Mass Defect

go back

Mass defect

• The total mass of a stable nucleus is always less than the combined masses of the nucleons
• Mass defect: difference between total mass of the individual nucleons, and the mass of the nucleus as a whole 
• The difference in mass is considered to be the binding energy of the nucleus

Binding Energy

• The difference in mass (mass defect) is considered to be binding energy
• If the mass of 4He was equal to the mass of 2 protons + 2 neutrons, the nucleus would fall apart without any input of energy
• To be stable, its mass must be less than that of its constituents combined 
• The nuclear binding energy represents the amount of energy that must be put into a nucleus to break it apart into its individual nucleons

Equations

SI units

• We can calculate the energy through $e=m{c}^2$
• e = energy in J
• m = mass in kg
• c = speed of light, or 300 000 000 m/s

Other units

• Sometimes, you’ll be given units in amu
• In this case, find the mass of all nucleons, and subtract the mass of the nucleus from that
• You should get a value in amu
• Multiply this by 931, and this will be the binding energy in MeV
• This can be converted into Joules using the data sheet formulas
• In short: $MeV=931\times amu$