NCERT Book Class 9 Science Chapter 4 Structure of the atom

C hapter 4
STRUCTURE OF THE ATOM
In Chapter 3, we have learnt that atoms and From these activities, can we conclude
molecules are the fundamental building that on rubbing two objects together, they
blocks of matter. The existence of different become electrically charged? Where does this
kinds of matter is due to different atoms charge come from? This question can be
constituting them. Now the questions arise: answered by knowing that an atom is divisible
(i) What makes the atom of one element
and consists of charged particles.
different from the atom of another element?
Many scientists contributed in revealing
and (ii) Are atoms really indivisible, as
proposed by Dalton, or are there smaller the presence of charged particles in an atom.
constituents inside the atom? We shall find It was known by 1900 that the atom was
out the answers to these questions in this indivisible particle but contained at least one
chapter. We will learn about sub-atomic sub-atomic particle – the electron identified by
particles and the various models that have J.J. Thomson. Even before the electron was
been proposed to explain how these particles identified, E. Goldstein in 1886 discovered the
are arranged within the atom. presence of new radiations in a gas discharge
A major challenge before the scientists at and called them canal rays. These rays were
the end of the 19th century was to reveal the positively charged radiations which ultimately
structure of the atom as well as to explain its led to the discovery of another sub-atomic
important properties. The elucidation of the
particle. This sub-atomic particle had a charge,
structure of atoms is based on a series of
equal in magnitude but opposite in sign to that
experiments.
One of the first indications that atoms are of the electron. Its mass was approximately
not indivisible, comes from studying static 2000 times as that of the electron. It was given
electricity and the condition under which the name of proton. In general, an electron is
electricity is conducted by dif ferent represented as ‘e–’ and a proton as ‘p+’. The
substances. mass of a proton is taken as one unit and its
charge as plus one. The mass of an electron is
4.1 Charged Particles in Matter considered to be negligible and its charge is
minus one.
For understanding the nature of charged It seemed that an atom was composed of
particles in matter, let us carry out the
protons and electrons, mutually balancing
following activities:
their charges. It also appeared that the protons
Activity ______________ 4.1 were in the interior of the atom, for whereas
electrons could easily be removed off but
A. Comb dry hair. Does the comb then
not protons. Now the big question was:
attract small pieces of paper?
B. Rub a glass rod with a silk cloth and what sort of structure did these particles of
bring the rod near an inflated balloon. the atom form? We will find the answer to
Observe what happens. this question below.