(a) 2H represents two atoms of hydrogen element.
(b) 4H2O represents four molecules of water.
(c) Cl2 represents a chlorine molecule.
(d) NaCl represents a molecule of compound sodium chloride.
(e) KNO3 represents a molecule of compound potassium nitrate.
(f) 2CuSO4 represents two molecules of copper sulphate.