Chemistry (matter)

  Chemistry (Matter)

     Matter; Anything that occupies space and has weight is called matter. A pure matter is called a substance. Eg salt, water, carbon and iron etc. A substance has a fixed composition. Impure matter is called a mixture. Eg stone, milk, tea and coal etc. Mixtures don't have definite compositions.
   If a substance can't be decomposed into simpler substances, then it's called an element. Eg carbon, iron and oxygen etc. If could, then it's called a compound. Eg H2O ( H and O) or NaCl (Na and Cl)

   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, or if a molecule can't be decomposed into simpler parts , then it is called an element. Eg He, Ne, H2, C, S etc.
  If a molecule is made up of different atoms or can be downloaded into simpler parts,  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. An individual atom is necessarily neither an element nor a compound. Like a cell is neither a plant nor an animal.  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.