How chemical compound forms?what is a molecule ?

There are two types of compound, ionic and molecular. Molecules are formed when share electrons and form what are called covalent bonds between them.

In , elements fully exchange their outermost electrons to form charged species called ions. For example when sodium and chlorine react they full exchange one electron so sodium loses one to become Na+ ions and chlorine gains one to be come Cl- ions. There no 'bonds' between these species, they simply associate together because negative charges attract positive charges. A bit like two magnets stuck together, south to north pole, you can relatively easily pull them apart.

If you dissolve an ionic compound (usually a salt) in water they fully separate and float around in solution as different species.

When this is not possible, because it requires too much energy, the elements instead 'share' their outermost electrons and form a covalent bond. This is the sort of you get in molecules. Now you have very strong bonds between the elements so that they cannot be easily separated. Ethanol is a molecule, if you dissolve that in water then the C, H and O atoms remain stuck together in the ##C_2H_5OH## molecules and do not separate from one another.