The phenomena of self disintegration by emitting certain invisible rays by the naturally occurring substances, elements and its compounds is called radioactivity. This invisible rays emancipating from the radioactive substances are called radioactive rays and also it can only occurs due to instability of the atoms.
The phenomenon of radioactivity was discovered by A.H Becquerel in 1886 purely by accident. He observed a peculiar property of uranium salt, affecting a photographic plate even when the plate was in a light proof package. He termed this property of uranium 'Radioactivity'. The phenomenon of spontaneous emission of radiation by heavy element is called radioactivity. Later Pierre and Madam Curie showed similar phenomenon in other metals like plutonium, Francium and Radium.
(a) Alpha particle
- A positively charged helium atom.
- Has very less penetrating power.
- Can be absorbed by a sheet of paper or stopped by aluminium foil.
(b) Beta particles
- A negatively charged light particle.
- Penetrating power greater than alpha ray.
(c) Gamma (r) rays
- These are electromagnetic radiations of low wavelength, high frequency and high energy.
- Their penetrating power is very great and can pass through several centimetres of lead.
The isotopes which are unstable (due to presence of extra neutrons in their nuclei) and emit various types of radiations are called a radioactive Isotope. For example- Carbon-14, Arsenic -74, Sodium -24 , Iodine -131 , Cobalt-60 and uranium - 235
- X-rays is a form of penetrating electromagnetic radiation similar to light.
- It has a shorter wavelength.
- Capable of penetrating solids.
- X-rays is produced when cathode ray fall on anti-cathodes (a metal of high atomic mass like tungsten)
Dense material like metal and bone absorb X-ray more strongly than material such as wood or flesh. Therefore it is possible to produce X- rays photographs for use in medical diagnoses.
A nuclear reaction is one in which a nucleus is bombarded with an elementary particle like neutron, proton etc.) or with another nucleus to produce other products in a very short time span. The first nuclear reaction was discovered by Rutherford in 1919 when he bombarded nitrogen with alpha particles.
Nuclear fission is the fragmentation of a large nucleus into two smaller nuclei and the liberation of large amount of energy. In 1939 the German scientists Otto Hahan and F. Steersman observed that when uranium was bombarded with slow neutrons then two smaller products were obtained with a tremendous amount of heat. The splitting of uranium is called nuclear fission.
Types of Nuclear Fission
- Controlled Nuclear Fission-Carried out in nuclear reactors in which rate of fission reaction is reduced and the energy produced can be used for constructive purposes.
- Uncontrolled Nuclear Fission- In an atom bomb uncontrolled fission takes place. A very large amount of heat is produced and the process continues until the entire amount of fissionable material is exhausted.
First Atom Bomb
On August 6, 1945, an atom bomb was dropped on Hiroshima city in Japan. The second bomb was dropped on Nagasaki, another city of Japan on August 9, 1945. The bomb was made of plutonium -239.
It is a nuclear reaction in which lighter nuclei fuse to form a nucleus of greater mass. In this reaction also an enormous amount of heat IS produced. By carrying on nuclear fusion under controlled conditions, the large amount of energy could be made available for useful purpose.
Atomic Energy (Nuclear Energy)
Energy produced by nuclear fission or nuclear fusion is called nuclear energy or atomic energy. In nuclear reactions there is loss of mass mechanical energy which can be used for various peaceful purposes. This mass is converted into energy.
Elements having same atomic number but different atomic mass. For Example:
(i) 8016, 8017, 8018
(ii) 1H1, 1H2, 1H3
Elements having the same atomic mass but different atomic number.
For Example: (i) 18Ar40, 19K40, 20Ca40, (ii) 6C14, 7N14
Elements having the same number of neutrons. For Example: (i) lH3, 2He4, (ii) 4Si32
Molecules & ions having same number of atoms as well as electrons. For Example: N2O, CO2
Chain reaction is the phenomenon in which neutrons liberated during a fission process lead to a further fission of atoms, releasing a large amount of energy. Half life is the time when the amount of radioactive element becomes half of the initial.
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