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After reading this article you will learn about:- 1. Meaning of Isotopes 2. Basic Concepts of Isotopes 3. Atomic Structure 4. Nucleus 5. Applications in Agriculture.
Meaning of Isotopes:
Isotopes are atoms of the same element whose nuclei contain the same number of protons, but different numbers of neutrons i.e. the isotopes of the same element having the same atomic number but they differ in mass numbers. Examples, 8O16, 8O17, 8O18, 18Ar36; 18Ar38, 18Ar40, 19K39, 19K40, 19K41; 92U233, 92U235,92U238,1H1, 1H21H3.
Basic Concepts of Isotopes:
The phenomenon of radioactivity was discovered by Henri Becquerel in the year 1896. The word radioactivity (meaning radiating activity) was first used by Madame Curie. For better understanding of radioactivity and isotopes, it is a pre-requisite to know about the elementary concepts of atomic structure and the basic terminology.
Atomic Structure of Isotopes:
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It is evident that an atom consists of a positively (+ve) charged nucleus surrounded by orbitals K, L, M, N shells of negatively (-ve) charged electrons (a constituent of all atoms) first proposed by G.J. Stoney (1891). Electron, the unit of electricity has a charge of 1.59 × 10-19coloumbs or 4.8 × 10-10e.s.u.
The orbit closest to nucleus represents the most stable state of the atom. On imparting energy to an atom, the electron absorbs the energy and passes to the outer orbits (away from the nucleus), resulting greater energy of the electrons that means the electron is at a higher energy level and the atom with the electron at one of higher energy level is called as an excited state.
The atoms absorb or emit energy when the electron jumps from one orbit to another. The electrons present in the outermost shell participate in chemical reactions or combinations and are called as valency electrons. Each orbit represents a definite energy level. For the first energy level n = 1; it is the lowest energy level and is nearest to the nucleus.
The electrons in the K orbit are, therefore, at the lowest energy level. The movement of electrons from one orbit to another create energy difference and such differences in energy results in the production of electromagnetic radiation namely, X-rays, ultraviolet or visible light.
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Max Planck in 1900 proposed that a black body radiates energy not continuously, but discontinuously in discrete energy packets, called quanta, given by the relation E as follows:
ε = hv = hc/γ where (v = c/γ)
where ε is the quantum of energy in joules, v is the frequency of the emitted radiation in cycles per sec; c is the velocity of light (3.0 × 108 m/sec), h is the Planck’s constant (6.63 × 10-34 Joules sec); λ is the wavelength of the electromagnetic radiation emitted in metres.
It is evident that the magnitude of the energy quantum is proportional to the frequency of the emitted radiation, the greater the frequency, the larger the size of the energy quantum. The sizes of the quanta are determined by frequency.
Nucleus in Isotopes:
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An atomic nucleus has protons and neutrons as major and fundamental components. The sum of neutrons and protons give mass number. The number of protons gives atomic number. A nucleus of mass number m and atomic number a contains (m – a) neutrons and ‘a’ protons. The relationship between mass number (m), atomic number (a) and the neutron number can be written as,
m = a + n
As protons and neutrons are the main constituents of the nucleus, they are combinedly known as nucleons.
Forces in the Nucleus of Isotopes:
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The nuclear binding energy, Eb/A, per nucleon for element of mass number, greater than 20 is approximately constant, about 8 MeV (million electron volt) i.e., Eb= a constant × A (atomic mass). The total nuclear binding energy is given by the expression:
Eb = 14.1 A – 13 A2/3– 0.6Z2/A1/3.
where, Z = atomic number.
Nucleons at the nuclear surface, however, do not; contribute a full 14.1 MeV to the nuclear binding energy. ‘The net value of the nuclear binding energy is about 18 MeV per particle. Nuclear attractive forces are not electrostatic as there are no oppositely charged particles and also the forces act over an exceedingly short range.
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In fact, the nuclear attractive force is independent of charge, as it binds proton to proton, neutron to neutron and neutron to proton. [Note: 1 MeV = 2.31 × 1010cal per mole].
Some important points are to be noted before discussing the radioactivity. The mass particles electron, protons, antiproton and position, and the energy particle neutrino are not subject to decay and are known as “stable particles”. Neutron and meson, on the other hand, are quite unstable, decaying into simpler products and are known as “unstable particles.”
Applications of Isotopes in Agriculture:
In recent years isotopes have been largely used as indication or tracers in the study of mechanism of chemical and biological processes, since their behaviour in course of reaction can be followed by their difference in mass and radioactivity.
The use of radioisotopes in agricultural research may be divided into two broad groups:
(i) As tracer atoms-property of radiation used for identification of the element. Radioactive atoms behave like stable atoms and are traced by the radiations.
(ii) As source of radiation mutation breeding, food preservation, sterile male technique, moisture determination from soils, bulk density of soil.
However, the basic principles of radioisotopes methodology are:
(i) Before decay the radioactive isotopes act in the same way to that of stable counterpart;
(ii) The rate of disintegration of the radioactive elements can be measured.
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