Nuclear Chemistry : Nuclear Reactions & Its Applications

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.

          92U235 + 0n156Ba140 + 36Kr93 + 2−3 0n1 + Huge amount of energy

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, namely U234(0.006%), U235(0.7%) and U238(99.3%). Of the three isomers of uranium, nuclear fission of U235 and U238 are more important. Uranium-238 undergoes fission by fast moving neutrons while U235 undergoes fission by slow moving neutrons; of these two, U235 fission is of much significance. Other examples are Pu239 and U233.

Uranium-238, the more abundant (99.3%) isotope of uranium, although it self does not undergo nuclear fission, is converted into plutonium-239.

92U238 + 0n192U239 ; 92U238 + 0n192U239 ; 92U238 + 0n192U239

Which when bombarded with neutrons undergo fission to emit three neutrons per plutonium nucleus. Such material like U-238 which themselves are non-fissible but can be converted into fissible material (Pu-239) are known as fertile materials.

Release of tremendous amount of energy : The importance of nuclear fission lies in the release of tremendous amount of energy during this process. During the U235 fission nearly 0.215 mass unit per uranium nucleus is found to be converted into energy.

\[\underbrace{\underset{235.124}{\mathop{{{U}^{235}}}}\,+\underset{1.009}{\mathop{_{0}{{n}^{1}}}}\,}_{236.133}\to \underbrace{\underset{138.955}{\mathop{X{{e}^{139}}}}\,+\underset{94.945}{\mathop{S{{r}^{95}}}}\,+\underset{2\times 1.009}{\mathop{{{2}_{0}}{{n}^{1}}+E}}\,}_{235.918}

The released energy is due to difference in the total sum of masses of the reactants and products, in according to the Einsten’s mass energy relation i.e. E = mc2.

Alternatively, Δm = 236.133 – 235.918 = 0.215 amu

∴ 1 amu = 931 MeV

0.215 amu = 931 × 0.215 MeV = 198 MeV = 198 × 2.3 × 107 kcal

∴ Energy released by the fission of 1 g of  

Recall that the combustion of 1 g of carbon releases only 94.0/12 = 7.83 kcal of energy while the fission of 1 g of U235 releases 1.9 × 107. Hence nuclear fission releases several million times higher energy than the ordinary chemical combustion.

Release of neutrons : During U235 fission it is obvious that 2-3 neutrons per uranium molecule are emitted. Some neutrons are ejected within an extremely short interval and are called prompt neutrons; fission products for an appreciable time fraction of a second to several seconds emit the rest after the fission. These are called delayed neutrons.

Note :     Each fission yields 3 neutrons each of which can cause further fission to give 3 neutrons goes on increasing in geometric progression 1, 3, 9, 27, 81, 243,…. and many geometric progression take place in a very small fraction of a second.

Chain reaction : With a small lump of U235, most of the neutrons emitted during fission escape but if the amount of U235 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 U235 fission.

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 nuclei of 92U235 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 likes 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 converts U-238 to a long-lived plutonium isotope, Pu-239 (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.

Production of radioactive isotopes by bombarding with neutrons from a nuclear reactor : These radioactive isotopes are used in medicine, industry and hospitals.

Nuclear reactors in India : India is equipped with the five nuclear reactors, namely Apsara (1952), Cirus (1960), Zerlina (1961), Purnima (1972) and R-5. Purnima uses plutonium fuel while the others utilize uranium as fuel.

Apsara the first nuclear reactor was completed on 14th August 1952 at Trombay under the guidance of the late Dr. H.J. Bhabha. It is the swimming pool reactor, which consists of a lattice of enriched uranium (fuel) immersed in a large pool of water. Water acts as a moderator, coolant and shield. This reactor is simple, safe, flexible, easily accessible and cheap.

(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 Thermo nuclear 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.

\[\underset{\text{Hydrogen}}{\mathop{{{4}_{1}}{{H}^{1}}}}\,\to \,\underset{\text{Helium}}{\mathop{_{2}H{{e}^{4}}}}\,+\,\underset{\text{Positron}}{\mathop{{{2}_{+1}}\,{{e}^{0}}}}\,\,\,+\,\,\text{Energy}

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.

Calculation of energy released in nuclear fusion : Let us write the reaction involving the fusion of four hydrogen nuclei to form helium nucleus.

    \[\underset{\begin{smallmatrix} \,\,\,Hydrogen\,\,Mass \\ 4\times 1.008144\,=\,4.032576\end{smallmatrix}}{\mathop{{{4}_{1}}{{H}^{1}}}}\,\to \underbrace{\underset{\begin{smallmatrix} Helium \\ 4.003873\end{smallmatrix}}\mathop{_{2}H{{e}^{4}}}}\,\,\,+\,\underset{\begin{smallmatrix} \,\,\,\,\,\,\,\,\,\,\,\,Positron \\ 2\,\times \,0.00558=0.001116\end{smallmatrix}}{\mathop{\,{{2}_{+1}}{{e}^{0}}}}\,}_{4.004989}\]

          ∴ Loss is mass, Δm = 4.032576 – 4.004989 = 0.027587 amu

          ∴ Energy released = 0.027587 × 931 MeV = 26.7 MeV

          ∴ Energy released/gm of hydrogen consumed

\[=\frac{26.7}{4}=6.7\,\,\,MeV=6.7\times 2.3\times {{10}^{7}}\,\,\,kcal=1.54\times {{10}^{8}}\,\,\,kcal

Controlled nuclear fusion : Unlike the fission process, the fusion process could not be controlled. Since there are estimated to be some 1017 pounds of deuterium (1H2) 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.

Comparision of nuclear fission and nuclear fusion : Now let us compare the efficiency of the energy conversion of the two processes, i.e. nuclear fission and nuclear fusion

Nuclear fission reaction,

92U235 + 0n156Ba141 + 36Kr92 + 2−3 0n1 + 200 MeV

If one atom of uranium is fissioned by one neutron, the percent efficiency in terms of mass converted into energy (where 1 mass unit = 931 MeV) will be : \[\frac{200\,\,MeV}{(235+1)\,\,\text{mass}\,\,\text{units}\times 931}\times 100=0.09%

Nuclear fusion reaction, 1H2 + 1H32He4 + 0n1 + 17.8 MeV

The percent efficiency of the reaction = \[\frac{17.8\,\,\,MeV}{(2+3\,\,\text{mass}\,\,\text{units)}\times 931}\times 100=0.35%

Thus it indicates that for these two fission and fusion reactions the percent efficiency is approximately four times greater for the fusion reaction.

Hydrogen bomb : Hydrogen bomb is based on the fusion of hydrogen nuclei into heavier ones by the thermonuclear reactions with release of enormous energy.

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 hydrogen bomb having power of 1 million tonnes of TNT

A hydrogen bomb is far more powerful than an atom bomb. Thus if it were possible to have sufficiently high temperatures required for nuclear fusion, the deuterium present in sea (as D2O) sufficient to provide all energy requirements of the world for millions of years.         

Note :     The first nuclear reactor was assembled by Fermi in 1942.

Difference between Nuclear fission and fusion

Nuclear fission Nuclear fusion
The process occurs only in the nuclei of heavy elements. The process occurs only in the nuclei of light elements.
The process involves the fission of the heavy nucleus to the lighter nuclei of comparable masses. The process involves the fission of the lighter nuclei to heavy nucleus.
The process can take place at ordinary temperature. The process takes place at higher temperature 108 oC.
The energy liberated during this process is high (200 MeV per fission) The energy liberated during the process is comparatively low (3 to 24 MeV per fusion)
Percentage efficiency of the energy conversion is comparatively less. Percentage efficiency of the energy conversion is high (four times to that of the fission process).
The process can be controlled for useful purposes. The process cannot be controlled. 


Isotopes, Isobars, Isotones, Isodiaphers, Isoelectronic species, Isosters and Nuclear isomers.

(1)     Isotopes : Atoms of a given element which have same atomic number (nuclear charge) but different mass number are called isotopes. In other words, isotopes are the atoms of the same element differing in mass number. Thus isotopes have same number of protons and electrons but different number of neutrons. They have same position in the periodic table, same chemical properties and same atomic charge. The term was first coined by Soddy. However, Aston using mass spectrometer first separated isotopes (Ne20 and Ne22).

Examples :      

(i)      \[\underset{\text{Hydrogen}\,\,\text{(Protium)}\,\,(p=1,\,\,e=1,\,\,n=0)}{\mathop{_{1}{{H}^{1}}}}\,\,\,\,\underset{\text{Deuterium}\,\,(p=1,\,\,\,e=1,\,\,n=0)}{\mathop{_{1}{{H}^{2}}}}\,\,\,\,\underset{\text{Tritium}\,\,(p=1,\,\,e=1,\,\,n=0)}{\mathop{_{1}{{H}^{3}}}}\,

(ii)     6c12, 6C12, 6C13 and 6C14

(iii)    8O16, 8O17, 8O18

(iv)    17Cl35 and 17Cl37

Of all the elements, tin has maximum number of stable isotopes (ten).

The fractional atomic weight (35.5) of chlorine is due to the fact that in the ordinary chlorine atom, Cl35 and Cl37 are present in the ratio of 3 : 1.

∴  Average atomic weight of Cl  \[=\frac{3\times 35+1\times 37}{4}=35.5\,\,amu

The percentage of a given isotope in the naturally occurring sample of an element is called Isotopic abundance. As the isotopic abundance of an element is constant irrespective of its source, atomic weight of an element is constant.

(2)     Isobars : Isobars are the atoms of different elements with the same mass number but different atomic numbers. In other words, isobars have different number of protons, neutrons and electrons but the sum of protons and neutrons (i.e., number of nucleons) is same.

Examples :

(i)      18Ar40, 19K40 and 20Ca40       

(ii)     52Te130, 54Te130, 54Xe130 and 56Ba130.

Since isobars are the atoms of different elements, they will have different physical and chemical properties.

(3)     Isotones : Isotones are the atoms of different elements with the same number of neutrons but different mass numbers, e.g. 14Si30, 15P31 and 16S32. Since the variable factor in isotones is the number of protons (atomic number), they must have different physical and chemical properties.

Examples :

(i)      14Si30, 14P31 and 16S32            (ii) 19K39 and 20Ca40    

(iii)    1H3 and 2He4                          (iv) 6C13 and 7N14

(4)     Isodiaphers : Atoms having same isotopic number are called isodiaphers.

Mathematically, isotopic number (isotopic excess) = (N ­– Z) or (A – 2Z)

Where, N = Number of neutrons; Z = Number of protons

Examples :

(i)      92U235 and 90Th231

(ii)     19K39 and 9F19

(iii)    29Cu65 and 24Cr55

(5)     Isoelectronic species : Species (atoms, molecules or ions) having same number of electrons are called isoelectronic.

Examples :

(i)      N3−, O2−, F, Ne, Na+, Mg2+, Al3+, CH4, NH3, H2O and HF have 10 electron each.

(ii)     P3−, S2−, Cl, Ar, K+ and Ca2+ have 18 electrons each.

(iii)    H, He, Li+ and Be2+ have 2 electrons each.

(iv)    CO, CN and N2 have 14 electrons each.

(v)     N2O, CO2 and CNO have 22 electrons each.

(6)     Isosters : Molecules having same number of atoms and also same number of electrons are called isosters.

Examples :

(i)      N2 and CO                  

(ii)     CO2 and N2O              

(iii)    HCl and F2        

(iv)    CaO nad MgS            

(v)     C6H6 (benzene) and inorganic benzene B6N6.

(7)     Nuclear isomers : Nuclear isomers (isomeric nuclei) are the atoms with the same atomic number and same mass number but with different radioactive properties. They have same number of electrons, protons and neutrons. An example of nuclear isomers is uranium-X (half-life 1.4 min) and uranium-Z (half-life 6.7 hours). Otto Hahn discovered nuclear isomers.

The reason for nuclear isomerism is the different energy states of the two isomeric nuclei. One may be in the ground state whereas the other should be in an excited state. The nucleus in the excited state will evidently have a different half-life.

Now-a-days as much as more than 70 pairs of nuclear isomers have been found. Few examples areas follows

(i) \[\underset{({{T}_{1/2}}=13.8\,\,hour)}{\mathop{^{69}Zn}}\,\,\,and\,\,\underset{({{T}_{1/2}}=57\,\,\min )}{\mathop{^{69}Zn}}\,              (ii)  \[\underset{({{T}_{1/2}}=4.4\,\,hour)}{\mathop{^{80}Br}}\,\,\,and\,\,\underset{({{T}_{1/2}}=18\,\,\min )}{\mathop{^{80}Br}}\,