Atoms and Moleculs
Law of conservation of mass states that
mass can neither be created nor destroyed in
a chemical reaction.
LAW OF CONSTANT PROPORTIONS
In a chemical substance the elements are
always present in definite proportions by
mass.
Dalton’s atomic theory
i) All matter is made of very tiny particles
called atoms, which participate in
chemical reactions.
(ii) Atoms are indivisible particles, which
cannot be created or destroyed in a
chemical reaction.
(iii) Atoms of a given element are identical
in mass and chemical properties.
(iv) Atoms of different elements have
different masses and chemical
properties.
(v) Atoms combine in the ratio of small
whole numbers to form compounds.
(vi) The relative number and kinds of
atoms are constant in a given
compound.
Atomic masses of a few elements
Element Atomic Mass (u)
Hydrogen 1
Carbon 12
Nitrogen 14
Oxygen 16
Sodium 23
Magnesium 24
Sulphur 32
Chlorine 35.5
Calcium 40
