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Chapter, , g ~ Radioactivity, , , , SYLLABUS, , (i) Radioactivity and changes in the nucleus; background radiation and safety precautions., , Scope of syllabus : Brief introduction (qualitative only) of the nucleus, nuclear structure, atomic number (Z), |, mass number (A), radioactivity as spontaneous disintegration. o, 8 and y - their nature and properties; changes |, within the nucleus. One example each of o and B decay with equations showing changes in Z and A. Uses of, , , , | radioactivity - radio isotopes. Harmful effects. Safety precautions. Background radiation., | Radiation : X-rays, radioactive fall out from nuclear plants and other sources, ae, Nuclear energy : Working on safe disposal of waste. Safety measures to be strictly reinforced. |, , (ii) Nuclear fission and fusion; basic introduction and equations., , , , , , , , (A) ATOMIC STRUCTURE AND RADIOACTIVITY, , , , 12.1. STRUCTURE OF THE ATOM AND, , NUCLEUS, , The discovery of electrons by Sir J.J., Thomson towards the end of the nineteenth, century and the scattering experiments of alpha, particles by Rutherford and others led to the, following structure of an atom and its nucleus., , Structure of an atom : An atom consists of, electrons, protons and neutrons. The protons and, neutrons reside inside the nucleus of the atom, which is at its centre, while the electrons revolve, around the nucleus in some specific orbits in, which they do not radiate out energy. Such orbits, are called the stationary orbits (or staionary, shells). In each stationary shell, the electron has, a definite energy. The number of shells varies in, atoms of different elements, depending upon the, total number of electrons present in the atom of, that element. The maximum number of electrons, in a shell can be 2n?, where n is the number of, that shell. The various shells around the nucleus, for n = 1, 2, 3, 4, 5, 6, 7, ... are named as, K, L, M, N, O, P, Q, ... respectively and these, shells can accommodate at the most 2, 8, 18, 32,, 50, 72, 98, ... electrons respectively. The, , electrons in different shells have different, energy which increases with the increase in the, number n of that shell., , The size of an atom is determined by the, radius of the shell of its outermost electron and, it is of the order of 10-!° m. The electron has a, negative charge equal to — 1-6 x 10-9 C (or - e), and its mass (m,) is nearly 9-1 x 10! kg which, is approximately 1/1840 times the mass of a, proton., , Structure of nucleus : The nucleus is at the, centre of atom whose size is of the order of, 10-5 m to 10-4 m (ie, 10° to 10~ times the, size of the atom). It consists of protons and, neutrons. The proton has a positive charge equal, to + 1-6 x 10°!9 C (or + e) and its mass (m,) is, nearly 1-67 x 10-°7kg. The total number of, protons in the nucleus determines the place of the, atom in the periodic table and is called the, atomic number of the element of that atom. The, atomic number of an element is denoted by the, symbol Z. The neutron is an electrically neutral, particle (i.e., charge = 0) and its mass (m,) is also, nearly 1-67 x 10” kg which is equal to that of, , 278
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a proton*. The protons and neutrons which are, the main constituents of the nucleus, are called, the nucleons. The total number of nucleons in the, nucleus is called the mass number of the element, and it is denoted by the symbol A. The nucleus, is thus positively charged and its total charge is, + Ze. The mass of nucleus is approximately A, times the mass of a proton., , , , Note : The mass of an electron is negligible, as compared to that of a proton (or a neutron)., Hence total mass of an atom can be considered, , , , , , to be same as the mass of its nucleus., , , , Thus we can define the atomic number and, man numbers as follows., , , , , , , Symbol, charge and mass of electron, proton and neutron, , 12.2 ATOMIC MODEL, , An atom is electrically neutral, therefore the, number of protons in the nucleus of an atom are, equal to the number of electrons revolving around, the nucleus of the atom., , If Z is the atomic number and A is the mass, number of an atom, then the atom contains, , ‘number of electrons = Z, , number of protons =Z, , number of neutrons = A - Z., , (12.1), , -6726 x 10” kg; m,, , , , 1-6749 x 10-7 kg, , , , The atom is specified by the symbol 4X, where X is the chemical symbol for the element., , The actual size of an atom is very small, (nearly 10-!° m) which is invisible, but just to, understand the distribution of its consitutents, we, can draw the model of an atom not to the scale., , Examples : (1) The-lightest atom is hydrogen, whose mass number A is 1 and atomic number Z, is also 1. It is represented as 1H and it has one, proton in the nucleus and one electron in the, K shell as shown in Fig. 12.1., , , , iH, Hydrogen, Fig. 12.1 Model of hydrogen atom, , (2) The helium atom has the mass number, A = 4 and the atomic number Z = 2. It is, represented as $He. It has 2 neutrons and, 2 protons inside the nucleus and 2 electrons in, the K shell., , , , Helium, , Fig. 12.2 Model of helium atom, , (3) The sodium atom has atomic number, Z = 11 and mass number A = 23. It will have, Z = 11 protons and A-Z = 23-11 = 12 neutrons, inside the nucleus and Z = 11 electrons distributed, , , , 23, qi Na, Sodium, , Fig. 12.3 Model of the sodium atom
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in the K, L and M shells (2 in the K shell, 8 in, the L shell and 1 in the M shell) as shown in, Fig. 12.3. It is represented as 2 Na., , , , Note : (1) If an atom undergoes a chemical, change, there is a change in the number of, orbital electrons of the atom, whereas if the, atom undergoes a nuclear change, there is a, change in the number of nucleons inside the, nucleus of the atom., , (2) A nuclear change requires much higher, energy, of the order of few MeV, which is, nearly 10° times as compared to the energy, required for a chemical change, which is of the, order of few eV only., , (3) In a nuclear reaction, the atomic, number (or number of protons) and the mass, number (or total number of protons and, neutrons) remain conserved. In other words,, the total sum of atomic numbers of the, reactants is equal to the sum of atomic, numbers of the products. Similarly, the total, sum of mass numbers of the reactants is equal, to the sum of mass numbers of the products., , , , , , , , 12.3 ISOTOPES, , , , Obviously, the isotopes have the same place, in the periodic table as it depends on the atomic, number Z., , The atoms of isotopes have the same number, of protons (Z), but different number of neutrons, (A -Z) in their nucleus. Since they have the same, , number of electrons outside the nucleus, so their, , chemical properties are also same., , In nature, different isotopes of an element, , occur in different proportion. In most of the cases,, the relative abundance of one isotope is very high, as compared to the others., , Examples : (1) Hydrogen has three isotopes,, namely, protium }H (or ordinary hydrogen, which is most abundent), deuterium ?H (or heavy, , , , hydrogen) and tritium }H. Each isotope in its, nucleus has one proton (Z = 1), but protium (iH), has no neutron, deuterium ( 7H) has one neutron, and tritium GH) has two neutrons. There is one, electron outside the nucleus in each isotope., , The table below gives the number of protons, and neutrons in the nucleus of isotopes of, hydrogen., , (2) Carbon has three isotopes 2C, '3C and, 4C, out of which C is most abundant. Bach, isotope in its nucleus has 6 protons (Z = 6)., The isotope C has 6 neutrons, the isotope >C, has 7 neutrons and the isotope \$C has, 8 neutrons inside the nucleus. The number of, electrons outside the nucleus in each isotope is 6., , (3) Chlorine has two isotopes *5Cl and 37Cl., Their relative abundance in nature is in, the ratio 3 : 1. Each isotope in its nucleus, has, 17 protons. The isotope *3Cl has 18 neutrons,, while #7Cl has 20 neutrons inside the nucleus., The number of electrons outside the nucleus of, each isotope is 17., , , , Note : (1) Tin (Sn) has the largest number, (= 10) of isotopes., , (2) The isotopes are of two kinds:, (a) stable isotopes which have the number of, neutrons nearly equal to the number of protons, in their nucleus, and (b) unstable or radioactive, isotopes which have number of neutrons much, more than the number of protons in their, nucleus. They undergo radioactive decay and, are of great medical and industrial use., , Example : In 733U and *°8,U, each isotope, has 92 protons inside the nucleus, but SU has, 143 neutrons while 7°8U has 146 neutrons, inside the nucleus. In both isotopes, the, number of neutrons are more than the number, , , , , , PS ee See 230 SS eee
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of protons, so both are the radio isotopes., Similarly '4C is a radio isotope out of the, three isotopes '2C, '3C and '4C of carbon., , , , 12.4 ISOBARS, , The atoms of isobars have the same number of, nucleons (A) in their nucleus, but different number, of protons (Z) and different number of neutrons, (A - Z). The number of electrons outside the, nucleus is always equal to the number of protons,, so isobars have different number of electrons., , Examples : (1) {{Na and ?3Mg are isobars., Na contains 11 protons and 12 neutrons inside, its nucleus, and 11 electrons outside the nucleus,, while ?3Mg contains 12 protons and 11 neutrons, inside its nucleus and 12 electrons outside the, nucleus. The total number of protons and neutrons, , is 23 in each., , (2) Similarly ‘$C and '4N are isobars. '4C has, 6 protons and 8 neutrons inside its nucleus, and 6 electrons outside the nucleus, while SN, has 7 protons and 7 neutrons inside the nucleus, and 7 electrons outside the nucleus., , , , Note : If the number of protons and, neutrons get interchanged inside the nucleus,, they are called mirror isobars e.g. 7}Na and, 33Mg are the mirror isobars., , , , , , , , 13.5 ISOTONES, , , , Examples : (1) *}Na and *5Mg are the isotones., Each nucleus has 12 neutrons. 7}Na nucleus has, 11 protons, while {Mg nucleus has 12 protons., , (2) 33K and #Ar are the isotones (each having, 20 neutrons in its nucleus)., , aS aes 281 ©, , 12.6 RADIOACTIVITY, , Henry Becquerel discovered the phenomenon, of radioactivity in 1896. In a dark room he once, left a uranium salt placed on a photographic plate, wrapped in a black paper. After some days, he, was surprised to found that the photographic plate, had been affected. Later on, the same observation, was made with the other salts of uranium. From, these observations, he concluded that uranium and, its salts by themselves emit some kind of, radiations which can pass through the cover (i.e.,, black paper, glass or wood, etc.) of the, photographic plate and affect it. These radiations, were called the Becquerel rays. On further, investigation, these radiations were found to be of, three types : (1) positively charged, (2) negatively, charged, and (3) uncharged which were named, as o (alpha), B (beta) and y (gamma) radiations, respectively. The substances which emit, these radiations were called the radioactive, , substances. Thus, , , , The isotopes of nearly all the elements of, atomic number higher than 82 (i.e., after lead in, the periodic table) are radioactive because in their, nucleus, the number of neutrons is much more, than the number of protons. These are called the, natural radioactive substances., , Any physical change (such as change in, pressure and temperature) or chemical change, (such as excessive heating, freezing, action of, strong electric and magnetic fields, chemical, treatment, oxidation etc.) does not change the, nature of radiation emitted by the substnace and, its rate of decay. This clearly shows that the, phenomenon of radioactivity cannot be due to the, orbital electrons which could easily be affected by, such changes. The radioactivity should therefore, be the property of the nucleus. Thus
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12.7 RADIOACTIVITY AS EMISSION OF, ALPHA (a), BETA (B) AND GAMMA (y), RADIATIONS, , In 1903, Rutherford experimentally studied the, nature of radiations emitted by the radioactive, substances. He found that on subjecting the, radiations given out by a radioactive substance to, a magnetic field in a direction perpendicular to, their path, they separate out into three distinct, constituents as shown in Fig. 12.4 in which, , the electromagnetic waves similar to light waves and, are therefore not affected by the magnetic field., , Similarly, if the radiations given out by a, , radioactive substance are subjected to an electric, , — = field in a direction perpendicular to their path, they, Note : From a radioactive substance | again separate out into the three constituents as, containing a very large number of atoms, there | shown in Fig. 12.5. Those which turn towards the, is no way to know (or predict) when or which | negative plate, are the positively charged alpha (a1), nucleus of the atom will decay at any moment, | particles. Those which turn towards the positive, hence the radioactive decay is a random | plate, are the negatively charged beta (B) particles., Phenomenon i.e., there is no law by which the | Those which pass undeviated, are the uncharged, disintegration of an individual nucleus can be | gamma (ry) radiations. The beta particles are, known (or predicted). deviated more than the alpha particles., , «— ELECTRIC FIELD, , , , , , RADIOACTIVE, SUBSTANCE, , Fig. 12.5 Deflection of radioactive, radiations in an electric field, , magnetic field is normal to the plane of paper 12.8 PROPERTIES OF ALPHA PARTICLES, , inwards. Those which turn to the left* must be, positively charged and are called the alpha (or a), particles. Those which turn to the right must be, negatively charged and are called the beta (or B), particles. The B particles are deviated more than, the © particles. Those which pass undeviated, must, be the uncharged (or neutral) and are called the, gamma (or Y) radiations. Gamma radiations are, , ata MAGNETIC FIELD, INWARDS, x x px, sg xXAx, x, x x x, x x xX Wx ae, x xX, RADIOACTIVE |, SUBSTANCE “J | 3p FEADIBOX, , , , , , , , Fig. 12.4 Deflection of radioactive, radiations in a magnetic field, , * The direction of deflection in a magnetic field is given by, Fleming’s left hand rule., , SSS eee 282, , Some properties of alpha particles are given, , below., (1) An alpha particle consists of two protons and, , two neutrons. It is same as a doubly ionised, helium atom*, i.e., a helium nucleus, containing two protons and two neutrons., , It is represented as 4He or He?*. Fig. 12.6(a), and (b) respectively represent the model of, , (a) HELIUM ATOM (b) ALPHA PARTICLE, Fig. 12.6 Helium atom and alpha particle, , , , * A helium atom which has lost its both the orbitat electrons, is, , called a doubly ionised helium atom.
