Why does calcium have a higher melting point than potassium?
Both potassium and calcium are metals. Thus, their bonding is metallic, where the atoms form a lattice shape and share their valence electrons throughout the structure. The electrostatic force between the positive ions (cations) and the delocalized electrons keeps the structure intact.
Calcium atoms have smaller radii than potassium atoms since calcium atoms have a greater nuclear charge. Each calcium atom will be closer to the delocalized electrons. Thus, the bonds in calcium will be stronger than that in potassium.
Another reason is, since calcium donates two electrons while potassium donates one, the calcium atom will have a charge of +2 while the potassium ion will have a charge of +1. Each atom in calcium has a stronger attraction to the delocalized electrons than potassium.
In addition, potassium atoms have only one valence electron, while calcium atoms have two. Since there will be more delocalized electrons per atom throughout calcium than potassium, the bond strength is stronger in calcium than potassium.
Since more energy is required to break the bonds, the melting point of calcium will be higher than that of potassium.