Mass of one proton = 1.0072765 amu

Mass of one neutron = 1.0086650 amu

Mass of one electron = 0.0005486 amu

In theory, the mass of a helium atom should be the sum of two protons, two neutrons and two electrons:

However, the actual observed mass is 4.0026033 amu.

Mass Defect - The difference between the mass of an atom and the sum of the masses of its protons, neutrons, and electrons.

• The mass defect is equal to the binding energy.
• Binding Energy (BE) - The energy released when the nucleus is formed from its constituent protons and neutrons.
• BE is also a measure of the difference between the stability of the products and the reactants in nuclear reactions.
  • Analogous to H in a chemical reaction.
  • The larger the BE, the more stable the nucleus.
  • Also equal to the energy that would be required to break the nucleus apart into its neutrons and protons.

where m is the mass of the atom and c is the speed of light (2.998 x 10⁸ m/s)

Binding Energy Per Nucleon - Total binding energy divided by the total number of protons and neutrons.

Next:  "Kinetics"