Which of these crystals is softest: P4, ZnS, CuCl, SiO2

I think the correct answer would be the first option. It is the solid phosphorus (P4) that will be the softest among these crystals. P4 is held by covalent bonding or sharing of electrons between atoms while ZnS, CuCl and SiO2 are being held by ionic bonding. Covalent bonding involves the overlapping of orbitals while ionic bonding has separation of the charges. Although covalent bonding is a strong type of bond, ionic bonding is much stronger as compared to the former. Being stronger, the compounds mentioned above with this type of bonding would tend to be harder than phosphorus which has covalent bonding only.