Nucleons are the protons and neutrons which compose the nucleus of an atom. Normally the number of protons and neutrons of an atom are equal to each other. But occasionally the number of protons and neutrons differ from each other. The various numbers of protons and neutrons in the nucleus of atoms is referred to as being the isotopes of that atom.
All the different combinations of unequal numbers of protons and neutrons are called nuclides. There are about 1,000 known nuclides. Of these, 25% are stable and 75% are unstable. An unstable isotope is one which seeks stability by giving off protons, neutrons, or electrons. A stable isotope does not seek stability by giving off protons, neutrons, or electrons.
Normally an atom is stable when it has an equal number of protons, neutrons, and electrons. Unstable isotopes become more stable by ejecting protons, neutrons or electrons. The process of ejecting protons, neutrons or electrons is called radioactive decay.
There are three kinds of radiation: alpha, beta, and gamma. Alpha radiation is when the atom radiates (ejects) 2 protons and 2 neutrons. Helium has a nucleus composed on 2 protons and 2 neutrons, so in alpha radiation the atom radiates one helium nucleus. Since protons have a positive charge and neutrons are neutral, alpha radiation has a positive electrical charge.
Beta radiation is when the atom radiates a neutron and an electron. Since neutrons have no electrical charge and electrons have a negative electrical charge, beta radiation has a negative electrical charge.
Gamma radiation is electromagnetic radiation (A form of light). Gamma radiation is the most dangerous form of nuclear radiation. Gamma radiation has enough energy to penetrate lead. Gamma radiation can alter the genetic code of living organisms, resulting in genetic defects and cancer. Beta radiation is less damaging than gamma radiation, but it can penetrate thin sheets of metal. Alpha radiation is the least damaging form of radiation. It cannot even penetrate a thin sheet of paper.
Radiation cannot be seen, but it can be detected in several ways. One way is by film. The badges once worn by people who worked around radiation used to contain film. The film was covered to protect it from ordinary light. However, dangerous radiation would penetrate this covering and expose the film. If the film was severely exposed, it served to let them know that they had been exposed to dangerous levels of radiation. Modern badges use more sophisticated detection methods.
Another way that radiation can be detected is by a Geiger tube, which is often called a Geiger Counter. The Geiger tube is filled with gas which becomes converted to ions whenever radiation strikes it. An electric current flows through the Geiger tube, and if more ions are present, there are bursts of additional electricity which are converted into clicking sounds. A computer counts the clicks to let you know the amount of radiation present.
You might remember that the atomic mass of an atom is equal to the number of protons plus the number of neutrons in the nucleus of an atom. The number of protons in the nucleus determines what that atom is. For example, any atom having 6 protons is carbon; any atom having 7 protons is nitrogen; any having one is hydrogen; any having 2 is helium; and any having 8 is oxygen. The isotopes of an atom are the various combinations of protons and neutrons. Included in the isotopes of an atom is the stable form in which the number of protons and neutrons are equal.
For example, carbon 12 is the stable isotope of carbon whose nucleus contains 6 protons and 6 neutrons. Carbon 13 is an unstable isotope of carbon whose nucleus contains 6 protons and 7 neutrons. The number which appears after the element name is the atomic mass number (approximately equal to protons plus neutrons). Remember, carbon can only be carbon if it has 6 protons in its nucleus. If it had 7, it would be nitrogen. If it had 5, it would be boron. So if the number of protons in the nucleus changes, the element becomes a different element. If, however, the number of neutrons changes, the element becomes a different isotope of the same element.
Isotopes of elements with more neutrons than protons are always radioactive. Hydrogen 3 has one proton and 2 neutrons. This isotope of hydrogen is called tritium. Tritium is the isotope of hydrogen used to make nuclear weapons. Isotopes of elements with more protons than neutrons are only sometimes radioactive.
Most elements with high atomic mass numbers have radioactive isotopes. Uranium, for example, has many radioactive isotopes. Uranium 238 is one of these. Since the atomic number of uranium is 92, this means that it has 146 neutrons in its nucleus. When uranium 238 undergoes radioactive decay, it gives off an alpha particle. Since an alpha particle is composed of 2 protons and 2 neutrons, the loss of the two protons changes the atomic number to 90. This makes uranium become thorium, because thorium is the element with an atomic number of 90.
If an isotope is radioactive, it is called a radioisotope. An example of a radioisotope is phosphorus 32. Phosphorus 31 is used in small amounts by plants and animals. If you substitute phosphorus 32 for 31, the path that phosphorus 32 takes in the plant or animal can be followed by using a device like a Geiger Counter. This has enabled biologist to learn how plants and animals use phosphorus. The replacement of a stable isotope with a radioisotope for the purpose of following the pathway taken by the element is called tagging.
The rate of radioactive decay is described by the term half-life. Half-life refers to the time required for half of the nuclei in a sample of radioisotope to decay. The half-life of carbon 14 is 5,930 years. This means that a ten gram sample of carbon 14 will become a 5 gram sample in 5,930 years. This 5 gram sample will become a 2.5 gram sample in another 5,930 years. This 2.5 gram sample will become a 1.25 gram sample in another 5,930 years.
It is known that the percent of carbon 14 in living organisms is
equal to the percent of carbon 14 in the atmosphere. If a once living
thing has remains which contain only half this amount, then these
remains are 5,930 years old. Through this technique, carbon 14 is used
to date the remains of once living things. This process is called
carbon dating. Uranium 238 has a half-life of 4.5 billion years. It
can be used to date the formation of nonliving things such as rocks.