Question

Which of the following is a strong base?
(A) $P{{H}_{3}}$
(B) $As{{H}_{3}}$
(C) $N{{H}_{3}}$
(D) $Sb{{H}_{3}}$

Answer 1

Hint: Lewis base is any substance, such as the $O{{H}^{-}}$ ion, that can donate a pair of nonbonding electrons. A Lewis base is therefore an electron-pair donor. One advantage of the Lewis theory is the way it complements the model of oxidation-reduction reactions.

Complete step by step solution: The hydrides of nitrogen families have one lone pair of electrons on their central atom. Therefore, they act as Lewis bases. As we go down the group, the basic character of these hydrides decreases. Nitrogen atoms have the smallest size among the hydrides. Therefore the lone pair is concentrated on a small region and electron density on it is the maximum. Consequently, its electron releasing tendency is maximum. As the size of the central atom increases down the family, the electron density also decreases. As a result, the electron donating capacity or the basic strength decreases down the group.
$N{{H}_{3}}$ ​ is a strong base. The decreasing order of the basic character is
\[N{{H}_{3}}>P{{H}_{3}}>As{{H}_{3}}>Sb{{H}_{3}}\]
 Basic character decreases down the group from N to Bi due to increase in atomic size.

Therefore the correct answer is the C option.

Note: Ammonia is a base because it is accepting hydrogen ions from the water. The ammonium ion is its conjugate acid - it can release that hydrogen ion again to reform the ammonia. The water is acting as an acid, and its conjugate base is the hydroxide ion. A Lewis acid is a chemical species that contains an empty orbital which is capable of accepting an electron pair from a Lewis base to form a Lewis adduct.