Dalton's atomic theory proposed that: 1) Matter is made of extremely small indivisible particles called atoms. 2) Atoms of a given element are identical in size, mass and properties. 3) Atoms of different elements vary in size, mass and properties. The theory had limitations such as not explaining Gay-Lussac's law of combining volumes or the nature of atomic bonding. Modern atomic theory has updated that: 1) Atoms can be subdivided into smaller particles like electrons and protons. 2) Atoms are destructible through nuclear processes converting mass to energy. 3) Atoms of a given element can differ in mass and properties as

1. atomic theory”
in year 1804.

2. Daltons atomic theory
 The basic postulates of the theory are:
a) Matter is made up of extremely small discrete particles
called atoms.
b) Atoms of a particular elements are similar in all
respects i.e. they possess same size , shape, mass,
and similar structure.
c) Atoms of one element differ from other elements in all
aspects i.e. they differ in size, shape, mass and
structure
d) Atoms are indivisible i.e. they can not further be
subdivided.
e) Atoms are indestructible i.e. they can neither be
created nor be destroyed.
f) Atoms is the unit of a chemical reaction i.e. it is
smallest particle taking part in a chemical reaction.
g) Atoms of different elements combine with each other in

3. Limitations of the Dalton’s atomic
theory
 The daltons atomic theory suffered following
limitations
a) It could not explain gay lussac’s law of
combining volumes.
b) It could not explain why and how do atoms
combine together to form compounds.
c) It could not explain the nature of force which
hold the atoms together.
d) It could not explain why atoms can not exist in
free state and why compound atoms(molecules)
can exist freely.

4. Modern day position of theory
 The present day condition of the theory is:
a) Atoms is the smallest particle which cannot exist in
free state
b) Atoms of a element can not be similar and may
differ in size, shape, mass and chemical properties.
E.g. carbon exist in three types one type has mass
of 12 amu. Second has mass of 13 amu and third
type has a relative mass of 14 amu. Such species
of atoms having an atom which possess different
atomic no. are known as isotopes.

5.  Atoms of one element may be similar to another in
relative masses. E.g.calcium and argon have similar
atomic masses i.e. 40 amu,such species of atoms of
different elements having identical relative masses are
known as isobars.
 Atom is divisible and further can be subdivided into
smaller particles ssuch electrons, protons and neutrons.
Beside these several other particles are also known to
exist in an atom.
 Atom is destructible. The mass can be converted into
energy according to Einstein equation i.e. E=mc2 . The
change is possible during nuclear fission ad fusion
processes. In ordinary reactions atoms are indivisible.
 Atom is still supposed to be the smallest unit of a reaction
 Atoms of different elements usually combine together in a