Nuclear Chemistry PDF

259 60 Nuclear Chemistry E3 Chapter 7 Nuclear Chemistry substance, the phenomenon is called induced or artificial radioactivity. ID “The branch of chemistry which deals with the study of composition of atomic nucleus and the nuclear transformations is known as nuclear chemistry”. Radioactivity can b...

259 60 Nuclear Chemistry E3 Chapter 7 Nuclear Chemistry substance, the phenomenon is called induced or artificial radioactivity. ID “The branch of chemistry which deals with the study of composition of atomic nucleus and the nuclear transformations is known as nuclear chemistry”. Radioactivity can be detected and measured by a number of devices like ionisation chamber, Geiger Muller counter, proportional counter, flow counter, end window counter, scintillation counter, Wilson cloud chamber, electroscope, etc. Radioactivity D YG U The common examples of nuclear processes are radioactivity, artificial transmutations, nuclear fission and nuclear fusion. The nuclear is also an important aspect of chemistry because the energies involved in some of these are million times greater than those in ordinary chemical reactions. “Radioactivity is a process in which nuclei of certain elements undergo spontaneous disintegration without excitation by any external means.’’ and the elements whose atoms disintegrate and emit radiations are called radioactive elements. U Henry Becquerel (1891) observed the spontaneous emission of invisible, penetrating rays from potassium uranyl sulphate K 2 UO2 (SO 4 )2 , which influenced photographic plate in dark and were able to produce luminosity in substances like ZnS. ST Later on, M.M. Curie and her husband P. Curie named this phenomenon of spontaneous emission of penetrating rays as, Radioactivity. Curies also discovered a new radioactive element Radium from pitchblende (an ore of U i.e. U3 O8 ) which is about 3 million times more radioactive than uranium. Now a days about 42 radioactive elements are known. The radioactivity may be broadly classified into two types, (1) If a substance emits radiations by itself, it is said to possess natural radioactivity. (2) If a substance starts emitting radiations on exposure to rays from some natural radioactive Nature and characteristics of radioactive emissions The phenomenon of radioactivity arises because Photographic plate of the decay of unstable  nuclei or certain element.   The nature of the radiations emitted from a radioactive substance was investigated Radioactive substance Lead block by Rutherford (1904) by applying electric and Fig. 7.1 magnetic fields. When these radiation were subjected to electric or magnetic field, these were split into three types ,  and  –rays. Characteristics of radioactive rays -Ray -Ray Charge and mass : It carries +2 charge and 4 unit mass. It carries charge and mass. Identity : Helium nuclei or helium Electron  1e 0 ion 2 He 4 -Ray -1 no It has no charge and negligible mass. High energy raditons. or He2+. Action of magnetic field : Deflected towards the cathode. Deflected anode. Velocity : 1/10th that of light. Same as that of light. to to Not deflected. Same as that of light. 260 Nuclear Chemistry Beta particle emission is due to the result of decay of Ionizing power : Very high nearly 100 times to that of -rays. Low nearly 100 times to that of rays. Very low. Effect on ZnS plate : They cause luminescence. Penetrating power : Low Very little effect. Very effect. 100 times that of -particles. 10 times that of -particles. Range : Very small. More than particles. Nature of product : Product obtained by the loss of 1 -particle has atomic number less by 2 units and mass number less by 4 units. Product obtained by the loss of 1 particle has atomic number more by 1 unit, without any change in mass number. More There is no change in the atomic number as well as in mass number. Theory of radioactivity disintegration -β    Daughter element Parent element Atomic mass : W Atomic number : Z W Z 1 (iii) -ray emission : -rays are emitted due to secondary effects. The excess of energy is released in the form of -rays. Thus -rays arise from energy rearrangements in the nucleus. As -rays are short wavelength electromagnetic radiations with no charge and no mass, their emission from a radioactive element does not produce new element. Special case : If in a radioactive transformation 1 alpha and 2 beta-particles are emitted, the resulting nucleus possesses the same atomic number but atomic mass is less by 4 units. A radioactive transformation of this type always produces an isotope of the parent element. ID Rutherford and Soddy, in 1903, postulated that radioactivity is a nuclear phenomenon and all the radioactive changes are taking place in the nucleus of the atom. They presented an interpretation of the radioactive processes and the origin of radiations in the form of a theory known as theory of radioactive disintegration. The main points of this theory are, The electron produced escapes as a beta-particleleaving proton in the nucleus. 60 little E3 - neutron into proton and electron. 0 n1 1 p 1  1e 0 α A W   Z 2B W 4 β   Z 1 C W 4 β   ZD W 4 A and D are isotopes. Group displacement law Soddy, Fajans and Russell (1911-1913) observed that when an -particle is lost, a new element with atomic number less by 2 and mass number less by 4 is formed. Similarly, when -particle is lost, new element with atomic number greater by 1 is obtained. The element emitting then  or -particle is called parent element and the new element formed is called daughter element. The above results have been summarized as, (1) When an -particle is emitted, the new element formed is displaced two positions to the left in the periodic table than that of the parent element (because the atomic number decreases by 2). (2) When a -particle is emitted, the new element formed is displaced one position to the right in the periodic table than that of the parent element (because atomic number increased by 1). (3) When a positron is emitted, the daughter element occupies its position one group to the left of the parent element in periodic table. Group displacement law should be applied with great care especially in the case of elements of lanthanide series (57 to 71), actinide series (89 to 103), VIII group (26 to 28; 44 to 46; 76 to 78), IA and IIA groups. To determine the number of - and - particles emitted during the nuclear transformation. It can be U (1) The atomic nuclei of the radioactive elements are unstable and liable to disintegrate any moment. Z D YG (2) The disintegration is spontaneous, i.e., constantly breaking. The rate of breaking is not affected by external factors like temperature, pressure, chemical combination etc. (3) During disintegration, atoms of new elements called daughter elements having different physical and chemical properties than the parent elements come into existence. (4) During disintegration, either alpha or beta particles are emitted from the nucleus. U The disintegration process may proceed in one of the following two ways, (i) -particle emission : When an -particle 4 (2 He ) is emitted from the nucleus of an atom of the ST parent element, the nucleus of the new element, called daughter element possesses atomic mass or atomic mass number less by four units and nuclear charge or atomic number less by 2 units because -particle has mass of 4 units and nuclear charge of two units. Parent element Atomic mass : W Atomic number : Z - α Daughter element  W 4 Z 2 (ii) -particle emission : -particle is merely an electron which has negligible mass. Whenever a beta particle is emitted from the nucleus of a radioactive atom, the nucleus of the new element formed possesses the same atomic mass but nuclear charge or atomic number is increased by 1 unit than the parent element. done in following manner, a  b  4 x or x  ab 4 a c X bd Y  x 42 He  y 1 e 0.....(i) Nuclear Chemistry.....(ii) Radioactive disintegration series Name series life of End product Number of lost particles 92 U 238 92 U 235 1. 39  10 10 10 4. 5  10 9 7.07  108 1. 39  10 10 2. 2  10 6 4. 5  10 9 13.5 Thorium (Natural ) Neptuniu m (Artificia l) Uranium (Natural ) Actinium (Natural ) 82 Pb n 241 94 Pu 208 52  6  4 83 Bi 209 52  8  5 82 Pb 206 51  8  6 82 Pb n1     n 2 n1  p 2  n1    p 2 n1  p 2 Binding forces between like nucleons (p - p or n n) result from the exchange of neutral mesons (0) as represented below. (a) p1 p 2   0 or p1   0 p2 (b) n1 n 2   or n1   n2 0 Nuclear stability 0 E3 232 4n + 3 58 (b) p1  n 2 Nuclides can be grouped on the basis of nuclear stability, i.e. stable and unstable nucleus. The most acceptable theory about the atomic nuclear stability is based upon the fact that the observed atomic mass of all known isotopes (except hydrogen) is always less from the sum of the weights of protons and neutrons present in it. Electron (- particle) from a radioactive nucleus may be regarded as derived from a neutron in the following way, U Half (yrs) 90 Th 4n + 2 59 D YG Parent element Half life (yrs) 4n + 1 59 meson (+ or   ) ID 4n 58 The binding forces between unlike nucleons (p and n) are explained by the oscillation of a charged (a) p1  n 2 The phenomenon of natural radioactivity continues till stable nuclei are formed. All the nuclei from the initial element to the final stable element constitute a series known as disintegration series. Further we know that mass numbers change only when -particles are emitted (and not when -particles are emitted) causing the change in mass of 4 units at each step. Hence the mass numbers of all elements in a series will fit into one of the formulae. 4n, 4 n  1 , 4 n  2 and 4 n  3 , hence there can be only four disintegration series. n force which is used in rapid exchange of Pi-mesons between nucleons are also called exchange forces. 60 c  d  2x  y where x = no. of -emitted, y = no. of -emitted substituting the value of x from eq. (i) in eq. (ii) ab a  b  we get c  d   2  y ; y  d   c  4   2  261 Neutron Proton  Electron Similarly, photons are produced from internal stresses within the nucleus. 207 51  7  4 Nuclear structure and Nuclear forces ST U According to an earlier hypothesis, the nucleus is considered as being composed of two building blocks, proton's and neutron's, which are collectively called nucleons. The forces, which hold the nucleons together means stronger proton – proton, neutron – neutron and even proton – neutron attractive forces, exist in the nucleus. These attractive forces are called nuclear forces. Nuclear forces operate only within small distance of about 1  10 15 m or 1 fermi (1 fermi = 10 13 cm ) and drops rapidly to zero at a distance of 1  10 13 cm. These are referred to as short range forces. Nuclear forces are nearly 10 21 times stronger than electrostatic forces. Yukawa in 1935, put forward a postulate that neutrons and protons are held together by very rapid exchange of nuclear particles called Pi-mesons (mesons have mass equal to 275 times of the mass of an electron and a charge equal to +1, 0 or –1. These are designated as + 0 and – respectively). The nuclear The stability of nucleus may be discussed in terms of any one of the following, (1) Nuclear Binding Energy and Mass defect : It is observed that atomic mass of all nuclei (except hydrogen) is different from the sum of masses of protons and neutrons. The difference is termed mass defect. Mass defect = Total mass of nucleons – obs. atomic mass The mass defect is converted into energy. This energy is called the binding energy. This is the energy required to break the nucleus into is constituents (p and n). Binding energy = Mass defect 931 MeV The stability of the nucleus is explained on the value of binding energy per nucleon and not on the basis of total binding energy. Binding energy per nucleon is maximum (8.7 MeV) in the case of iron (56). The value of binding energy per nucleon can be increased either by fusion of lighter nuclei or fission of heavier nuclei. Value of binding energy predicts the relative stability of the different isotopes of an element. If the 262 Nuclear Chemistry 4  0 n1  2 He 5  0.8 MeV Therefore, 2 He 5 4 2 He 4 is more stable than 2 He 3 and. (2) Packing fraction : The difference of actual isotopic mass and the mass number in terms of packing fraction is defined as, Actual isotopicma ss  M ass number Packing fraction   10 4 M ass number ID The value of packing fraction depends upon the manner of packing of the nucleons with in the nucleus. Its value can be negative, positive or even zero. A negative packing fraction generally indicates stability of the nucleus. (4) Neutron-proton ratio or causes of radioactivity It has been found that the stability of nucleus depends upon the neutron to proton ratio (n/p). If we plot the number of neutrons against number of protons for nuclei of various elements, it has been observed that most of the stable (non-radioactive) nuclei lie in a belt shown by shaded region in figure this is called stability belt or stability zone. The nuclei Stability whose n/p ratio does not lie in the belt are unstable and belt undergo spontaneous radioactive disintegration. Neutron rich nuclei n/p=1 Proton rich nuclei 20 40 60 80100 120 Atomic number (p)7.2 Fig. It has been observed that, (i) n / p ratio for stable nuclei lies quite close to D YG U In general, lower the packing fraction, greater is the binding energy per nucleon and hence greater is the stability. The relatively low packing fraction of He, C and O implies their exceptional stability, packing fraction is least for Fe (negative) and highest for H (+78). 60 2 He  0 n 2 He  20.5 MeV 1 E3 2 He 3 series terminate into stable isotope of lead (magic number for proton = 82, magic number for neutron = 126). Nuclei with nucleons just above the magic numbers are less stable and hence these may emit some particles to attain magic numbers. Neutron number (n) value of binding energy is negative, the product nucleus or nuclei will be less stable than the reactant nucleus. Thus the relative stability of the different isotopes of an element can be predicted by the values of binding energy for each successive addition of one neutron to the nucleus. (3) Magic number : Nucleus of atom, like extranuclear electrons, also has definite energy levels (shells). ST U Nuclei with 2, 8, 20, 28, 50, 82 or 126 protons or neutrons have been found to be particularly stable with a large number of isotopes. These numbers, commonly known as Magic numbers are defined as the number of nucleons required for completion of the energy levels of the nucleus. Nucleons are arranged in shells as two protons or two neutrons (with paired spins) just like electrons arranged in the extra-nuclear part. Thus the following nuclei 2 He 4 , 8 O16 , 20 Ca 40 and 82 Pb 208 containing protons 2, 8, 20 and 82 respectively (all magic numbers) and neutrons 2, 8, 20 and 126 respectively (all magic numbers) are the most stable. Magic numbers for protons : 2, 8, 20, 28, 50, 82,114 Magic numbers for neutrons : 2, 8, 20, 28, 50, 126, 184, 196 When both the number of protons and number of neutrons are magic numbers, the nucleus is very stable. That is why most of the radioactive disintegration unity for elements with low atomic numbers (20 or less) but it is more than one for nuclei having higher atomic numbers. Nuclei having n / p ratio either very high or low undergo nuclear transformation. (ii) When n / p ratio is higher than required for stability, the nuclei have the tendency to emit   rays i.e., a neutron is converted into a proton. (iii) When n / p ratio is lower than required for stability, the nuclei increase the ratio, either by emitting   particle or by emitting a position or by Kelectron capture. Rate of radioactive decay “According to the law of radioactive decay, the quantity of a radio-element which disappears in unit time (rate of disintegration) is directly proportional to the amount present.” The law of radioactive decay may also be expressed mathematically. Suppose N0 be the number of atoms of the radioactive element present at the commencement of observation, t  0 and after time t, the number of atoms remaining unchanged is N t. The rate of disintegration  dN t     at any time t is directly proportional to N.  dt  Then,  dN t = N dt Nuclear Chemistry where  is a radioactive constant or decay constant. Various forms of equation for radioactive decay are, This equation is similar to that of first order reaction, hence we can say that radioactive disintegration are examples of first order reactions. However, unlike first order rate constant (K), the decay constant () is independent of temperature. Rate of decay of nuclide is independent of temperature, so its energy of activation is zero. (1) Half-life period (T1/2 or t1/2) : The half-life period of a radioelement is defined, as the time required by a given amount of the element to decay to  1.44  Half life(T1 / 2 )  2  t1 / 2 Thus, the average life period of a radioisotope is approximately under-root two times of its half life (3) Activity of population or specific activity : It is the measure of radioactivity of a radioactive substance. It is defined as ' the number of radioactive nuclei, which decay per second per gram of radioactive isotope.' Mathematically, if 'm' is the mass of radioactive isotope, then Specific activity  Rate of decay N Avogadro number    m m Atomic mass in g where N is the number of radioactive nuclei which undergoes disintegration. (4) Radioactive equilibrium : Suppose a radioactive element A disintegrates to form another radioactive element B which in turn disintegrates to one-half of its initial value. ID 0.693  (T) period. N0 t N 2.303 ;   log 0 t Nt Nt 2. 303 t1 / 2  life 60 log ; log N 0  log N t  0.4343 t Average E3 N t  N0e  t Thus, 263 still another element C. A  B  C B is said to be in radioactive equilibrium with A if its rate of formation from A is equal to its rate of decay into C. D YG U Now since  is a constant, we can conclude that half-life period of a particular radioelement is independent of the amount of the radioelement. In other words, whatever might be the amount of the radioactive element present at a time, it will always decompose to its half at the end of one half-life period. Let the initial amount of a radioactive substance be N 0 Amount of radioactive substance left after n halflife periods n 1 N    N0 2 U Total time T  n  t1 / 2 where n is a whole number. ST (2) Average-life period (T) : Since total decay period of any element is infinity, it is meaningless to use the term total decay period (total life period) for radioelements. Thus the term average life is used. Average life (T)  Sum of lives of the nuclei Total number of nuclei Average life (T) of an element is the inverse of its 1 decay constant, i.e., T  , Substituting the value of   in the above equation, T t1 / 2 0.693  1.44 t1 / 2 It is important to note that the term equilibrium is used for reversible reactions but the radioactive reactions are irreversible, hence it is preferred to say that B is in a steady state rather than in equilibrium state. At a steady state, N A  B T A t 1 / 2  A    N B  A T B t 1 / 2 B Thus at a steady state (at radioactive equilibrium), the amounts (number of atoms) of the different radioelements present in the reaction series are inversely proportional to their radioactive constants or directly proportional to their half-life and also average life periods. (5) Units of radioactivity : The standard unit in radioactivity is curie (c) which is defined as that amount of any radioactive material which gives 3.7  10 10 disintegration’s per second (dps), i.e.,1c = Activity of 1g of Ra 226  3.7  10 10 dps The millicurie (mc) and microcurie (c) are equal to 10 3 and 10 6 curies i.e. 3.7  10 7 and 3.7  10 4 dps respectively. 1c  10 3 mc  10 6 c ; 1c  3.7  10 10 dps 1mc  3.7  10 7 dps ; 1c  3.7  10 4 dps 264 Nuclear Chemistry But now a day, the unit curie is replaced by rutherford (rd) which is defined as the amount of a radioactive substance which undergoes 10 6 dps. i.e., 1 rd  10 6 dps. The millicurie and microcurie correspondingly rutherford units are millirutherford (mrd) and microrutherford (rd) respectively. 1 c  3.7  10 10 dps  37  10 3 rd recent accelerating instrument is called the synchrotron or bevatron. It is important to note that this instrument cannot accelerate the neutrons, being neutral. When a target element is bombarded with neutrons, product depends upon the speed of neutrons. Slow neutrons penetrate the nucleus while a highspeed neutron passes through the nucleus. 92 U However, in SI system the unit of radioactivity is Becquerel (Bq) 1 Bq = 1 disintegration per second = 1 dps = 1rd, 10 6 Bq  1 rd , 3.7  10 10 Bq  1 c (6) The Geiger-Nuttal relationship : It gives the relationship between decay constant of an radioactive substance and the range of the -particle emitted. D YG The conversion of one element into another by artificial means, i.e., by means of bombarding with some fundamental particles, is known as artificial transmutation. The phenomenon was first applied on nitrogen whose nucleus was bombarded with particles to produce oxygen. 14  2 He 4 7N Nitrogen isotope Alpha particle 17  1 H1 8O Oxy gen isotope Proton ST U The element, which is produced, shows radioactivity, the phenomenon is known as Induced radioactivity. The fundamental particles which have been used in the bombardment of different elements are, 1 0n high speed 4 Be 9 ( , n) 6 C 12 i.e. 94 94 4 Be 9  2 He 4 6 C 12  0 n1 Pu 239 ( , n) 96 Cm 242 i.e. Pu 239  2 He 4 94 Cm 242  0 n1 (b) , p type 9F 19 N 7 ( , p) 10 Ne 22 ie. 9 F19  2 He 4 10 Ne 22  1 H 1 14 ( , p) 8 O 17 i.e., 7 N 14  2 He 4 8 O17  1 H 1 (c) ,  type 26 Fe 59 ( ,  ) 29 Cu 63 i.e., 26 Fe 59  2 He 4 29 Cu 63  1e 0 (ii) Transmutation by protons (a) p, n type 15 P 31 ( p, n) 16 S 31 i.e., P 31  1 H 1 16 S 31  0 n1 15 (b) p,  type 6C 12 ( p,  ) 7 N 13 i.e., 6 C 12  1 H 1 N 13   (c) p, d type 4 Be 9 ( p, d ) 4 Be 8 i.e., 4 Be 9  1 H 1 4 Be 8  1 H 2 (d) p,  type 8O 16 ( p,  ) 7 N 31 i.e., 8 O 16  1 H 1 7 N 13  2 He 4 -particle : 2 He 4 ; Proton : 1 H 1 (iii) Transmutation by neutrons (a) n,p type Deutron : 1 H 2 or 1 D 2 ; Neutron : 0 n1 13 Since -particles, protons and deutrons carry positive charge, they are repelled by the positively charged nucleus and hence these are not good projectiles. On the other hand, neutrons, which carry no charge at all, are the best projectiles. Cyclotron is the most commonly used instrument for accelerating these particles. The particles leave the instrument with a velocity of about 25,000 miles per second. A more 92 U 237  20 n1 Thus slow neutrons, also called thermal neutrons are more effective in producing nuclear reactions than high-speed neutrons. Alchemy : The process of transforming one element into other is known as alchemy and the person involved in such experiments is called alchemist. Although, gold can be prepared from lead by alchemy, the gold obtained is radioactive and costs very high than natural gold. (i) Transmutation by -particles (a) , n type U Where R is the range or the distance which an particle travels from source before it ceases to have ionizing power. A is a constant which varies from one series to another and B is a constant for all series. It is obvious that the greater the value of  the greater the range of the -particle. Artificial transmutation of elements 92 U 239 ; 92 U 238  ID log   A  B log R 1 0n slow speed E3 1 c  3.7  10 4 dps  37 mrd  238 60 1 mc  3.7  10 7 dps  37 rd Al 27 (n, p) 12 Mg 27 i.e., 13 Al 27  0 n1 12 Mg 27  1 H 1 (b) n, type 8O 16 (n,  ) 12 Mg 27 i.e., 8 O 16  0 n1 6 C 13  2 He 4 n,  type (c) 238 (n,  ) 92 U 239 i.e., 92 U 92 U 238  0 n1 92 U 238   (d) n, type 8O 18 (n,  ) 9 F19 i.e., 8O 18  0 n1 9 F19  1e 0 (iv) Transmutation by deutrons Nuclear Chemistry d,p type 6 6 2 7 7 1 3 Li (d , p ) 3 Li i.e., 3 Li  1 H 3 Li  1 H 32 As 75 (d , p) 32 As 76 i.e., 32 As 75  1 H 2 32 As 76  1 H 1 (v) Transmutation by -radiations (a)  , n type 4 Be 9 (, n) 4 Be 8 i.e., 4 Be 9   4 Be 8  0 n1 Synthetic elements : Elements with atomic number greater than 92 i.e. the elements beyond uranium in the periodic table are not found in nature like other elements. All these elements are prepared by artificial transmutation technique and are therefore known as transuranic elements or synthetic elements. Nuclear chain reaction : With a small lump of U 235 , most of the neutrons emitted during fission escape but if the amount of U 235 exceeds a few kilograms (critical mass), neutrons emitted during fission are absorbed by adjacent nuclei causing further fission and so producing more neutrons. Now since each fission releases a considerable amount of energy, vast quantities of energy will be released during the chain reaction caused by U 235 fission. Uranium 60 (a) E U 235  0 n 1 56 Ba 140  36 Kr 93  3 0 n 1 + Huge amount of D YG Spallation reactions are similar to nuclear fission. However, they differ by the fact that they are brought by high energy bombarding particles or photons. Elements capable of undergoing nuclear fission and their fission products. Among elements capable of undergoing nuclear fission, uranium is the most common. The natural uranium consists of three isotopes, U 238 E3 Fig. 7.3 Atomic bomb : An atomic bomb is based upon the process of that nuclear fission in which no secondary neutron escapes the lump of a fissile material for which the size of the fissile material should not be less than a minimum size called the critical size. There is accordingly a sudden release of a tremendous amount of energy, which represents an explosive force much greater than that of the most powerful TNT bomb. In the world war II in 1945 two atom bombs were used against the Japanese cities of Hiroshima and Nagasaki, the former contained U-235 and the latter contained Pu-239. Atomic pile or Nuclear reactor : It is a device to obtain the nuclear energy in a controlled way to be used for peaceful purposes. The most common reactor consists of a large assembly of graphite (an allotropic form of carbon) blocks having rods of uranium metal (fuel). Many of the neutrons formed by the fission of U energy 9 E ID 92 Neutron 3E Nuclear fission and Nuclear fusion (1) Nuclear fission : The splitting of a heavier atom like that of uranium – 235 into a number of fragments of much smaller mass, by suitable bombardment with sub-atomic particles with liberation of huge amount of energy is called Nuclear fission. Hahn and Startsman discovered that when uranium-235 is bombarded with neutrons, it splits up into two relatively lighter elements. 265 namely U 234 (0.006 %) , U 235 (0.7%) and (99.3%). Of the three isomers of uranium, nuclear fission of U 235 and U 238 are more important. Uranium238 undergoes fission by fast moving neutrons while U U 235 undergoes fission by slow moving neutrons; of these two, U 235 fission is of much significance. Other ST examples are Pu 239 and U 233. Uranium-238, the most abundant (99.3%) isotope of uranium, although itself does not undergo nuclear fission, is converted into plutonium-239. 92 U 93 238  0 n 1 92 U 239 ; 92 U 239 92 NP 239  1 e 0 Np 238 94 Pu 239  1 e 0 Which when bombarded with neutrons, undergo fission to emit three neutrons per plutonium nucleus. Such material like U-238 which themselves are nonfissible but can be converted into fissible material (Pu239) are known as fertile materials. nuclei of 92 U 235 escape into the graphite, where they are very much slow down (from a speed of about 6000 or more miles/sec to a mile/sec) and now when these low speed neutrons come back into the uranium metal they are more likely to cause additional fissions. Such a substance like graphite, which slow down the neutrons without absorbing them is known as a moderator. Heavy water, D2O is another important moderator where the nuclear reactor consists of rods of uranium metal suspended in a big tank of heavy water (swimming pool type reactor). Cadmium or boron are used as control rods for absorbing excess neutrons. Plutonium from a nuclear reactor : For such purposes the fissile material used in nuclear reactors is the natural uranium which consists mainly (99.3%) of U-238. In a nuclear reactor some of the neutrons produced in U-235 (present in natural uranium) fission 266 Nuclear Chemistry 4 2 He Helium Positron Nuclear fusion The process occurs only in the nuclei of light elements. The process involves the fission of the lighter nuclei to heavy nucleus. The process takes place at higher temperature E3 60 Nuclear fission The process occurs only in the nuclei of heavy elements. The process involves the fission of the heavy nucleus to the lighter nuclei. The process can take place at ordinary temperature. ( 10 8 o C). The energy liberated during this process is high (200 MeV per fission) Percentage efficiency of the energy conversion is comparatively less. The process can be controlled for useful purposes.  2 1e 0  Energy D YG This is not a simple reaction but involves a set of the thermonuclear reactions, which take place in stars including sun. In other words, energy of sun is derived due to nuclear fission. Controlled nuclear fusion : Unlike the fission process, the fusion process could not be controlled. Since there are estimated to be some 10 Difference between Nuclear fission and fusion The energy liberated during the process is comparatively low (3 to 24 MeV per fusion) Percentage efficiency of the energy conversion is high (four times to that of the fission process). The process cannot be controlled. U 4 1H1 Hydrogen hydrogen bomb having power of 1 million tonnes of TNT A hydrogen bomb is far more powerful than an atom bomb. Thus if it is possible to have sufficiently high temperatures required for nuclear fusion, the deuterium present in sea (as D2O) is sufficient to provide all energy requirements of the world for millions of years. The first nuclear reactor was assembled by Fermi in 1942. ID converts U-238 to a long-lived plutonium isotope, Pu239 (another fissionable material). Plutonium is an important nuclear fuel. Such reactors in which neutrons produced from fission are partly used to carry out further fission and partly used to produce some other fissionable material are called Breeder reactors. Nuclear reactors in India : India is equipped with the five nuclear reactors, namely (i) Apsara (1952) (ii) Cirus (1960) (iii) Zerlina (1961) (iv) Purnima (1972) and R-5 Purnima uses plutonium fuel while the others utilize uranium as fuel. (2) Nuclear fusion : “Oposite to nuclear fission, nuclear fusion is defined as a process in which lighter nuclei fuse together to form a heavier nuclei. However, such processes can take place at reasonable rates only at very high temperatures of the order of several million degrees, which exist only in the interior of stars. Such processes are, therefore, called Thermonuclear reactions (temperature dependent reactions). Once a fusion reaction initiates, the energy released in the process is sufficient to maintain the temperature and to keep the process going on. 17 pounds of deuterium ( 1 H 2 ) in the water of the earth, and since each pound is equivalent in energy to 2500 tonnes of coal, a controlled fusion reactor would provide a virtually inexhaustible supply of energy. Radioisotopes find numerous applications in a variety of areas such as medicine, agriculture, biology, chemistry, archeology, engineering and industry. (1) Age determination : The age of earth has been determined by uranium dating technique as follows. Samples of uranium ores are found to contain - and -decays. Now if it is assumed that the ore sample contained no lead at the moment of its formation, and if none of the lead Pb 206 as a result of long series of formed from As mentioned earlier the above nuclear reactions can take place only at very high temperatures. Therefore, it is necessary to have an external source of energy to provide the required high temperature. For this purpose, the atom bomb, (i.e., fission bomb) is used as a primer, which by exploding provides the high temperature necessary for successful working of hydrogen bomb (i.e., fusion bomb). In the preparation of a hydrogen bomb, a suitable quantity of deuterium or tritium or a mixture of both is enclosed in a space surrounding an ordinary atomic bomb. The first hydrogen bomb was exploded in November 1952 in Marshall Islands; in 1953 Russia exploded a powerful U 238 decay has been lost then the measurement of the Pb 206 / U 238 ratio will give the value of time t of the mineral. No. of atoms of Pb 206 ST U Hydrogen bomb : Hydrogen bomb is based on the fusion of hydrogen nuclei into heavier ones by the thermonuclear reactions with release of enormous energy. Application of radioactivity No. of atoms of U 238 left  e  t 1 where  is the decay constant of uranium-238 Alternatively, t 2.303  log Initialamount of U 238 Amount of U 238 in the mineral present tilldate Similarly, the less abundant isotope of uranium, U 235 eventually decays to Pb 207 ; Th 232 decays to Pb 208 and thus the ratios of Pb 207 / U 235 and Pb208 / Th 232 can be used to determine the age of rocks and minerals. Nuclear Chemistry (half-life 5760 years) was used by Willard Libby (Nobel lauret) in determining the age of carbonbearing materials (e.g. wood, animal fossils, etc.) Carbon-14 is produced by the bombardment of nitrogen atoms present in the upper atmosphere with neutrons (from cosmic rays). 7 Thus carbon-14 is oxidised to CO 2 and eventually ingested by plants and animals. The death of plants or animals puts an end to the intake of C 14 from the 14 atmosphere. After this the amount of C in the dead tissues starts decreasing due to its disintegration. 14 7 N 14  1 e 0 N N 2. 303 2.303 log 0 log 0 or t   t Nt Nt radioactivity (in the fresh wood), Nt = D YG Radioactivity in the fossil The above formula can be modified as, t 2.303  log Initialratio of C 14 / C 12 (in fresh wood) Similarly, dating purposes. C 14 / C 12 ratio in the old wood tritium 1 C is used to study the kinetics of photo synthesis. (iii) In industry : Radioisotopes are used in industry to detect the leakage in underground oil pipelines, gas pipelines and water pipes. Radioactive carbon has been used as a tracer in studying mechanisms involved in many reactions of industrial importance such as alkylation, polymerization, catalytic synthesis etc. (iv) In analysis : Several analytical procedures can be used employing radioisotopes as tracers. (a) A small amount of radioactive isotope is mixed with the inactive substance and the activity is studied before and after adsorption. Fall in activity gives the amount of substance adsorbed. (b) The solubility of lead sulphate in water may be estimated by mixing a known amount of radioactive lead with ordinary lead. (c) Ion exchange process of separation is readily followed by measuring activity of successive fractions eluted from the column. U where t = Age of the fossil,  = Decay constant, N 0 = Initial phosphorus 32 P in fertilizers has revealed how phosphorus is absorbed by plants. This study has led to an improvement in the preparation of fertilizers. ID It has been observed that on an average, one gram of radioactive carbon emits about 12 -particles per minute. Thus by knowing either the amount of C-14 or the number of -particles emitted per minute per gram of carbon at the initial and final (present) stages, the age of carbon material can be determined by using the following formulae.  (ii) In agriculture : The use of radioactive 14 N 14  0 n1 6 C 14  1 H 1 6C noted that the radioactive isotopes used in medicine have very short half-life periods. 60 14 E3 6C 267 H 3 has been used for ST U (2) Radioactive tracers (use of radio–isotopes) : A radioactive isotope can be easily identified by its radioactivity. The radioactivity can, therefore act as a tag or label that allows studying the behaviour of the element or compounding which contains this isotope. An isotope added for this purpose is known as isotopic tracer. The radioactive tracer is also known as an isotopic tracer. The radioactive tracer is also known as an indicator because it indicates the reaction. Radioisotopes of moderate half-life periods are used for tracer work. The activity of radioisotopes can be detected by means of electroscope, the electrometer or the Geiger-Muller counter. Tracers have been used in the following fields, (i) To diagnose many diseases : For example, Arsenic – 74 tracer is used to detect the presence of tumours, Sodium – 24 tracer is used to detect the presence of blood clots and Iodine –131 tracer is used to study the activity of the thyroid gland. It should be (d) By labelling oxygen of the water, mechanism of ester hydrolysis has been studied. (e) The efficiency of analytical procedures may be measured by adding a known amount of radio-isotopes to the sample before analysis begins. After the completion, the activity is again determined. The comparison of activity tells about the efficiency of separation. (3) Use of  rays :  rays are used for disinfecting food grains and for preserving food stuffs. Onions, potatoes, fruits and fish etc., when irradiated with  rays, can be preserved for long periods. High yielding disease resistant varieties of wheat, rice, groundnut, jute etc., can be developed by the application of nuclear radiations. The  rays radiations are used in the treatment of cancer. The  radiations emitted by cobalt –60 can burn cancerous cells. The  radiations are used to sterilize medical instruments like syringes, blood transfusion sets. etc. These radiations make the rubber and plastics objects heat resistant. Hazards of radiations The increased pace of synthesis and use of radio isotopes has led to increased concern about the effect of radiations on matter, particularly in biological systems. 268 Nuclear Chemistry as it occupies small space and have low absorption cross-section.  The total life span of a radioactive element is infinite.  The  - radiation of total energy 1.02 MeV, 60 emitted when a positron and an electron interact are known as annilation radiation. ID (1)Somatic or pathological damage : This affects the organism during its own life time. It is a permanent damage to living civilization produced in body. Larger dose of radiations cause immediate death whereas smaller doses can cause the development of many diseases such as paralysis, cancer, leukaemia, burns, fatigue, nausea, diarrhoea, gastrointestinal problems etc. some of these diseases are fatal. Many scientists presently believe that the effect of radiations is proportional to exposure, even down to low exposures. This means that any amount of radiation causes some finite risk to living civilization. one ton of coal.  Beryllium has been found to be the best moderator E3 The accident of Chernobyl occurred in 1986 in USSR is no older when radioisotopes caused a hazard there. The nuclear radiations (alpha, beta, gamma as well as Xrays) possess energies far in excess of ordinary bond energies and ionisation energies. Consequently, these radiations are able to break up and ionise the molecules present in living organisms if they are exposed to such radiations. This disrupts the normal functions of living organisms. The damage caused by the radiations, however, depends upon the radiations received. The resultant radiation damage to living system can be classified as, D YG U (2) Genetic damage : As the term implies, radiations may develop genetic effect. This type of damage is developed when radiations affect genes and chromosomes, the body's reproductive material. Genetic effects are more difficult to study than somatic ones because they may not become apparent for several generations. U  The particle like mesons, positron, neutrino, etc, about 20 in number are created by stresses in nucleus but do not exist as component of nucleus. ST  Highest degree of radioactivity is shown by radium.  The nuclear forces are not governed by inverse square law.  About 42 radioactive nuclides (Z > 82) occur in nature. Each of these gives stable end product of an isotope of lead.  The half life is independent of physical or chemical state of a radioactive element.  The average life of the natural radioactive element vary from 10–6s 1010 years or more.  It has been observed that fusion of 45 mg of hydrogen produce as much energy as obtained from