Explanation:
The smallest part of element or compound which don't take part in chemical reaction is known as molecule. They are similar to atom but difference is that they don't have valancy. They are formed by the chemical reaction of two or more elements in a fixed ratio. Suppose..Water is formed by reaction of hydrogen and oxygen..hence due to the reaction the atoms of oxygen and hydrogen complete their octet or duplet state..therefore the smallest part of water is a molecule.
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