Question

Li nucleus has three protons and four neutrons. Mass of the lithium nucleus is 7.036005 amu. Mass of the proton is 1.007277 amu and the mass of the neutron is 1.008665 amu. Mass defect for lithium nucleus in amu is
A. 0.02048
B. 0.04050
C. 0.04052
D. 0.04055

Answer 1

For this question we will find the combined mass of all the protons and neutrons that make up the lithium nucleus and then subtract from it the mass of the nucleus the difference between them is the mass defect which is converted to the attractive potential energy of the nucleus.

Complete step-by-step answer:
The total mass of three protons used in the nucleus is $3\times 1.007277=3.021831$
The total mass of four neutrons used in the nucleus is $4\times 1.008665=4.03466$
The total mass of the protons and neutrons is $3.021831+4.03466=7.056491$
The mass of lithium nucleus is given as 7.036005
The difference between them is $7.056491-7.036005=0.020486$.

So, the correct answer is “Option A”.

Additional Information: When protons and neutrons come together to form a neutron, they lose some of their mass because the lost mass appears as the attractive energy between the particles, which is known as binding energy. It is noticeable in this case because the strong nuclear force in effect is 100 times stronger than the electromagnetic force. We will need to provide the same amount of energy as the mass defect to separate the protons and neutrons making the nucleus. The mass energy conversion will be done by the mass-energy equation given by Einstein. The mass defect is different for different atoms. We take mass defect per nucleon (term used collectively for protons and neutrons) and it is the difference in mass defect per nucleon which is used in nuclear reactions to give energy.

Note: We must take all the decimal places provided in the question for our calculations as accuracy is needed here and our answer is a very small number. We can use the mass energy equivalence given by Einstein to calculate the energy generated by the loss of mass. The energy is used to bind the nucleons together.