atoms combine to form molecules , same type of molecules combine to form elements , different molecules of different element combine to form compound and all these can we broken down to simplest form which are the atoms so atom is the smallest particle of an element/compound though it is not stable for all the elements that are known to us.

i hope it helps.......... 
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An atom is the smallest part of an element. A group of atoms combine to form a molecule, these mocules combine to form an element, these elements combine to form a compound.
Thus we came to know the presence of atoms.