# Whether sum of weight of proton and neutron is equal to atomic mass?

1. Feb 24, 2012

### newbe

Dear friend,
I am getting confused in this question;what i learned till now is that number of proton+number of neutron=atomic mass. But while searching the net i got this equation
atomic mass = mass a x fract a + mass b x fract b. can any one tell me which is the correct (not approximate) method for finding atomic weight.

and my second question

is weight of proton+weight of neutron= atomic mass is this equation correct

advance thanks for your help

2. Feb 24, 2012

### Staff: Mentor

Apparently both these equations can't be right at the same time, number of objects and weight of objects are not the same thing.

Mass of the nucleus equals mass of the neutrons plus mass of the protons MINUS binding energy.

Neutrons and protons hold very strong to each other in nucleus. When you take several separated neutrons and several protons and you fuse them into a nucleus a lot of energy is emitted (that's where the energy in stars and hydrogen bombs comes from). You have probably heard about Einstein equation E=mc2 - it means energy is equivalent to mass. When the energy is emitted, mass of the remaining nucleus is smaller than the sum of masses of protons and neutrons, this missing mass is called "mass deficit" or "binding energy".

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