Where Does the Mass Come From??

• There is a "Strong Nuclear Force" that hold the neutrons and protons of a nucleus together.
• Because energy must be added to take the nucleus apart (overcome the strong nuclear force), the mass of the assembled nucleus is less than the sum of all the neutrons and protons that make up the nucleus.
• The energy required to pull the nucleus apart is called ‘binding energy’
• the difference in mass between the assembled nucleus and its individual parts is called the ‘mass defect’
• we use the equation E=mc2 to find the binding energy.
• We use the symbol u to represent atomic mass units

 

Example :

The mass of a proton is 1.007825 u, and a neutron is 1.008665 u. The mass of the nucleus of radioactive hydrogen tritium is 3.016049 u. Tritium has 2 neutrons and 1 proton. A) what is the nuclear mass defect of this isotope, and b) what is the binding energy of tritium?

1.007825 u + 2(1.008665 u) = 3.025255 u

3.025155 u - 3.016049 u = 0.009106 u

• conversion factor : 1 u = 1.6605 x 10^-27 kg

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0.009106 u x 1.6605 x 10^—27 kg = 1.512 x 10^-29 kg

1 u

E = mc² = (1.512 x 10^-29 kg)(3.00 x 10⁸ m/s)²

= 1.36 x 10^-12 Joules

So for 3 grams of Tritium, we would get

(6.02 x 10²³)(1.36 x 10^-12) = 8.16 x 10¹¹ Joules of energy