Understanding atomic mass units (amu) and weight of particles

[solarcopper]

I read that a C12 atom has an atomic mass unit of 12 amu's. A C12 atom has 6 protons, 6 neutrons, and 6 electrons. If the mass of a proton is 1.00728 amu, the mass of a neutron is 1.00867 amu, and the mass of an electron is .000549, how can it's total amu be 12?

6*1.00728+6*1.00867+6*.000549= 12.098994 amu

How did they determine that a C12 atom is 12 amu's? Are they giving an approximate answer?

 

[solarcopper]

I found the answer to this question:

"The carbon-12 atom has a mass of 12.000 u, and yet it contains 12 objects (6 protons and 6 neutrons) that each have a mass greater than 1.000 u, not to mention a small contribution from the 6 electrons.

This is true for all nuclei, that the mass of the nucleus is a little less than the mass of the individual neutrons, protons, and electrons. This missing mass is known as the mass defect, and represents the binding energy of the nucleus."

from http://physics.bu.edu/~duffy/sc546_notes10/mass_defect.html

 

[Borek]

Yes, mass defect (or a binding energy) is the answer.