Question

Why does lithium have a high melting and boiling point?

Answer 1

The element lithium is a soft, silvery-white alkali metal. It is the lightest metal and also the lightest solid element. Lithium is highly reactive and also flammable. The high reactivity of this element is due to the presence of one unpaired electron in its valence shell.

Complete answer:
The element lithium has higher ionization enthalpy than other alkali metals. This is due to the small size of lithium compared to other elements of that group. The binding energy of lithium in the crystal lattice is very high. Due to the high ionization and binding energies, lithium has high melting and boiling points. Note that the melting point of alkali metals decreases as we go down the group in the modern periodic table. This is due to a decrease in the charge density (i.e. as the atomic radius increases down the group and charge stays constant, as the increase in shielding from more inner electron shells is more significant than the increase in the proton number) meaning that the delocalized electrons are far away from the metal cations, which leads to weaker electrostatic forces of attraction which require less amount of energy to break down, causing a decrease in the melting point.

Note:
Remember that the first ionization energy decreases as we move down the first group. Because as the atomic size increases, the outermost electrons will be farther from the nucleus. That means there is an increased shielding of the nuclear charge by the electrons in the inner levels.