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Solution

The young\'s modulus depends on two factors: bond stiffness and the density of bonds per unit volume.

On the other hand the density of a solid depends on three factors: the atomic weight of its atoms or ions, their size and the way they are packed.

1) Metals are dense because they are made of heavy atoms, packed densely. Metals have high Young\'s Module because of close-packing gives a high bond density and the bonds are strong.

2)Ceramics have lower densities than metal because they contain light O, N or C atoms. But ceramics like diamond has very high modulus because the carbon atom is small, giving a high bond density and atoms are linked very strongly.

3)Polymers have low densities because they are largely made of carbon and hydrogen in low density amorphous or crystalline packings. Polymers contains both strong diamond-like covalent bonds and weak hydrogen Van-der-Waals bonds and it is the weak bonds that stretch when the polymer is deformed, giving low modulus.

Note: For more information check: Ashby\'s Materials Selection in Mechanical Design.