Q.Which one of these is more elastic, steel or rubber? Why?

In a solid, interatomic forces of attraction and repulsion hold every atom at a fixed equilibrium separation from its neighbours -- its mean position. Applying a deforming force pulls some atoms further apart or pushes others closer together than this equilibrium separation. The moment the deforming force is withdrawn, the same interatomic forces that originally held the lattice together act to pull every atom back to its own mean position. A material that does this completely -- regaining its exact original size and shape once the deforming force disappears -- is called ELASTIC, and the underlying property is ELASTICITY (rubber, metal wires, and steel ropes are everyday elastic materials, at least up to a limit). A material that does NOT spring back -- that stays permanently deformed once the force is removed -- is called PLASTIC, and the property is PLASTICITY (glass is the standard example: it shatters or deforms permanently rather than recovering). At the atomic scale, elastic behaviour is simply the direct, mechanical consequence of interatomic bonding forces trying to restore the lowest-energy (equilibrium) configuration of the lattice; plastic behaviour occurs when the applied deformation is large enough to permanently rearrange t …