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The Lewis bases have atleast one lone pair of electrons that can be donated to the other deficient group.

The non-bonding electrons, anions or double and triple bonded element that has two pi electrons.

a. ${\mathrm{NH}}_3$

The nitrogen atom in the given molecule is the fifteenth group element. Therefore, it has 5 valence electrons.

The three electrons of nitrogen are shared with the three hydrogen atoms, and the two remaining electrons are present on the nitrogen atom that are lone pair. Therefore, ${\mathrm{NH}}_3$acts as a Lewis base.

b. ${\mathrm{CH}}_3{\mathrm{CH}}_2{\mathrm{CH}}_3$

The carbon atom in the given molecule is the fourteenth group element. Therefore, it has 4 valence electrons. All four electrons are shared with hydrogen and carbon. Therefore, no lone pair is present on any carbon. Therefore, ${\mathrm{CH}}_3{\mathrm{CH}}_2{\mathrm{CH}}_3$does not act as a Lewis base.

c. ${\mathrm{H}}^{-}$

The given hydride ion is an ion of the hydrogen atom. The hydrogen atom has 1 valence electron. So, by accepting an electron, the hydrogen atom becomes a hydride ion with a lone pair. Therefore, ${\mathrm{H}}^{-}$acts as a Lewis base.

d. $\mathrm{H}-\mathrm{C}≡\mathrm{C}-\mathrm{H}$

In the given molecule, there is a triple bond. The triple bond is formed by the two pi electrons, which can be transferred due to sidewise overlapping (weak overlapping). Therefore, $\mathrm{H}-\mathrm{C}≡\mathrm{C}-\mathrm{H}$ acts as a Lewis base.