 You are undoubtedly familiar with the concept of mass, and will understand how to measure and describe masses in units of kilograms. But only perhaps more recently have you learned about the concept of energy, and how to measure energies in units of joules. However, since nuclei are such tiny objects, it usually makes sense to use a different system of units when solving nuclear problems. Furthermore, since nuclear processes involve regular conversion between mass and energy, we will want to allow for the relationship between energy and mass, that is, we will need to use Einstein's equation, equals mc squared. On the macroscopic real world scale, we generally use the units of kilograms for mass, and joules for energy. An alternative unit for energy is the electron volt, with the symbol EV. One EV is equivalent to 1.602 times 10 to the minus 19 joules, and is equal to the amount of energy that a single electron receives as it accelerates through an electric potential difference of one volt. An electron is much closer to the scale of a nucleus than, for example, a brick, so rather than using energy units of joules, we will use these energy units of EV. Furthermore, rather than using mass units of kilograms, we will use units of EV on c squared. We can do this because if we rearrange the equation E equals mc squared, we can see that m is equal to E on c squared, and hence the units of mass are equal to the units of energy divided by c squared. There is one last caveat. While an electron is closer to the nucleus than a brick, it is still substantially smaller than a nucleus. The typical energy scale for nuclei is actually millions of electron volts, so we will generally use units of 1 million electron volts, written and pronounced as mev. As some examples of typical energies and masses measured in these units, the masses of a bare neutron and a bare proton are 939.6 mev over c squared and 938.3 mev over c squared respectively. If we look at the alpha decay of polonium-212 into lead 208, we find that the energy of the emitted alpha particle is 8.95 mev. Finally, if we look at the excited states that can be observed in the nucleus lead 208, we find that the first excited state occurs at an energy of 2.615 mev. If this state were to decay, it would emit a gamma ray with an energy of 2.615 mev. In textbooks and papers about nuclear and particle physics, the vast majority of the time you will find masses and energies described in terms of mev, or sometimes other multiples of electron volts, such as kev, meaning thousands of electron volts, or gev, meaning billions of electron volts.