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combination of more than two atoms (Figure 5.3). The smallest
particles which can exist independently are called molecules. Like
elements, compounds also have molecules. Molecules of
compounds consist of atoms of two or more different elements.

Molecules are the smallest particles which can exist
independently.
Method of representing Atoms and Molecules
The size of Atom
You have studied the method of assigning symbols for
Can you imagine the size of an
the elements. Now, let us see how the atoms and atom? The word, 'atom' has
molecules are represented. been derived from the Latin
word, Atomos. It means
The symbol of helium is 'He'. When we write 'He', it
something indivisible. The
represents one atom of the element helium. What does
diameter of a gold atom is
2He represent? 0.0000000254 cm, which
Elements like helium, neon, argon etc., are seen in nature means around 3.5 crores gold
atoms arranged in a row would
as single atoms. They are monoatomic molecules. Their
be as long as just 1cm. Ancient
molecules can also be represented as He, Ne, Ar etc. But Indian thinker Kanadan and
elements like hydrogen exist as diatomic molecules. Greek thinker Democritus have
Hydrogen atom is represented as H, and how about its talked about the smallest
particles of substances
molecule?
centuries ago. It was John
H represents a hydrogen molecule. Dalton, an English Scientist
2
Some elements exist as molecules of more than two who formulated the modern
theory of atom.
atoms. Phosphorus (P ), sulphur (S ) etc., are examples
4 8
of such elements.
Molecules with only one atom are called monoatomic
molecules. Those with two atoms each are called diatomic
and molecules with more than two atoms are called
polyatomic molecules.

In monoatomic molecules, the number given on the left John Dalton (1766-1844)
side of the symbol indicates the number of molecules
and atoms. In polyatomic molecules, the subscript on
the right side of the symbol indicates the number of
atoms within one molecule. The total number of
molecules is indicated on the left side.

Classify the following molecules into monoatomic,
diatomic and polyatomic.

H , Cl , P , O , S , He, Ar
2 2 4 2 8

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