Why do refractory metals have high melting points?
Refractory metals have high melting points because they have strong electrostatic bonds.
The refractory metals are niobium, molybdenum, tantalum, tungsten and rhenium which have melting points over 2750K.
Melting occurs when the thermal vibrations of atoms overcomes the bonds holding the solid crystal together. Technically the amplitude of the vibration exceeds the distances the atoms are apart.
In the case of metals, the electrons which are free for conduction form a cloud which causes atoms to be bonded together by electrostatic forces. The conduction electrons are usually the two outermost electrons in the s shell.
In the case of refractory metals theses have 3, 4 or 5 electrons in the d shell. These electrons form covalent bonds with neighbouring atoms. It is the strength of these bonds which gives these metals a high melting point.