ATOM & MOLECULES
The smallest particle of a matter that takes part in a chemical reaction is called an atom. The atom of all
gases except those of noble gases, cannot exist in free state. These exist in molecular form. The molecules
of hydrogen, nitrogen, oxygen and halogens are diatomic (H2, N2). Phosphorus molecule is tetratomic and
that of sulphur is octa atomic.
The smallest particle of a matter that can exist in free state in nature, is known as a molecule.
Some molecules are composed of homoatomic atom, e.g., H2, O2, N2, Cl2, O3 etc., while the molecules of
compounds are made up of two or more heteroatomic atoms e.g., HCl, NaOH, HNO3, CaCO3, etc.
DALTON’S ATOMIC THEORY
The concepts put forward by John Dalton regarding the composition of matter are known as Dalton’s atomic
theory. Its important points are as follows.
Every matter is composed of very minute particles, called atoms that take part in chemical reactions.
Atoms cannot be further subdivided.
The atoms of different elements differ from each other in their properties and masses, while the atoms of the
same element are identical in all respects.
The atoms of different elements can combine in simple ratio to form compounds. The masses of combining
elements represent the masses of combining atoms.
Atom can neither be created nor destroyed.
MODERN CONCEPT :
Many of the concepts of Dalton’s atomic theory cannot be explained. Therefore, foundation of modern
atomic theory was laid down by the end of nineteenth century. The modern theory is substantiated by the
existence of isotopes, radioactive disintegration, etc. The important points of the modern atomic theory are
Prof. Henri Bacquerel discovered the phenomenon of radioactivity and found that an atom is divisible.
An atom is mainly composed of three fundamental particles, viz. electron, proton and neutron.
Apart from the aforesaid three fundamental particles, many others have also been identified, viz. positron,
meson, neutrino, a