Dalton Atomic Theory

Dalton Atomic Theory

Based upon laws of chemical combination, Dalton’s Atomic Theory
provided an explanation for the Law of Conservation of Mass and Law of
Constant Composition.
Postulates of Dalton’s atomic theory are as follows :

  • All matter is made up of very tiny particles called ‘Atoms’.
    Atom are indivisible particles, which can’t be created or destroyed in a
    chemical reaction. (Proves ‘Law of Conservation of Mass’)
  • Atoms of an element have identical mass and chemical properties.
  • Atoms of different elements have different mass and chemical properties.
  • Atom combine in the ratio of small whole numbers to form compounds.
    (Proves ‘Law of Constant Proportion’)
  • The relative number and kinds of atoms are constant in a given compound.


According to modern atomic theory, an atom is the smallest particle of
an element which takes part in chemical reaction such that during the
chemical reaction, the atom maintain its identity, throughout the chemical
or physical change.

• Atoms are very small and hence can’t be seen even through very powerful

Atomic radius of smallest atom in hydrogen is 0.37 × 10-10 m
or 0.037 nm.


The mass of an atom of an element is called its atomic mass.
In 1961, IUPAC have accepted ‘atomic mass unit’ (u) to express atomic
and molecular mass of elements and compounds.

Atomic Mass Unit
The atomic mass unit is defined as the quantity of mass equal to 1/12 of mass
of an atom of carbon-12.


How do atoms exist ?
• Atoms of most of the elements are very reactive and does not exist in free

Only the atoms of noble gases (such as He, Ne, Ar, Kr, Xe and Rn) are
chemically unreactive and can exist in the free state as single atom.

Atoms of all other elements combine together to form molecules or ions.


A molecule is a group of two or more atoms which are chemically bonded
with each other.

• A molecule is the smallest particle of matter (except element) which is
capable of an independent existence and show all properties of that
E.g., ‘H2O’ is the smallest particle of water which shows all the properties of

A molecule may have atom of same or different elements, depending
upon this, molecule can be categorized into two categories :

Homoatomic molecules (containing atom of same element) and
Heteroatomic molecules or compounds (containing atoms of different

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