mass difference

In fact we should call it "Mass defect" which can be explained by the Mass-Energy relatioship proposed by Einstein:
Energy E and mass m are equivalent and inter-related by E = mc2.
where c is the speed of light.
From this equation, it can be deduced that mass can be converted to energy and vice versa.
Therefore, when a nucleus is formed from its respective nucleons (neutrons and protons), owing to the strong repulsion force between the positive charge of protons, work has to be done in order to overcome the repulsive force between them and hence the energy is originated from the mass of those nucleons, implying that some mass has been converted to energy, which is known as the binding energy.
So conclusively, the mass of an atom is always less than the total mass of the nucleons and the outer electrons which form the atom.
Taking an helium atom as an example, it is composed of 2 neutrons, 2 protons and 2 electrons. And in accordance to the carbon scale:

Mass of a neutron = 1.008665 u (where u is the atomic mass unit)
Mass of a proton = 1.007276 u
Mass of an electron = 0.000549 u
So invidually counting, the total mass of those particles of a helium atom is 4.03298 u.
And by the experimental result that mass of a helium atom = 4.00258 u, we can calculate the mass defect = 0.0304 u.
So from the above mass-energy equation, we can get that a mass defect of 1 u is equavalent to an energy value of 931 MeV where eV is the energy unit electron-volt and 1 eV is equivalent to 1.6 × 10-19 J of energy.
Therefore, the binding energy of a helium atom is:
0.0304 × 931 = 28.3 MeV or 4.54 × 10-12 J

e = m c^2

電子同質子互相吸引,
位能比較佢地分開時少左
e 細左, m 就同時細左.
所以你係正確的.

the mass of an atom should > the mass of its nucleons and electrons.

can you quote your source , where you know that??

2007-02-17 01:42:10 補充：
all other are wrong in concept the equation is E=mc^2not m=E/c^2the math is the same, but different in physics!, we never use E=mc^2 to explain fusion!

2007-02-17 01:44:19 補充：
fission, heavy nulcear split into 2 part, the total mass B4 is > after, because mass used as blinding energy B4, and now we use E=mc^2 to calcalate the energy released by fission.fusion, extra energy force 2 nulcears overcome the potential. and the mass created is calculatd by m=E/c^2.

2007-02-17 01:45:53 補充：
in an equation, the LHS is what we want to find, the RHS is what we haev known.

2007-02-17 01:46:58 補充：
so, the mass of atom is always n p e indiviually.