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The Discovery of Radioactivity


In 1896, Henri Becquerel was using naturally fluorescent minerals to study the properties of x-rays, which had been discovered in 1895 by Wilhelm Roentgen. He exposed potassium uranyl sulfate to sunlight and then placed it on photographic plates wrapped in black paper, believing that the uranium absorbed the sun’s energy and then emitted it as x-rays. This hypothesis was disproved on the 26th-27thof February, when his experiment "failed" because it was overcast in Paris. For some reason, Becquerel decided to develop his photographic plates anyway. To his surprise, the images were strong and clear, proving that the uranium emitted radiation without an external source of energy such as the sun. Becquerel had discovered radioactivity.

Why are Isotopes Radioactive?

The strong nuclear force

What holds the nucleus together? The nucleus is tiny, so the protons are all very close together. The gravitational force attracting them to each other is much smaller than the electric force repelling them, so there must be another force keeping them together. This other force is known as the strong nuclear force; it works only at small distances. The strong nuclear force is a very strong attractive force for protons and neutrons separated by a few femtometers, but is basically negligible for larger distances.

The tug-of-war between the attractive force of the strong nuclear force and the repulsive electrostatic force between protons has interesting implications for the stability of a nucleus. Atoms with very low atomic numbers have about the same number of neutrons and protons; as Z gets larger, however, stable nuclei will have more neutrons than protons. Eventually, a point is reached beyond which there are no stable nuclei: the bismuth nucleus with 83 protons and 126 neutrons is the largest stable nucleus. Nuclei with more than 83 protons are all unstable, and will eventually break up into smaller pieces; this is known as radioactivity.



Nuclear binding energy and the mass defect

A neutron has a slightly larger mass than the proton. These are often given in terms of an atomic mass unit, where one atomic mass unit (u) is defined as 1/12th of the mass of a carbon-12 atom.

Particle mass in kg mass in atomic mass units
electron 9.11 x 10-31kg 5.486 x 10-4 u
proton 1.673 x 10-27 kg 1.0073 u
neutron 1.675 x 10-27 kg 1.0087 u

Something should probably strike you as being a bit odd here. The carbon-12 atom has a mass of 12.000 u, and yet it contains 12 objects (6 protons and 6 neutrons) that each have a mass greater than 1.000 u. The fact is that these six protons and six neutrons have a larger mass when they're separated than when they're bound together into a carbon-12 nucleus.

This is true for all nuclei, that the mass of the nucleus is a little less than the mass of the individual neutrons and protons. This missing mass is known as the mass defect, and is essentially the equivalent mass of the binding energy.

Einstein's famous equation relates energy and mass:

E=mc2   (m= mass in kg; c=speed of light 3.0 x 108m/s)

If you convert some mass to energy, Einstein's equation tells you how much energy you get. In any nucleus there is some binding energy, the energy you would need to put in to split the nucleus into individual protons and neutrons. To find the binding energy, then, all you need to do is to add up the mass of the individual protons and neutrons and subtract the mass of the nucleus:

mass defect=Dm=mass of individual nucleons - mass of the nucleus

The binding energy is then:

binding energy = mass defect x c2

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