Mass as a Form of Energy

It may seem strange to consider mass as a form of energy, however Einstein proposed the famous relationship:

In chemical reactions the amount of mass that is transformed to other forms of energy is very small and mass and energy conservation may appear to be separated. However when we study nuclear reactions the link between mass and energy is clear. The energy released is large (about a million times larger than a chemical reaction) and change in mass can be easily measured.

When we talk about the energy and mass of atomic particles we use units that are most useful for this scale. Energy of macroscopic objects is usually measured in Joules (J), and the mass in kilograms (kg). To talk about objects such as electrons, protons or neutrons we use different units.

Energy - Electron volts (eV). This is the amount of energy that an electron receives if it moves through one volt.

Mass - Atomic mass unit (amu). One atomic mass unit is approximately the mass of the proton or neutron.

If we use Einstein's equation we can determine the energy-mass equivalence for any object, for example:

Example of mass as a form of energy
When a Radium atom decays we get a Radon atom and an alpha particle. The masses of these objects are:

Calculate the difference in mass before and after the interaction (the decay), and hence the amount of energy released by this interaction.

Now apply the ideas of momentum and energy conservation to the decay of Radium. Which of the objects (alpha particle or Radon atom) will have the greater speed?

In the 1930s scientists studying the decay of a Cesium atom noticed that the electrons that were emitted as a result of the interaction has a range of kinetic energies. The maximum kinetic energy detected was 205.0 keV and the average kinetic energy was 56.3 keV.

A Cesium atom decays to give a Barium atom and an electron. The mass difference between the objects after the interaction is 0.0002892 amu which is equivalent to 269.4 keV. To solve this apparent dilemma we must consider both momentum conservation and energy/mass conservation of the system.

In the same way as mass can be converted into energy, energy can be converted into mass.

Two protons moving fast and colliding produce two protons and two mesons called pions (or Pi meson). The energy of the two original protons has changed form to mass.

Proton mass = 1 amu
Meson mass = 0.15 amu