The Basics Edit
General Rules Edit
Over the years, scientists have managed to determine some general rules to predict how likely it is that an atom will undergo radioactive decay. An element will undergo decay if:
- It has more than 83 protons.
- The ratio of protons to neutrons is not very close. For example, Carbon, having 6 protons and 6 neutrons (therefore having a 1:1 ratio of protons and neutrons) is a very stable atom. On the other hand, Bismuth, an unstable atom, has 126 neutrons and 83 protons, giving it a 1.52:1 ratio.
- Nuclides containing certain numbers of protons or neutrons (2, 8, 20, 50, 82, and 126) are more stable than others.
- Nuclides with even numbers of both protons and neutrons and more stable than those with odd numbers of both.
Of course, there are exceptions to these rules, but in general they are accurate and it shouldn't be hard to make an educated guess!
The History Edit
The concept of radioactive decay was first discovered in 1896 by Henri Becquerel as he was working the element uranium compounds. In his first experiment, he placed the uranium on top of photographic film wrapped in dark paper and placed the crystals in the sunlight. The film darkened in response to some type of energy, which Bequerel assumed to be the prescence of the sun. However, Becquerel's second experiment revealed something much more interesting. After placing the uranium in an enclosed spot with no sunlight at all, Becquerel expected no dark spots to appear on the film. Much to his surprise, the paper was darkened as it had when being exposed to the sun. Becquerel concluded that the uranium must be emitting some sort of high energy. He had discovered radioactivity. Later, Becquerel demonstrated that the radiation emitted by uranium was similar to X rays but, unlike X rays, could be deflected by a magnetic field and therefore must consist of charged particles. For his discovery of radioactivity, Becquerel was awarded the 1903 Nobel Prize for physics.
With the help of Madame Curie and her husband Pierre, Becquerel was able to isolate two other radioactive substances: polonium and radium. In fact, it was found that radium was over 300,000 times stronger than uranium.
Following this discovery, in 1899 the scientist Ernest Rutherford (1871-1937) discovered alpha, beta, and gamma radiation. A substance, by emitting any of these particles, changes itself by a process called "transmutation".
Alpha Decay Edit
An alpha particle, represented by the notation He2+, consists of two protons and two neutrons, meaning that it must come from the nucleus of an atom. The loss of these charges causes the nucleus to transmute: the mass number of the decaying nucleus is reduced by 4 while the atomic number is reduced by two.
Image taken from http://education.jlab.org/glossary/alphadecay.html
For example, in the decay of gold:
Gold, which has an atomic number of 79, becomes an element with an atomic number of 77, and changes from an atomic weight of 185 to an atomic weight of 181. Using a Periodic Table of elements, the element fitting those requirements is Iridium.
Alpha particles are emitted from the nucleus atom at 10 percent the speed of light. Because the penetrating power of alpha particles is so low, it can't even go through a sheet of paper! However, the alpha particle will do severe damage to the cells if taken into the body, such as through inhalation.
Beta Decay Edit
Beta decay is caused by electrons and antineutrinos being emitted by the nucleus. For this to happen, a neutron within the nucleus must be changed into a proton. In beta decay, the atomic number increases by 1 but the mass remains unchanged. Following the Law of Conservation of Charge, another negative charge must be created. Consequently, charge is conserved.
In the beta decay of Helium, a beta particle is emitted and the element becomes Lithium.
Beta particles are emitted from the nucleus at 0.9 times the velocity of light. Although beta particles have a greater penetrating power than alpha particles, 1 cm of aluminum foil can stop them. Beta particles are also harmful if ingested into the body.
Gamma Radiation Edit
When exposed to gamma radiation a nucleus changes from a higher energy state to a lower energy state through the emission of photons. Because of this, gamma rays move at the speed of light. However, neither mass nor atomic number is changed when the nucleus emits gamma rays. Gamma radiation usually occurs with either beta or alpha decay.
Half-Life and Carbon Dating Edit
Believe it or not, radioactive decay has become quite a useful tool in modern science. Remember when I said that atoms that are have equal ratios of protons and neutrons, like Carbon, tend to be more stable than those that don't? In fact, Carbon is so stable that it is used in a modern process called "carbon dating". Scientists are able to use Carbon to compare with other elements, determining the age of various materials, such as fossils.
Scientists are able to do this using a special property of all radioactive materials called "half-life". A half-life is the amount of time it takes for half of the reactant to be converted into its product. This half-life can also be calculated using the equation:
- AE is the amount of substance left
- A0 is the original amount of substance
- t is the elasped time
- t1/2 is the half-life of the substance
As well as being used for scientific purposes, Carbon dating can also be used for health reasons. When there are accidents at nuclear power plants, it is imperative for the people to know exactly when environments are safe to return to. Carbon dating can accurately tell us the minimun amount of time needed to stay clear until the radioactive substance is past the point of being dangerous to humans. Here is a chart showing some half-lives of radioactive substances: http://www.agiweb.org/news/evolution/figures/dating3.jpg Image from: http://www.agiweb.org/news/evolution/dating3.html
Practice Problems Edit
Practice Problems: Fill in the spaces with the appropriate elements or particles.