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SAI International School
Class-IX

Subject- Chemistry
Topic-Atoms and Molecules
Topic-Atomicity& molecular mass of the compounds

LESSON NOTE
Atoms: These are the smallest particles of matter which may or may not exist
independently and take part in chemical reactions.
Molecules:These are the smallest particles of matter which can exist independently.

Atomic mass or Relative atomic mass:Atomic mass of an element may be
th
defined as the average relative mass of an atom of the element as compared with 1/12
of the mass of an atom of carbon (C-12 isotope) taken as 12 amu.

Molecular mass or relative molecular mass of the substance:
The relative molecular mass of the substance can be defined as the mass of one
th
molecule of the substance compared with 1/12 of the mass of one atom of carbon
which is taken as a standard. It is expressed in terms of amu or u.

Molecular mass of an element is defined as the sum of the masses of the elements
present in the molecule. It is expressed in atomic mass unit (u).
e.g. H 2O [H = 1, 0 = 16]
1 × 2 + 16 = 18 u
H 2SO 4 [H=1, S=32, O=16]
=2X1 + 32+4X16 = 98 u

Formula Unit Mass
It is the sum of the atomic masses of all atoms in a formula unit of a compound.
The constituent particles are ions.
–
+
e.g., Na + Cl → NaCl
1 × 23 + 1 × 35.5 = 58.5 u

Atomicity:
The number of atoms present in a molecule of an element or a compound is known as
its atomicity.
For example, atomicity of oxygen (O 2) is 2 while atomicity of ozone (O 3) is 3.

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