Answer the question on basis of passage given below:

All the members of Group 15 elements form covalent hydrides with hydrogen as $NH_3, PH_3, AsH_3, SbH_3, BiH_3$. Group 16 elements form covalent hydrides with hydrogen as $H_2O, H_2S, H_2Se, H_2Te, H_2PO$. Group 17 elements form covalent hydrides with hydrogen as $HF, HCI, HBr, HI$. Based on these facts answer the following question.

Which of the following should be a Lewis base?

$NH_3$

The correct answer is Option 3. $NH_3$.

The concept of a Lewis base is based on its ability to donate an electron pair to a Lewis acid, which accepts that electron pair.

Let us break down the compounds:

1. \(H_2O\) (Water):

Water has two lone pairs of electrons on the oxygen atom, which could technically be donated, making it a potential Lewis base. However, compared to \(NH_3\), its Lewis basicity is lower because the lone pairs are held more tightly by the electronegative oxygen atom. 

2. \(HCl\) (Hydrochloric acid):

\(HCl\) is primarily known as a strong acid. It dissociates in water to release \(H^+\) ions (protons). It does not donate an electron pair, so it cannot act as a Lewis base. In fact, \(HCl\) is more likely to act as a Lewis acid because the chloride ion \((Cl^-)\) could accept protons.

3. \(NH_3\) (Ammonia):

Ammonia has a lone pair of electrons on the nitrogen atom, which is readily available for donation. Since nitrogen is less electronegative than oxygen, the lone pair is more accessible, making \(NH_3\) a strong Lewis base. It often donates this lone pair to form bonds with Lewis acids, such as in reactions where it interacts with \(H^+\) ions to form \(NH_4^+\).

4. \(H_2S\) (Hydrogen sulfide):

Similar to water, hydrogen sulfide has lone pairs on sulfur. However, sulfur is less electronegative than oxygen, so its lone pairs are more available than water’s but still less accessible compared to NH₃. This makes H₂S a weaker Lewis base than NH₃.

Why \(NH_3\) is the Best Lewis Base:

The nitrogen atom in \(NH_3\) is less electronegative than oxygen (in \(H_2O\)) or sulfur (in \(H_2S\)), meaning it holds onto its lone pair of electrons less tightly. Therefore, ammonia can donate its lone pair more easily than water or hydrogen sulfide, making \(NH_3\) the strongest Lewis base out of the options provided.

Thus, option 3: \(NH_3\) is the correct answer because it's the most effective at donating its lone pair of electrons, fitting the definition of a Lewis base perfectly.