Why calcium is harder and has high melting point than potassium?

Calcium is harder and has a higher melting point than potassium due to its metallic bonding and crystal structure. Calcium, being an alkaline earth metal, has a higher atomic number and stronger metallic bonds because of its greater number of delocalized electrons. This leads to a more tightly packed atomic structure, contributing to its hardness and elevated melting point compared to potassium, which is an alkali metal with weaker metallic bonds and a less compact structure.