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An atom can be best defined with the help of its sub-atomic particles; electrons, protons and neutrons. These fundamental particles are arranged in certain manner. The number of sub-atomic particles is unique for each atom therefore it is best way to differentiate atoms from each other. Today we have modern periodic table or long form of periodic table in which all known elements are arranged in increasing order of their atomic number.

The number of protons and neutrons in an atom determines the stability of an atom. As the number of nucleons increases, the stability of atom decreases. That is the reason, elements with more than 82 atomic numbers are unstable and involve in radioactive decay. Atoms of same elements with same atomic number but different mass number are called as isotopes. Since isotopes of same elements have same atomic number therefore they do not have separate position in the periodic table.

Some of the isotopes are stable whereas some of the isotopes are unstable and emit some radiations to convert into stable isotopes such as C has 3 naturally occurring isotopes,  C-12, C-13, and C-14 out of which C-14 is unstable and exhibits radioactivity. Radioisotopes can be natural or artificial in nature. Elements with atomic number more than 82, exhibit radioactivity naturally therefore known as natural radioactive isotopes. Radioisotopes with smaller atomic numbers can produce artificially with the help of artificial transmutation in which a stable nucleus is bombarded with energetic particles such as protons, neutrons or alpha particles.

It results the formation of new nucleus which is unstable and exhibits radioactivity such as bombardment of energetic neutron with the nitrogen forms C-14 which is unstable and shows radioactivity as it emits beta particles. The radioactive decay of C-14 again forms nitrogen which is a stable nucleus.

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Radioisotopes have several applications in different fields like medicines, industries, agriculture etc.

Radioisotopes of Co are used to detect the internal flaws in a cast material and some of the radioisotopes are also used to dispel static electricity acquired by some materials. Cobalt-60 is also used for gamma sterilization and industrial radiography. Naturally occurring radioisotopes like C-14 is used to estimate the age of wood and other carbon-containing materials. Cl-36 is used to detect the sources of chloride and also the age of water. Similarly Pb-210 is used to estimate the age of layers of sand and soil. Tritium which is an isotope of hydrogen with mass number 3 is used to estimate the age of groundwater up to 30 years. Artificial radioisotopes also involve in various industries such as Am-241 is used in backscatter gauges, smoke detectors and fill height detectors.

Radioisotope of Cesium (Cs-137) is used in radiotracer technique for detection of sources of soil erosion and deposition. It is also used to measure the density, to fill height level switches and for low-intensity gamma sterilization. Cr-51 is used to study coastal erosion and in study of blood. Some other radioisotopes like Co-60, Ln-140, Sc-46, Ag-110, Au-198 are used in blast furnaces to determine resident times and to measure the performance of furnace.

Gold-198 and Technetium-99 are used to study liquid waste movements and tracing factory waste that is causing ocean pollution. They can trace the sand movement in river beds and ocean floors. Gold-198 is also used to label sand to check the coastal erosion. Radioisotope of Iridium ( Ir-192) involves gamma radiography that helps in the location of flaws in metal components. Kr-85 is used for industrial gauging and Mn-54 is involved in the prediction of behavior of heavy metal components in mining waste water. Ni-63 is one of the major components of light sensors in cameras and plasma display. It is also used in electronic discharge prevention and in electron capture detectors. Strontium-90 and Thallium-204 are used for industrial gauging whereas Ytterbium-169 is used in gamma radiography. Zinc-65 helps in the prediction of heavy metal components in industrial waste water.

## Uses of Radioisotopes in Medicine

Like industrial uses, radioisotopes are also involved in medical industries. They mainly involve in diagnosis and therapeutic purposes. Usually radioisotopes are introduced into the body to detect the effected part of body. Radioisotopes are also used in the study of different stages of metabolism that helps in determination of existence of disease.

For example, thyroid gland can be studied by with the help of small dose of radioiodine ( I131). This radioisotope is also used in the therapy of hyperthyrodium. Radioisotope of Fe is used in the detection and treatment of bone marrow whereas Radio calcium involves in the diagnosis and treatment of bone cancer. Na-24 and P-32 are useful in heart and kidney diseases. Similarly radiations from Co-60 are used in the treatment of cancer.

## Uses of Radioisotopes in Agriculture

Carbon dating is one of the most important applications of radioisotopes. In Carbon dating the radioisotope of carbon ( C-14) is used to measure the age of ancient biological materials. Both isotopes of carbon; C-12 and C-14 combine with oxygen to form $CO_{2}$ which is absorbed by plants during photosynthesis. When animals eat these plants, both isotopes are passed to them.