The hardness of the metals depend upon the structure of the metals. Basically metals are made up of atoms which are attached with each other through metallic bonds. The metals which are very closely packed have 12-coordination. This means that each atom of the metal is attached with 12 other atoms in the metal layers. In some metals there is 8-coordination which means than each atom in the metal layer is attached with 8 touching neighbors. The arrangement of the atoms determine the malleability and ductility of the metals. On the basis of the arrangement of the atoms, a metal can be more harder than others. The metals which are soft, their layers can slide over the other layers and those metals which do not have gain boundaries, their layers do not get attachable atoms when they slide and therefore, these metals fracture.
Hardness in metals
Hardness in metals
