The Nucleus Help (page 3)

By — McGraw-Hill Professional
Updated on Sep 5, 2011

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 12 C 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, 14 C, is found in trace amounts in virtually all carbon-containing substances. This fact has proven quite useful to geologists and archaeologists. The isotope 14 C is radioactive, whereas 12 C is not. The radioactivity of 14 C diminishes with time according to a well-known, 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 = mc 2

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 c 2 is gigantic: approximately 90 quadrillion meters squared per second squared (9 × 10 16 m 2 /s 2 ). This is why so much energy can be produced by an atomic reaction between two elemental samples of modest mass.

An excellent source of information concerning all the known elements, including atomic number, atomic mass, and various other characteristics, can be found at the following Web site:

If you have a computer with Internet access, it would be a good idea spend a while exploring this Web site right now.

The Nucleus Practice Problem


Suppose that the nucleus of an oxygen atom, which has eight protons and usually has eight neutrons, were split exactly in two. What element would be the result? How many atoms of this element would there be? Neglect, for simplicity, any energy that might be involved in the reaction.


This reaction would produce two atoms of beryllium, each with four protons and four neutrons. This would not be the most common isotope, however; it turns out that beryllium usually has five neutrons in its nucleus.

Practice problems of these concepts can be found at: Particles of Matter Practice Test

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