Chemistry: An atom capable of independent existence, or group of atoms bonded by covalent bond is called a molecule. Eg He, Ne, H2, O2, C, H2O, S, H2O, H2SO4 etc.
If in a molecule all the atoms are chemically identical, then it is called an element. Eg He, Ne, H2, C, S etc.
If a molecule is made up of different atoms then its called a compound. Eg H2O, H2SO4 and NaOH etc.
Metals are elements since here all the atoms are chemically identical. Eg Na, K, Rb etc but not molecules because they don't have covalent bonds. Nor their atoms are capable of independent existence. They have a special type of bonding called metallic bond.
Group of ions (bonded by ionic bond) are called ionic compounds, eg NaCl, MgCl2 etc. They are compounds because the atoms (ions) are not same but not molecules because they don't have covalent bonds.
Atoms are the building blocks of all matter, mixture, molecules, elements and compounds. And molecules are the building blocks of elements and compounds.
If we say H2 is a compound then carbon will also be a compound which is not the case
