Page 57 - Understanding NCERT Science 09

• Rutherford’s alpha-particle scattering experiment led to the
discovery of the atomic nucleus.
• Rutherford’s model of the atom proposed that a very tiny
nucleus is present inside the atom and electrons revolve
around this nucleus. The stability of the atom could not be
explained by this model.
• Neils Bohr’s model of the atom was more successful. He
proposed that electrons are distributed in different shells with
discrete energy around the nucleus. If the atomic shells are
complete, then the atom will be stable and less reactive.
• J. Chadwick discovered presence of neutrons in the nucleus of
an atom. So, the three sub-atomic particles of an atom are:
(i) electrons, (ii) protons and (iii) neutrons. Electrons are
negatively charged, protons are positively charged and neutrons

1
have no charges. The mass of an electron is about times
2000
the mass of an hydrogen atom. The mass of a proton and a
neutron is taken as one unit each.
• Shells of an atom are designated as K,L,M,N,….
• Valency is the combining capacity of an atom.
• The atomic number of an element is the same as the number
of protons in the nucleus of its atom.

• The mass number of an atom is equal to the number of nucleons
in its nucleus.
• Isotopes are atoms of the same element, which have different
mass numbers.
• Isobars are atoms having the same mass number but different
atomic numbers.
• Elements are defined by the number of protons they possess.

Exercises

1. Compare the properties of electrons, protons and neutrons.
2. What are the limitations of J.J. Thomson’s model of the atom?
3. What are the limitations of Rutherford’s model of the atom?
4. Describe Bohr’s model of the atom.
5. Compare all the proposed models of an atom given in this
chapter.
6. Summarise the rules for writing of distribution of electrons in
various shells for the first eighteen elements.
7. Define valency by taking examples of silicon and oxygen.

46 SCIENCE

Rationalised 2023-24