Page 52 - Understanding NCERT Science 09
P. 52
4.2.3 BOHR’S MODEL OF ATOM uestions
In order to overcome the objections raised 1. On the basis of Thomson’s model
against Rutherford’s model of the atom, Q of an atom, explain how the atom
Neils Bohr put forward the following is neutral as a whole.
postulates about the model of an atom: 2. On the basis of Rutherford’s
(i) Only certain special orbits known as model of an atom, which sub-
discrete orbits of electrons, are allowed atomic particle is present in the
inside the atom. nucleus of an atom?
(ii) While revolving in discrete orbits the 3. Draw a sketch of Bohr’s model
electrons do not radiate energy. of an atom with three shells.
4. What do you think would be the
observation if the α-particle
Neils Bohr (1885–1962) scattering experiment is carried
was born in Copenhagen out using a foil of a metal other
on 7 October 1885. He was than gold?
appointed professor of
physics at Copenhagen 4.2.4 NEUTRONS
University in 1916. He got
the Nobel prize for his work In 1932, J. Chadwick discovered another sub-
atomic particle which had no charge and a
on the structure of atom in
mass nearly equal to that of a proton. It was
1922. Among Professor eventually named as neutron. Neutrons are
Bohr’s numerous writings, three appearing present in the nucleus of all atoms, except
as books are:
hydrogen. In general, a neutron is
(i) The Theory of Spectra and Atomic represented as ‘n’. The mass of an atom is
Constitution, (ii) Atomic Theory and, therefore given by the sum of the masses of
(iii) The Description of Nature. protons and neutrons present in the nucleus.
Q 2. Helium atom has an atomic mass
These orbits or shells are called energy uestions
levels. Energy levels in an atom are shown in 1. Name the three sub-atomic
Fig. 4.3. particles of an atom.
of 4 u and two protons in its
nucleus. How many neutrons
does it have?
4.3 How are Electrons Distributed
in Different Orbits (Shells)?
The distribution of electrons into different
orbits of an atom was suggested by Bohr
and Bury.
Fig. 4.3: A few energy levels in an atom The following rules are followed for writing
the number of electrons in different energy
These orbits or shells are represented by levels or shells:
the letters K,L,M,N,… or the numbers, (i) The maximum number of electrons
n=1,2,3,4,…. present in a shell is given by the
STRUCTURE OF THE ATOM 41
Rationalised 2023-24
