Imagine going to an ice cream store. Let's say that they have 30 different flavors of ice cream. Those are elements, the things that I have available to build my dessert from. The smallest amount of ice cream that the store will sell to me is a scoop. This is an atom. If I want, I can put two or more scoops of ice cream together. This is a molecule. If my molecule has more than one flavor of ice cream, I can call it a compound.

So, in summary:

element - a basic substance that can't be simplified (hydrogen, oxygen, gold, etc...)

atom - the smallest amount of an element

molecule - two or more atoms that are chemically joined together (H2, O2, H2O, etc...)
...more

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An element is a substance which cannot be spilt into two or more simpler substance by any sort of chemical means. They are made up of particles which can either be atoms, molecules or ions.

An atom is the smallest particle that can take part in a chemical change. They are extremely small, about 10-10 meter in size.
Now coming to the explanation of molecules. A molecule is a smallest particle of a compound which is formed of a group of atoms. The atoms in a molecule are shown by a chemical formula.
A compound on the other hand is defined as a pure substance consisting of similar type of molecules which can be made up of different elements.