John Dalton developed atomic theory in the early 1800s based on careful chemical measurements. The main postulates of Dalton's atomic theory were that matter is composed of very tiny indivisible particles called atoms, atoms of a given element are identical in mass and properties, and atoms of different elements have different properties and combine in small whole number ratios. While some aspects have been updated, Dalton's atomic theory of atoms as the basic building blocks of matter remains valid in modern chemistry.

1. Dalton’s Atomic Thoery
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2. Who Is Dalton?
John Dalton FRS (6 September
1766 – 27 July 1844) was an
English chemist, meteorologist
and physicist. He is best known
for his pioneering work in the
development of modern atomic
theory, and his research into
colour blindness (sometimes
referred to as Daltonism, in his
honour).

3. What Is DALTON'S ATOMIC THEORY?
It was in the early 1800s that John Dalton,
an observer of weather and discoverer of
color blindness among other things, came
up with his atomic theory.
The idea of atoms had been proposed
much earlier. The ancient Greek
philosophers had talked about atoms, but
Dalton's theory was different in that it had
the weight of careful chemical
measurements behind it. It wasn't just a
philosophical statement that there are
atoms because there must be atoms.

4. The postulates of Dalton's atomic
theory were used to explain
earlier observations of the
behaviour of matter.
Although two centuries old,
Dalton's atomic theory remains
valid in modern chemical thought.

5. Matter is composed of very tiny or
microscopic particles called
"Atom".

6. Atom is an indivisible particle.

7. Atom can neither be created nor it
is destroyed.

8. Atoms of an element are identical
in size, shape, mass and in other
properties.

9. Atoms of different elements are
different in their properties.

10. Atoms combine with each other
in small whole numbers.

11. All chemical reactions are due
to combination or separation of
atoms.

12. With the discovery of sub-atomic particles, e.g.,
electrons, neutrons and protons, the atom can no
longer be considered indivisible.
 The atoms of same elements are not similar in all
respect. They may vary in mass and density.
These are known as Isotopes. For example:
chlorine has two isotopes having mass numbers
35 a.m.u and 37 a.m.u.

13. Atoms of different elements are not different in all
respects. Atoms of different elements that have the same
atomic mass are called Isobar.
Different elements combine in simple whole
number ratio to form compounds. This is not
seen in complex organic compounds like sugar
C12H22O11.
The theory fails to explain the existence of
Allotropes.

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