Disappearing Mass
The mass of a nucleus is less than the mass of the protons and neutrons that it is made of.
(mass of protons + mass of neutrons) – mass of nucleus = ∆m
∆m is the difference in the masses and is called the mass defect.
Let us look at the nucleus of a Helium atom to see this in action. It is made up of 2 protons and 2 neutrons:
Mass of nucleons = 2 x (mass of proton) + 2 x (mass of neutron)
Mass of nucleons = 2 x (1.673 x 10-27) + 2 x (1.675 x 10-27)
Mass of nucleons = 6.696 x 10-27 kg Mass of nucleus = 6.648 x 10-27 kg
Mass defect = mass of nucleons – mass of nucleus
Mass defect = 6.696 x 10-27 – 6.648 x 10-27 = 0.048 x 10-27 kg
| Particle | Mass (kg) | Mass (u) |
|---|---|---|
| Proton | 1.673 x 10-27 | 1.00728 |
| Neutron | 1.675 x 10-27 | 1.00867 |
| Electron | 9.11 x 10-31 | 0.00055 |
As we can see, we are dealing with tiny masses. For this reason we will use the atomic mass unit, u
1u = 1.661 x 10-27 kg
The mass defect now becomes = 0.029 u