An atom of carbon 12 is made of six protons, six neutrons, and six electrons, which when added together have a mass of 12.095646 amu. But carbon 12 is defined as having an atomic weight of exactly 12.000000 — in fact this is the standard by which all other atomic weights are defined. How do we account for the extra 0.095646 amu?
This mass discrepancy is called the mass defect, and is one of the fundamental properties of nuclear physics. When nucleons come together to form an atomic nucleus, a fraction of their mass is lost as it is converted to energy and released as electromagnetic radiation. This is the source of the energy released in the fusion process, and must be supplied back to the nucleus in order to separate its components again. It is called, therefore, the binding energy that holds the atomic nucleus together. Binding energy can be found with Einstein's famous E=mc2 equation after measuring the mass of the atom and comparing it to the mass of its component parts.
Up to a point, binding energy increases with atomic weight. Iron 56 is the element with the highest binding energy per nucleon (with a mass defect of 0.52872, more than half a proton!). Iron is therefore the largest atom which can be formed by a star's normal nuclear fusion process, because the formation of larger elements would not release energy, they require energy. In general, elements and isotopes above iron 56 can only be created by the awesome forces generated by a supernova, and even then only up to uranium. Elements with a higher atomic number are created by beta decay of uranium, or high energy nuclear physics experiments.
Calculating mass defect:
- Proton: 1.007277 amu
- Neutron: 1.008665 amu
- Electron: 0.000548597 amu
- 1 amu = 1.6606E-27 kg
- c = 299.792458E6 m/s2
- 1 Joule = 6.2415E12 MeV
Silicon 28 has an atomic weight of 27.9769271 amu, and is composed of 14 protons, 14 neutrons, and 14 electrons. Therefore, its mass defect is:
14(1.007277) + 14(1.008665) + 14(0.000548597) - 27.9769271 = 28.2308684 - 27.9769271 = 0.2539413 amu
Its binding energy is then:
(421.6793638E-30 kg)(299.8E6 m/s2)2 = 3.78986512E-11 J
We see that 1 amu of mass defect is equal to 931.4908918 MeV, MeV being the standard unit of binding energy.
Thanks to RPGeek for some corrections.