What is a mass defect in a nucleus? Why is it observed?

Mass defect is the difference between the sum of the masses of individual nucleons (protons and neutrons) and the actual mass of the nucleus.

Definition in simple terms:

Mass defect = (Mass of protons + Mass of neutrons) − Actual mass of nucleus

Why is mass defect observed?

When protons and neutrons come together to form a nucleus:

• A part of their mass is converted into energy
• This energy is called nuclear binding energy

According to Einstein’s equation:

E = mc²

This shows that mass can be converted into energy, so the nucleus weighs less than the total mass of its particles

What does it indicate?

• The mass defect represents the binding energy that holds the nucleus together
• Greater mass defect → more stable nucleus

Key Takeaway:

Mass defect occurs because some mass is converted into binding energy when a nucleus forms, making the nucleus lighter than the sum of its protons and neutrons.