Nuclear Fission and Fusion - Part I

The phenomenon of radioactivity was discovered accidentally by Henry Bacquerel in 1896. He observed that a salt of uranium was emitting various subatomic particles. Interest in nuclear physics was kindled by research by Pierre and Marie Currie. Rutherford, while performing some experiments noticed that if nitrogen nuclei were bombarded with energetic alpha particles, oxygen nuclei were being formed!! This was the first evidence of nuclear transmutation. This led to a lot of research in the way nuclei react with each other and an entire branch of nuclear physics was born which dealt with nuclear reactions alone. Nuclei react with each other when bombarded together at high energies. There are two ways in which nuclei react either by fusing together (nuclear fusion) or by first fusing and then breaking up (nuclear fission).

What we will study in this chapter :
1. Structure of atomic nuclei and representation
2. Forces inside the nucleus
3. Nuclear fusion and nuclear fission reactions
4. Energy released during fission reaction, chain reaction
5. Types of fission reactions, difference between nuclear fusion and fission reactions
6. Difference between nuclear reactions and chemical reactions
7. Nuclear reactors, atoms bomb
8. How sun produces its energy, production of heavy elements in the stars

1. Structure of atomic nuclei and representation
Atom is made up of three basic particles : positively charged protons, negatively charged electrons and neutral neutrons. Neutrons and protons make up the nucleus of an atom. The electrons circulate around the nucleus. Mass of a neutron is nearly equal to a mass of a proton. But an electron mass is 1800 times smaller than the proton mass. In an atom, the number of electrons is equal to the number of protons. Thus an atom as such is always electrically neutral.

Number of protons in an atom is known as the atomic number of the atom. Letter Z represents atomic number. Z = number of protons. Each element in the periodic table of elements has different Z. For example : Hydrogen (H)  has 1 proton. Therefore its atomic number is Z = 1. Calcium (Ca) has 20 protons. Therefore its atomic number Z = 20. An atom of Gold (Au) has 79 protons. Therefore its atomic number Z = 79. Number of neutrons in an atom is denoted by N. N = number of neutrons.

Mass number is defined as the total number of protons and neutrons present in an atom.
Mass number is denoted as A.

A =  Z + N

For example : Hydrogen (H)  has 1 proton and no neutrons Therefore its atomic number Z = 1, N = 0, A = 1.

Calcium (Ca)  has 20 protons and 20 neutrons. Therefore its atomic number Z = 20, N=20 and A = 40.

An atom of Gold (Au) has 79 protons and 118 neutrons. Therefore its atomic number Z = 79, N = 118 and A = 197.

It has to be remembered that atomic mass and atomic weights are not exactly the same. Atomic weight takes into account the actual weight of an atom, including the weight of the electrons, their binding energies, etc.

We have now seen that there are three numbers which describe an atom : Z, N and A. The number of electrons is same as the number of protons Z, hence no extra number to show the electrons is required while representing an atom.

An atom  X is represented as    ^A_ZX_N  , ^A_ZX  or  as _ZX^A .

Even though the number N is not specifically written sometimes, from the values of A and Z, N can be calculated N = A ñ Z.

Examples :  Hydrogen is represented as ¹₁H,  Calcium is written as ⁴⁰₂₀Ca, Gold is written as ¹⁹⁷₇₉Au.

Atoms of an element having the same atomic number Z but different atomic masses A, are called isotopes of each other. Isotopes differ in their neutron numbers only. Chemically isotopes are identical. The chemical symbol for isotopes is same as that of its atom since they have the same Z.

Example 1 : Hydrogen has two isotopes deuterium and tritium.

Hydrogen : Z = 1, N = 0 , A = 1, represented as ¹₁H
Deuterium : Z = 1, N = 1, A = 2, represented as ²₁H
Tritium : Z = 1, N = 2, A = 3, represented as ³₁H.

Example 2 : Carbon has two isotopes. The number of protons in carbon is 6. The two isotopes have neutron numbers 6 and 8 and care called carbon-12 and carbon-14 respectively.

Example 3 : The number of protons in an uranium atom is 92. But some of the uranium atoms have N= 143 and some have N = 146. Thus there are two isotopes of Uranium. They are represented as 
²³⁵₉₂U  and    ²³⁸₉₂U.   

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