ATOMIC MASS
 

The atomic mass, sometimes called the atomic weight, of an element is approximately equal to the sum of the number of protons and the number of neutrons in the nucleus. This quantity is formally measured in atomic mass units (amu), where 1 amu is equal to exactly 1/12 the mass of the nucleus of the carbon isotope having six neutrons. This is the most common isotope of carbon and is symbolized ¹²C or carbon-12. Any proton or any neutron has a mass of approximately 1/12 amu, but neutrons are a little more massive than protons.
Elements are uniquely defined by their atomic numbers, but the atomic mass of an element depends on the particular isotope of that element. A well-known isotope of carbon, ¹⁴C, is found in trace amounts in virtually all carbon-containing substances. This fact has proven quite useful to geologists and archaeologists. The isotope ¹⁴C is radioactive, whereas ¹²C is not. The radioactivity of ¹⁴C diminishes with time according to a wellknown, predictable mathematical function. This makes it possible for researchers to determine when carbon-containing compounds were created and thus to find out how old various rocks, fossils, and artifacts are. In nuclear reactions capable of producing energy, such as the reactions that take place inside stars, atomic bombs, and nuclear power plants, a certain amount of mass is always given up—and converted into energy—in the transactions between the atoms. This amount of mass can be exceedingly small yet produce an enormous burst of energy. The first person to formalize this relation was Albert Einstein, using his famous equation
E = mc²
where E is the energy produced in joules, m is the total mass in kilograms lost during the reaction, and c is the speed of light in meters per second. The value of c² is gigantic: approximately 90 quadrillion meters squared per second squared (9 × 10¹⁶ m²/s²). This is why so much energy can be produced by an atomic reaction between two elemental samples of modest mass.
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