In nuclear physics, the mass defect of a nucleus is mainly the result of what process occurring when the nucleus is formed?

Introduction / Context:

This question relates to nuclear binding energy and Einstein mass energy relation. When individual nucleons, that is protons and neutrons, come together to form a nucleus, the combined mass of the bound nucleus is slightly less than the sum of the separate masses. This difference is called mass defect. Understanding why this mass difference occurs is fundamental for explaining nuclear energy release in fission and fusion.

Given Data / Assumptions:

• We are dealing with stable or bound nuclei made of protons and neutrons.
• Mass defect refers to the difference between the sum of masses of separate nucleons and the actual mass of the nucleus.
• The reasoning involves the relation E = m * c^2, where m is mass and c is the speed of light.

Concept / Approach:

According to relativity, mass and energy are equivalent. When nucleons bind together, they occupy a lower energy state because energy is released as the nucleus forms. This released energy appears as gamma rays or kinetic energy of particles. Since energy has been given out, the bound system has less total energy and therefore less equivalent mass than the initial separate nucleons. The missing mass is the mass defect, and it corresponds to the binding energy that holds the nucleus together.

Step-by-Step Solution:

Verification / Alternative check:

As a check, examine nuclear fission and fusion processes. In both cases, the total mass of products is less than the total mass of reactants. The mass difference appears as large amounts of energy, which is exploited in nuclear reactors and stars. The logic is the same as for individual nuclei. The formation of more tightly bound configurations releases energy and reduces the total mass. This supports the explanation that mass defect arises from conversion of mass into binding energy during the binding process.

Why Other Options Are Wrong:

• Protons and electrons being attracted in orbitals describe atomic structure, not nuclear mass defect, and involve much smaller energy changes.
• Energy being converted into moles of atoms when neutrons break apart protons is not a correct description of nuclear structure and misstates the direction of conversion.
• Mass being converted to energy when protons and neutrons break apart refers to high energy reactions that disrupt the nucleus, not to the formation of a bound nucleus and the observed mass defect.

Common Pitfalls:

Students often confuse mass defect with simple measurement error or think that mass disappears without explanation. Others may reverse the cause and effect and think that energy is absorbed when the nucleus forms. The correct picture is that the nucleus is a lower energy configuration; energy is released, and the corresponding mass is lost from the system. Remembering that binding energy is the energy required to separate a nucleus back into free nucleons provides a useful perspective.

Final Answer:

The mass defect results from mass being converted to energy when protons and neutrons bind together in a nucleus.