Question

properties of a compound are altogether different from those of the constituting elements. comment on the statement and elaborate

Answer 1

As to "chemical properties". All chemical reactions involve the sharing of or transfer of "electrons". One atom comes along which has higher electronegativity than another and takes an electron from the lower EN atom. More precisely..."one atom withdraws electron density from another atom due to differences in electronegativities". This creates electrostatic attractions between the atoms and the result is a combination of the atoms that has a lower energy than the two original atoms seperately do. Which is the goal of all chemical reactions. To create products with lower energy than the reactants. 

Once that happens, once this "chemical bond" between the atoms is formed, the atoms electronegativities and desire to create bonds with other atoms changes. And we see that as a change in chemical reactivity. or "chemical properties". 

As to physical properties, that varies. But things like melting points and boiling points depend upon intermolecular forces (forces between molecules.. as opposed to forces within molecules). And these are usually again electrostatic forces. As elements react to form new products, their intermolecular interactions change because the electrostatics (like polarity of the molecules) have changed. In addition, you might see differences in packing ability. ie.. density. etc