False.
• Statement B: True.
• Statement C: False.
• Statement D: True.

The official answer is answer option D, because statements B and D are true while statements A and C are false.

How does a radius–volume mix-up produce wrong answer option C?

Answer option C results if you transfer the radius exponent directly to volume but judge the mass-defect statements correctly. The faulty step skips cubing the radius.

The remembered relation is correct: $r\propto A^{1/3}$

The next step is not:

That error accepts statement A and rejects statement B. If statement C is correctly rejected and statement D accepted, the selected pair becomes statements A and D, producing answer option C.

Repair this error by writing the geometry before substituting the mass-number dependence: $V\propto r^3$

An exponent for a length cannot be transferred unchanged to a volume. Use the volume relation first rather than relying on a remembered exponent.

Which three related Atoms and Nuclei questions should you practise next?

Practise radius and volume ratios, nuclear density, and binding energy. These are original related practice questions, not additional verified PYQs. Attempt each before reading its working.

How do you find radius and volume ratios for two nuclei?

Question 1: For two nuclei, the radius ratio is the cube root of their mass-number ratio; the volume ratio equals their mass-number ratio. Express both ratios for mass numbers:

Using the same radius constant for both nuclei:

Does nuclear density increase with mass number?

Question 2: Nuclear density is approximately independent of mass number. Derive this using the expressions below, treating proton and neutron masses as approximately equal and neglecting binding-related mass differences:

Divide mass by volume:

Mass number cancels. A larger nucleus has proportionally more mass and more volume, not a higher density in this approximation.

How do you calculate mass defect and binding energy from nuclear mass?

Question 3: Subtract nuclear mass from the free-nucleon total, then multiply the defect by the square of the speed of light. Write both quantities for a nucleus with:

The worked answer is:

$E_b=\Delta m{c}^2$ $c=\text{speed of light}$ Before substituting, check whether the supplied mass is nuclear mass or neutral atomic mass. Do not substitute an atomic mass directly into this nuclear-mass formula without accounting for electrons.

Next step: the past-paper archive on NEET JEEnius AI and search past NEET papers by year, subject or chapter, each with a worked solution (100 free searches a month).

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