NCERT Book Class 11 Chemistry Chapter 2 Structure of Atom

Unit 2

structure of atom

The rich diversity of chemical behaviour of different

Objectives 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 The existence of atoms has been proposed since the time
electron, proton and neutron and of early Indian and Greek philosophers (400 B.C.) who
their characteristics; were of the view that atoms are the fundamental building
• describe Thomson, Rutherford blocks of matter. According to them, the continued
and Bohr atomic models; subdivisions of matter would ultimately yield atoms
which would not be further divisible. The word ‘atom’
• understand the important features
has been derived from the Greek word ‘a-tomio’ which
of the quantum mechanical model
means ‘uncut-able’ or ‘non-divisible’. These earlier ideas
of atom;
were mere speculations and there was no way to test
• understand nature of them experimentally. These ideas remained dormant for
electromagnetic radiation and a very long time and were revived again by scientists in
Planck’s quantum theory; the nineteenth century.
• explain the photoelectric effect The atomic theory of matter was first proposed
and describe features of atomic on a firm scientific basis by John Dalton, a British
spectra; school teacher in 1808. His theory, called Dalton’s
• state the de Broglie relation and atomic theory, regarded the atom as the ultimate
Heisenberg uncertainty principle; particle of matter (Unit 1). Dalton’s atomic theory was
able to explain the law of conservation of mass, law of
• define an atomic orbital in terms
constant composition and law of multiple proportion
of quantum numbers;
very successfully. However, it failed to explain the results
• state aufbau principle, Pauli of many experiments, for example, it was known that
exclusion principle and Hund’s substances like glass or ebonite when rubbed with silk
rule of maximum multiplicity; and or fur get electrically charged.
• write the electronic configurations In this unit we start with the experimental observations
of atoms. made by scientists towards the end of nineteenth and
beginning of twentieth century. These established that
atoms are made of sub-atomic particles, i.e., electrons,
protons and neutrons — a concept very different from
that of Dalton.

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