Radioactivity for std 11 & 12 GSEB course based

Review
An atom is made up of three fundamental particles namely proton, neutron and electron.
Protons and neutrons are bound together is the central part of an atom, called the nucleus. 
Electrons revolve round the nucleus in different imaginary orbits. 
The number of protons in an atom is called its atomic number, represented by Z.

Protons and neutrons is a nucleus have a common name ‘nucleons’ the total number of nucleons in an tom is known as the atomic mass number represented by A. Mass number is very close to the atomic mass.
Electron (-1eo) is a negatively charged particle with negligible mass.
Proton (+1p1) is a positively charged particle with unit mass considering the mass of hydrogen atom to be ‘one unit’.
Neutron (0N1) is an electrically neutral particle with unit mass.

INTRODUCTION

In 1896, Henri Becquerel (1852-1908) a French scientist, accidentally discovered radioactivity. He found that a photographic plate, wrapped well to protect is from visible light, was blackened by a mineral containing uranium. He concluded that the mineral emitted a new kind of radiations which occurred without any external cause. The emission of these radiations is not affected by external factors like temperature, pressure, electric field and magnetic field. The radiations were called Becquerel rays.

Radioactivity

All elements which emit Becquerel rays are said to be radioactive and the elements are called radioactive elements. 

All the elements from atomic number 81 to 92 are found to be radioactive. Experiments show that atoms of radioactive elements disintegrate into atoms of other elements. It became clear that the radiations must emanate from the nucleus and radioactivity is a nuclear phenomenon. We can conclude that the phenomenon is due to disintegration of an unstable nucleus accompanied by some type of radiation. 

The property of spontaneous disintegration of certain unstable. This is referred to as natural radioactivity. Radioactivity produced artificially or induced, is called artificial or induced radioactivity. The radiations emitted by radioactive elements are called radioactive radiations.

Note:

In 1898, Marie Curie (1867-1934) and her husband Pierre Cune (1959-1906) discovered a new element named Polonium and later on, another element named Radium, In 1903 Nobel prize in physics was shared by H. Becquerel, M. Curie and P. Curie, for their discovery of radioactivity. In 1911, M. Curie was awarded Nobel prize in chemistry for the extraction of radium in its purest form.