Question

Out of $N{{H}_{3}}$ and $B{{F}_{2}}$ which one is Lewis base and why?

Answer 1

Hint : We know that a Lewis base is a substance that can donate a pair of non-bonding electrons. A Lewis base is therefore called electron-pair-donor. In Lewis base molecules the orbitals used to be highly occupied so they have a tendency to donate electrons.

Complete Step By Step Answer:
The central atom in Ammonia, i.e. Nitrogen is hybridized. It is a gas at normal conditions and is known as Azane. The molecular weight of Ammonia is $\,17\text{ }g/mol$ . It is a colourless alkaline gas. Starting with the Lewis dot structure of Ammonia, Nitrogen has five valence electrons and each hydrogen has one valence electron. So, the total valence electrons are eight. Hydrogen always goes on the outside, so Nitrogen is the central atom. After the three valence electrons of Nitrogen have bonded with three Hydrogens, we still have two valence electrons left, which make up one lone pair of Nitrogen. In ammonia the molecule consists of a nitrogen atom as a central metal atom. Ammonia is Lewis base because nitrogen has a lone pair of electrons which can be donated; hence it acts as Lewis base. It can give a lone pair of electrons to protons to form ammonium ions. This reaction shows its Lewis base tendency. The Lewis dot structure of ammonia is give as:
$H-\overset{\centerdot \centerdot }{\mathop{N}}\,(H)-H$
A Lewis acid-base reaction can be done when a Lewis base donates a pair of electrons to a Lewis acid. Let’s see an example of Lewis acid-base reaction. In the following reaction each of two ammonia molecules which is a Lewis base donates a pair of electrons to a silver ion which acts as Lewis acid in the reaction.
Therefore, Out of $N{{H}_{3}}$ and $B{{F}_{2}}$ , $N{{H}_{3}}$ is Lewis Base.

Note :
Remember that students might not consider the lone pair of nitrogen in the hybridization of the central atom of Ammonia. The hybridisation in that case would come out to be. Ammonia has a filled orbital containing an electron pair which does not involve bonding and Lewis acid contains an empty orbital which is capable of accepting electrons.