Question

Why do metals often have high melting and boiling point?

Answer 1

Hint: We know that in metals, the electrons leave the valence shell of metal atoms to form positive metal ions and a sea of delocalised electrons which tend to form strong electrostatic forces among each other and the type of bonding is known as metallic bonding. Boiling or melting of a metal is a change of physical state which can be explained on the basis of metallic bonding.

Complete answer:
For melting or boiling of a substance, energy is transferred to overcome the forces of attraction between the metal ions and the delocalised electrons which form strong metallic bonding. The more energy transferred, higher will be the melting or boiling point of a substance.
Metals consist of giant lattice structures in which strong electrostatic force of attraction exist between metal ions and valence electrons from s and p orbitals which move freely around positively charged ions as a sea of electrons. The energy required to break this strong electrostatic force of attraction within the lattice of metal is very high and thus, the melting and boiling points of metals is high. Basically, the melting and boiling point of metals are similar to ionic compounds as compared to covalent compounds. For example, a comparison chart is prepared for melting and boiling points of magnesium, chlorine and magnesium chloride which is as per following table:

Substance	Formula	Melting point	Boiling point	Type of bonding
Magnesium	$Mg$	${650}^{o}C$	${1091}^{o}C$	Metallic
Chlorine	$C{l}_2$	$-{101}^{o}C$	$-{34}^{o}C$	Covalent
Magnesium chloride	$MgC{l}_2$	${714}^{o}C$	${1412}^{o}C$	Ionic

Therefore, from the table it is clear that the metals have high melting and boiling points similar to that of ionic compounds and the reason for this property is same in both the cases i.e., strong electrostatic forces of attraction acting between oppositely charged ions.
Hence, we can conclude that the metals often have high melting and boiling points.

Note:
It is important to note that alkali metals i.e., metals belonging to group 1 have comparatively lower melting and boiling points because each element in group 1 consist of one electron in the valence shell which is loosely held by nuclear force and hence, each of one electron contributes to form sea of electrons which is weakly attracted to the nuclei of each atom and thus, have low melting and boiling point.