NCERT Book Class 12 Physics Chapter 12 Atoms

Physics

Chapter Twelve

ATOMS

12.1 INTRODUCTION
By the nineteenth century, enough evidence had accumulated in favour of
atomic hypothesis of matter. In 1897, the experiments on electric discharge
through gases carried out by the English physicist J. J. Thomson (1856 –
1940) revealed that atoms of different elements contain negatively charged
constituents (electrons) that are identical for all atoms. However, atoms on a
whole are electrically neutral. Therefore, an atom must also contain some
positive charge to neutralise the negative charge of the electrons. But what
is the arrangement of the positive charge and the electrons inside the atom?
In other words, what is the structure of an atom?
The first model of atom was proposed by J. J. Thomson in 1898.
According to this model, the positive charge of the atom is uniformly
distributed throughout the volume of the atom and the negatively charged
electrons are embedded in it like seeds in a watermelon. This model was
picturesquely called plum pudding model of the atom. However
subsequent studies on atoms, as described in this chapter, showed that
the distribution of the electrons and positive charges are very different
from that proposed in this model.
We know that condensed matter (solids and liquids) and dense gases at
all temperatures emit electromagnetic radiation in which a continuous
distribution of several wavelengths is present, though with different
290 intensities. This radiation is considered to be due to oscillations of atoms