Hardness of matter

The hardness of solids can vary considerably, dependent on the threedimensional structure and on the forces that hold this structure toghether.
The more ionic the substance, the harder it will be.
In ionic and in metalic lattices ions cannot move or displace internally.
The hardest substance seems to be diamond that has an 'atomic lattice' of Carbon atoms. Every C-atom is connected with four other C-atoms, so is you look well, the diamond actually is one big molecule. It has the perfect structure; every particle is connected through atomic bonds / covalent bonds.

Polymers are built up of macromolecules; they can have a more or less spherical shape, but not always. The molecules can be long, without interconnections and then we deal with soft plactic.
On the other hand, if there are intermolecular connections between these macromolecules, we could have hard plastic. soft plastic, at heating, can more or less melt; hard plastic cannot melt. In chemistry we speak about thermoplast and thermohard.