Molecules of elements contain the atoms of only one kind i.e only of 1 substance/element.
whereas molecules of compounds contain the atoms of different kind i.e. of different elements
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molecules of a compound is defined as pure substance which consists of two or more elements chemically combined by fixed mass are called as molecules of compounds .
molecules of elements  is defined as the substances which are can't be split up into two or more simpler substances by the use of any chemical methods to apply electric energy and heat.
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