why does 2Co2 mean two molecules of carbon dioxide whereas 2He stands for two atoms of helium?

CO2 is a molecule and He is an atom. An atom is a basic unit of matter. It is the smallest particle of an element that can take part in chemical reaction while molecule is a chemical combination of two or more atoms and the atoms can be of same or two different elements. Since He has inert gas configuration therefore it does not combine with any atom while in CO2, C and O chemically combine to give a molecule CO2.
 

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Because 2 here is the cofficient of the element in the case of He and 2 is the cofficient of whole CO2 !!

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