Amongst the trihalides of nitrogen, which one is least basic? (a) \(\mathrm{NF}_{3}\) (b) \(\mathrm{NCl}_{3}\) (c) \(\mathrm{NBr}_{3}\) (d) \(\mathrm{NI}_{3}\)

When it comes to nitrogen trihalides, it is essential to consider that these compounds consist of a nitrogen atom bonded to three halogen atoms. The basicity of these compounds is fundamentally linked to the ease with which they can donate their lone pair of electrons. A nitrogen trihalide that can readily donate its lone pair is considered more basic.

Each halogen brings its own unique properties to the trihalide. For example, as one moves from fluorine to iodine in the halogen group, the resulting trihalides exhibit differing reactivities and basic strengths due to changes in electronegativity and atomic size. This means that NF₃, NCl₃, NBr₃, and NI₃ can show significant differences in their behaviour as Lewis bases.

The concept of electronegativity is crucial when discussing basicity. Electronegativity refers to the ability of an atom in a molecule to attract shared electrons to itself. In the context of nitrogen trihalides, the basicity decreases as the electronegativity of the halogen increases.

Here's why: a highly electronegative halogen, such as fluorine, exerts a strong pull on the shared electrons, including those in the lone pair on nitrogen. This reduces the electron density around the nitrogen and makes its lone pair less available for donation. Consequently, compounds like NF₃ are much less basic compared to trihalides with less electronegative halogens like NCl₃, NBr₃, or NI₃. As a result, among nitrogen trihalides, NF₃ is the least basic due to the high electronegativity of fluorine.

The concept of lone pair donation plays a pivotal role in the basicity of nitrogen compounds. A nitrogen atom typically has five valence electrons, three of which are used for bonding with halogens in trihalides, leaving one lone pair on the nitrogen. This lone pair is a key factor in nitrogen's ability to act as a Lewis base, as it can be donated to a Lewis acid during a chemical reaction.

The availability of this lone pair for donation is influenced not just by the electronegativity of the bonded halogens, as previously discussed, but also by steric factors and the overall electron distribution in the molecule. In simpler terms, the lone pair on nitrogen will be more accessible in a molecule where it's less crowded and less pulled upon by adjacent atoms. Understanding this concept is essential when evaluating the basicity of nitrogen trihalides and explains why electronegativity is often a more determining factor than steric hindrance in these compounds.