Structure of Atom

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CHEMISTRY

UNIT 2

STRUCTURE OF ATOM

The rich diversity of chemical behaviour of different elements can be traced to the differences in the internal structure of atoms of these elements.
After studying this unit you will be
able to


know about the discovery of
electron, proton and neutron and
their characteristics;



describe Thomson, Rutherford
and Bohr atomic models;



understand the important
features of the quantum
mechanical model of atom;



understand
nature
of
electromagnetic radiation and
Planck’s quantum theory;



explain the photoelectric effect
and describe features of atomic
spectra;



state the de Broglie relation and
Heisenberg uncertainty principle;



define an atomic orbital in terms
of quantum numbers;



state aufbau principle, Pauli
exclusion principle and Hund’s
rule of maximum multiplicity;



write the electronic configurations
of atoms.

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The existence of atoms has been proposed since the time
of early Indian and Greek philosophers (400 B.C.) who
were of the view that atoms are the fundamental building
blocks of matter. According to them, the continued
subdivisions of matter would ultimately yield atoms which
would not be further divisible. The word ‘atom’ has been derived from the Greek word ‘a-tomio’ which means ‘uncutable’ or ‘non-divisible’. These earlier ideas were mere speculations and there was no way to test them

experimentally. These ideas remained dormant for a very
long time and were revived again by scientists in the
nineteenth century.
The atomic theory of matter was first proposed on a
firm scientific basis by John Dalton, a British school
teacher in 1808. His theory, called Dalton’s atomic
theory, regarded the atom as the ultimate particle of
matter (Unit 1).
In this unit we start with the experimental
observations made by scientists towards the end of
nineteenth and beginning of twentieth century. These
established that atoms can be further divided into subatomic particles, i.e., electrons, protons and neutrons— a concept very different from that of Dalton. The major
problems before the scientists at that time were:
• to account for the stability of atom after the discovery of sub-atomic particles,
• to compare the behaviour of one element from other
in terms of both physical and chemical properties,

C:\Chemistry XI\Unit-2\Unit-2(2)-Lay-2.pmd 12.1.6 (Final)14.1.6,16.1.6, 17.1.6, 24.1.6

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STRUCTURE OF ATOM





to explain the formation of different kinds
of molecules by the combination of
different atoms and,
to understand the origin and nature of the
characteristics of electromagnetic
radiation absorbed or emitted by atoms.

2.1 SUB-ATOMIC PARTICLES
Dalton’s atomic theory was able to explain
the law of conservation of mass, law of
constant composition and law of multiple
proportion very successfully. However, it failed
to explain the results of many experiments,
for example, it was known that substances
like glass or ebonite when rubbed with silk or
fur generate electricity. Many different kinds
of sub-atomic particles were discovered in the
twentieth century. However, in this section
we will talk about only two particles, namely
electron and proton.
2.1.1 Discovery of Electron
In 1830, Michael Faraday showed that if
electricity is passed through a solution of an
electrolyte, chemical reactions occurred at the
electrodes, which resulted in the liberation
and deposition of matter at the electrodes. He
formulated certain laws which you will study
in class XII. These results suggested the
particulate nature of electricity.
An insight into the structure of atom was
obtained from the experiments on electrical
discharge through gases. Before we discuss
these results we need to keep in mind a basic
rule regarding the behaviour of charged
particles : “Like charges repel each other and
unlike charges attract each other”....
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