Select‌ ‌the‌ ‌least‌ ‌basic‌ ‌compound‌ ‌among‌ ‌the‌ ‌following‌ ‌
 ‌1.‌ ‌\[N{I_3}\]‌
‌2.‌ ‌\[NB{r_3}\]‌
‌3.‌ ‌\[NC{l_3}\]‌ ‌
4.‌ ‌\[N{F_3}\]‌

Hint: According to Lewis theory, the ion or molecule or compound which can donate a pair of electrons is known as Lewis base and the ion or molecule or compound which can accept a pair of electrons is known as Lewis acid. The ionic radii increases down the group and electronegativity decreases down the group, that is it decreases with the increase in the atomic or ionic radii.

Complete answer:
Nitrogen is a group 15 element and has one lone pair of electrons. Nitrogen readily reacts with halogens which are group 17 elements and can have up to 3 lone pairs of electrons. As the size of the halide ion increases the electronegativity of halide ion decreases and the basic character increases. Hence in the given question the least basic will be \[N{F_3}\] because fluorine is the most electronegative among all the halogens and it will attract the lone pair of \[N\] atom and thus the electrons will be less available for donation causing the compound to be least basic. The \[N{I_3}\] compound is most basic among the following as the atomic radii of iodine is greater than all the halogens.
Hence \[N{F_3}\] is the least basic compound.

Note:
The least basic compound will be a stable compound. Here \[N{F_3}\] is stable and all the other three are unstable in nitrogen halides. This is because nitrogen is very small in size and cannot form bonds with larger halogen atoms like \[Cl\],\[Br\],\[I\] and will be having low polarity. Generally, \[N{H_3}\] is the most basic in nitrogen compounds as it can donate more pairs of electrons.