An atom is the smallest particle of an element that can take part in a chemical reaction;however, it may or may not exist independently.
A molecule is the smallest particle of an element or a compound that can exist by itself; it never breaks up except for taking part in a chemical reaction.
An element is a pure substance which cannot be converted or broken down into two or more simpler substances by any physical or chemical process.
An ion is a charged particle which is formed due to the gain or the loss of one or more electrons by an atom.
A compound is a pure substance, composed of two or more elements combined chemically in a fixed proportion.