(a) Physical properties of ionic compounds are :
(i) Ionic compounds are usually crystalline solids because their oppositely charged ions attract one another strongly and form a regular crystal structure.
(ii) Ionic compounds have high melting and high boiling points. The ionic compounds are made up of positive and negative ions. There is a strong force of attraction between the oppositely charged ions, so a lot of heat energy is required to break this force of attraction and melt or boil the ionic compounds.
(b) Gold and platinum metals are found in free state in the earth's crust. These metals are located at the bottom in the activity series.
(c) Metals towards the top of the activity series are highly reactive. The oxides of highly reactive metals are very stable and cannot be reduced by 'carbon' to obtain free metals because these metals have more affinity for oxygen than carbon.