RADIOACTIVITY IN CHEMISTRY
Radioactivity is defined as the spontaneous emission of radiation by an element. That type of an element is called a radioactive element.
Becquerel in 1896 observed that a crystal of a uranium salt spontaneously emitted radiation, which could penetrate through opaque material to affect a photographic plate. Further experiments showed that the metal uranium and all it’s compounds possessed the same property. Becquerel concluded that the uranium atoms were responsible for the emission of this unknown radiation and called it phenomenon radioactivity.
Pierre and Marie curie in 1898 detected some radioactivity in the element thorium . They also noted that the level of radiation in a naturally occurring uranium ore, called pitchblende, was much higher than could be explained by it’s uranium content .Suspecting the presence of other radioactive elements in pitchblende, they analyzed it and isolated two new radioactive elements, polonium and radium. Then the latter was found to be several million times as radioactive as the same mass of uranium. These discoveries led to further investigations by scientists. Since then, more than 40 naturally occurring radioactive substances have been found. Studies on radioactivity soon revealed that this phenomenon could not be explained by ordinary chemical reactions, which involved only the electrons of atoms. Instead, it could only be explained in terms of special changes which involved the nuclei of atoms.
CHARACTERISTICS OF RADIOACTIVITY
A radioactive substance emits radiation continually and spontaneously. Temperature and pressure have no effect on the rate at which this radiation is emitted. The radiation, unlike light rays ,can penetrate through opaque matter. However, like visible light rays, it affects photographic plates. It also ionizes the gases through which it passes, causing fluorescence in certain substances , e.g.zinc sulphide, and leaving tracks in a cloud chamber. Radioactivity is always associated with a release of energy. The energy of radioactivity is about a million times as great as that liberated during any chemical reaction. This type of energy is known as nuclear energy.
TYPES OF RADIATION
Radioactive radiation consists of three main components of different penetrating power:
1) alpha rays
3)and gamma rays .
These three components can be separated and distinguished by their behavior in an electrostatic field.
Alpha are fast_moving streams of positively charged particles, each having a mass number of four , an atomic number of two and two units of positive charge. Thus , each particle is actually a helium nucleus. Alpha rays have very low penetrating power. They travel only a few centimetres in air and are stopped or absorbed by a thin sheet of paper . The exert is a very powerful ionizing effect upon any gas through which they pass. In addition, alpha rays can cause fluorescence in some materials. e.g. zinc sulphide.
Beta rays are very fast moving streams of electrons. Since they are negatively charged and have a relatively small mass, they are quite markedly deflected towards the positive plate in an electrostatic field. Each particle has a mass number of zero and a charge of _1.
Beta rays are much more penetrating than alpha rays. Their range is about 3 m in air , and about 4mm in aluminium. In air, the ionization power of beta particles is only about one_thousandth of that of a _particles. They also cause fluorescence in certain substances like anthracene.
Always remember that the y-rays are not particles but electromagnetic waves that is similar to visible light and X-rays , but with very short wavelengths. They travel at the speed of light,and are unaffected by an electrostatic field. Of the three types of radioactive emissions , the y- rays have the least ionization power and they are the most penetrating. Bear in mind that they can penetrate about 100m through air and can also pass through 0.5m of iron or lead. They can also cause fluorescence in certain substances like sodium iodide.
X_rays are electromagnetic waves, like visible light but with a shorter wavelength. They are produced by allowing fast moving electrons to bombard metals such as tungsten. The fast moving electrons knock electrons out of the inner shells of the metal atoms . The dislodged electrons are replaced by electrons moving in from the outer shells. This movement of electrons is accompanied by the emission of x-rays
X- rays can penetrate easily through most solid substances which are opaque to visible light, such as metal foils, flesh, wood and paper. Hard X- rays have a greater penetrating ability than soft X rays. Meanwhile the Soft X- rays are used in medicine to photograph human body parts. The X- rays pass through the flesh shadow photograph of the bones. Hard X- rays are used for destroying cancerous cells.
DETECTION OF RADIATION
Different types of devices have been developed over the years for detecting radiation. The most commonly used detectors are the Geiger-Muller counter, the scintillation counter and the diffusion cloud chamber.
BIOLOGICAL EFFECTS OF RADIATION
Exposure to radioactive radiation has harmful physiological effects, especially since some of these effects are cumulative. Mild doses of radiation can cause changes in cell structure and body chemistry. Anaemia, cancer, especially leukaemia and genetic mutations are common in these cases . Death usually results from heavier doses. Thus, great care has to be taken when handling radioactive material. And moreover the best shields against most penetrating radioactive rays are thick blocks of lead , iron and also high density concrete. Workers in radiological laboratories are checked regularly to ensure that they have not been exposed to a dangerous dose of radiation. We are also exposed to some radiation from the radioactive material in the earth’s crust and from cosmic rays from outer space, but this amount is so small that there are no harmful effects. This form of radiation is known as background radiation.
USES OF RADIOISOTOPES
Medical uses: Intense y- radiation can be used to destroy cancerous growths. However , great care must be taken to regulate the dosage and confine the irradiation to only cancerous tissues.
Sterilization: When an object is irradiated with y-radiation , germs are killed, leaving it perfectly sterile with no trace of radioactivity. This is particularly used for sterilizing surgical equipment which can be irradiated after it is sealed so that there is no risk of further contamination.
The Industrial uses: B and y- radiations are being used to monitor and control the thickness of a sheet material, such as plastic, paper and metal as well, during production by detecting for variations in the intensity of the radiation passing through the material. In the same way, these radiations are used to measure the wall thickness of pipes to check for internal corrosion.
Agricultural purposes: Radioactive radiation is employed in agricultural research to induce mutations (modifications in the genetic constitution) in plants and animals in order to obtain new and improved varieties with desired characteristics such as earlier maturity, and productivity and greater resistance to diseases. Radiation is also uses in insect and pest control, for example, the male pupae of an undesired insect are sterilized by irradiation so that sterile male adults are produced. However, they’re released in large numbers to mate with the native female adults, which will then lay unfertilized eggs. Consequently, the next generation of this insect will be greatly reduced in number.
Radioactive tracers: The movement or behavior of a radioactive atom can be traced because it emits radiation. However, the radioactive isotope of an element may be used as a marker or tracer to trace what happens to the element during a chemical change. Furthermore making use of this technique, many metabolic processes in both plants and animals have been studied.
Dating techniques: The presence of very long _lived radioisotopes in the earth’s crust is utilized to estimate the age of rocks. This is done by comparing the radioactivity of rocks now with that which they were presumed to have had when first formed. From the proportions of radioactive uranium, radium and lead occurring in some of the oldest rocks.