Question

why are metals hard whereas non metals soft?

Answer 1

Not all metal are hard as some exception are their but since metal have electron in the s orbital having high symmetry orbital with electron near to nucleus due to which density of the electron of shell is very high due to which they form compact structure with their other atoms of metal and give hardness.
The Hardness of metals also depends upon the cohesive enrgy and as the cohesive energy decreases metals become soft. Dow the group size of metal increases and thus the cohesive energy decreases due to which metals become soft.

Similarly, non-metal are soft due to presence of electron in the p orbital that electron density is not as high as that in case of metals due to which non-metal atom do not associate themselves easily as compare to metals. Hence non-metals are soft. But their are also exceptions where non-metal are hard also.