Why can a metal not be used as a dielectric in a capacitor?

The potential gradient at which the dielectric of a condenser just gets punctured is called

When a dielectric is placed in an external electric field,the electric field inside the dielectric is

$A$ cube of side $1 \ cm$ contains $100$ molecules,each having an induced dipole moment of $0.2 \times 10^{-6} \ C \cdot m$ in an external electric field of $4 \ N \cdot C^{-1}$. The electric susceptibility of the material is .... $C^2 \cdot N^{-1} \cdot m^{-2}$.

The dielectric strength of air is . . . . . . .

Explain the difference in the behaviour of a conductor and a dielectric in the presence of an external electric field.