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Nuclear Safeguards Education Portal

Energy and Mass Relationships

Albert Einstein
Albert Einstein 

The most fundamental relationship in nuclear physics is Albert Einstein's relationship between mass and energy:


This relationship tells us that mass can be converted to energy and vice-versa. While to Einstein this was probably quite obvious, to the rest of us it is likely not so. 

Before jumping into some simple example problems based on this concept, let's review some basic definitions including some units and prefixes to units that we frequently use:

A mole is defined as the amount of substance that contains as many elementary entities (e.g., atoms, molecules, ions, electrons) as there are atoms in 12 g of the isotope carbon-12 (12C). Thus, by definition, one mole of pure 12C has a mass of exactly 12 g. Therefore, an atomic mass unit (amu) is typically in units of grams/mole: Carbon-12 has a mass of 12 amu or one mole of pure Uranium-235 has a mass of 235.043923 grams.

Avogadro's number is a universal constant commonly used in nuclear physics for the number of atoms (or molecules) in one 1 mole of substance. The value is 6.022×1023. This number is extensively used in calculating number densities and energy conversions from mass.

If a mass is given in amu and it must be converted to grams, then dividing by Avogadro's number will give grams/atom.

If you are still confused about Avogadro's number, take a look at this video.

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