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of Spectral Lines
chemical elements in the form of gas or vapour
are excited by either heat or a strong electric discharge, they emit
radiations of only certain wavelengths. When the radiation emitted by an
element is observed by means of a spectroscope, bright-coloured lines (line spectrum) are seen. Each element
emits its own spectrum.
It is observed that hydrogen is the lightest element and emits a line
spectrum, which exhibits a definite regularity. The line spectrum of
hydrogen consists of four lines in the visible region. These lines form a
series because as the wavelength of line decreases, the separation
between the lines decreases. Many attempts were made to explain these
lines on the basis of classical theory.
According to Thomson's atom model, an atom is a sphere of positive
charge in which electrons are embedded at definite sites. These sites are
supposed to be distributed in such a way that the mutual electrostatic
repulsion of the electrons is exactly balanced by their effective
attraction towards the centre of the sphere which the positive charge
exerts on them and the atom becomes stable.
atom model failed because,
Model of an Atom
Note : The radius of
an atom is 10,000 times that of a nucleus.
Defects of Rutherford Model
revolve around the nucleus in circular orbits. The circular motion is an
accelerated motion. According to classical theory, accelerated charge
radiates energy continuously. Therefore, orbiting electron which is an
accelerated charge, radiates energy. Therefore,
it loses energy and finally falls into the nucleus. Thus, the atom should
be unstable, however, the atom is stable.
2. As the electron
loses energy, it moves along a spiral path and radius of the orbit
decreases. As a result, the velocity and frequency of the electron go on
increasing. Therefore, it should emit radiation continuously which should
give rise to a continuous spectrum. However, one gets a line spectrum.