Atomic Mass Unit 

or 

Unified Mass Unit

  The proton and neutron have very nearly equal mass, albeit very small, approximately . This is not a very convenient number, so a new unit was defined the atomic mass unit or sometimes called unified mass unit u. One unified mass unit is defined as one-twelfth the mass of a carbon-12 atom. Carbon-12 has 6 protons and 6 neutrons.

 This is call an atomic mass unit is defined to be the mass of exactly one-twelfth of a carbon-12 atom. That works out to be:

 

ParticleMass (u)
Electron (me)0.000549
Proton (mp)1.007277
Neutron (mn)1.008665
Helium Atom4.002602

     It turns out that unbounded protons and neutrons don't weigh 1u, instead there mass is more unbound then bound, which leads to one of the most important equations in all nuclear physics. 

    This may seem odd that such a simple equation is so important, and an equation that is clearly wrong.  But what this equation is trying to say is that when you attach a proton to neutron the total mass is less then mass of the particles when separated.  This oddity sets up one of the most important idea in the last two centuries, and that is mass and energy are interchangeable, that when two or more particles bind themselves together part of their masses are converted to energy that holds them together.  This mass that is converted to energy is called the mass defect.

Atomic Mass Unit 

or 

Unified Mass Unit

 The proton and neutron have very nearly equal mass, albeit very small, approximately . This is not a very convenient number, so a new unit was defined the atomic mass unit or sometimes called unified mass unit u. One unified mass unit is defined as one-twelfth the mass of a carbon-12 atom. Carbon-12 has 6 protons and 6 neutrons.

)
1 u = \frac{mass \: of \: one \: carbon \: 12 \: atom}{12}

This is call an atomic mass unit is defined to be the mass of exactly one-twelfth of a carbon-12 atom. That works out to be:

(2)
1u = 1.66 \times 10^{-27} kg

Since the neutron and proton have approximately the same mass, a proton and neutron have approximately mass’s of approximately 1u.

 

ParticleMass (u)
Electron (me)0.000549
Proton (mp)1.007277
Neutron (mn)1.008665
Helium Atom4.002602

Mass Defect 

Binding Energy 

 The difference between the unbound system calculated mass and experimentally measured mass of nucleus is called mass defect. It is denoted by Δm. It can be calculated as follows:

Mass defect = (unbound system calculated mass) - (measured mass of nucleus)
i.e, (sum of masses of protons and neutrons) - (measured mass of nucleus)

In nuclear reactions, the energy that must be radiated or otherwise removed as binding energy may be in the form of electromagnetic waves, such as gamma radiation, or as heat. Again, however, no mass deficit can in theory appear until this radiation has been emitted and is no longer part of the system.

The energy given off during either nuclear fusion or nuclear fission is the difference between the binding energies of the fuel and the fusion or fission products. In practice, this energy may also be calculated from the substantial mass differences between the fuel and products, once evolved heat and radiation have been removed.

When the nucleons are grouped together to form a nucleus, they lose a small amount of mass, i.e., there is mass defect. This mass defect is released as (often radiant) energy according to the relation E = mc2; thus binding energy = mass defect × c2.

This energy is a measure of the forces that hold the nucleons together, and it represents energy which must be supplied from the environment if the nucleus is to be broken up. It is known as binding energy, and the mass defect is a measure of the binding energy because it simply represents the mass of the energy which has been lost to the environment after binding.

Definition Provided by Wikipedia