Thomson’s Model of an Atom was proposed in 1903
All matter is made of atoms and all the atoms are electrically neutral and it was known that all atoms contain the electrons. Based on these facts, Thomson concluded that –
(a) an atom could be considered as a sphere of uniform positive charge in which electrons are embedded.
(b) the negative and positive charges are equal in magnitude. It makes the atom as a whole electrically neutral.
This model is similar to a water-melon according to which an atom can be thought of as a sphere of positive charge (red, edible part) in which the electrons are embedded like seeds. This model is also called plum pudding model or raisin pudding model because the electrons resembled the raisins dispersed in a pudding (an English dessert).
Thomson’s model explained successfully that atoms are electrically neutral but it failed to explain the results of various experiments carried out by the other scientists such as Rutherford’s experiments.
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What is radioactivity and alpha particles?
During this period only, the phenomenon of radioactivity was also being studied by the scientists. This is the phenomenon in which certain elements like Uranium, Thorium, Radium etc. are broken and element converted to other elements. They emit radiation which can cause fogging of photographic plate. The elements which omit these rays are called radioactive elements There are about 40 radioactive elements. This phenomenon of spontaneous emission of rays from atoms of certain elements also proved that the atom was divisible and it contained sub –atomic particles.
Henri Becquerel (1896) discovered radioactivity. Madam Curie also gave valuable contribution in this field. The radiation could have three types of particles – ‘α’ (alpha), ‘β’ (beta) and ‘γ’ (gamma).
Alpha particles –
- They are helium nuclei, He²+. They are doubly positively charged with a mass of 4u. They are obtained by the removal of two electrons from the helium atom.
- They move with speed equal to the 1/10th of the velocity of the light i.e. they are fast moving particles having considerable amount of energy.
Beta particles – They are negatively charged. They move with the speed equal to the velocity of light.
Gamma particles – They are electromagnetic radiations. They carry no charge. They travel with speed equal to velocity of light.