A nitrogen atom has a lone pair of electrons because it has five valence electrons in its outermost energy level. Nitrogen belongs to Group 15 of the periodic table, which means it has five valence electrons (2s^2 2p^3 configuration).
In order to achieve a stable electron configuration, nitrogen can form three covalent bonds by sharing its three unpaired electrons with other atoms. This allows it to complete its octet, following the "octet rule" for main group elements.
However, nitrogen still has two remaining electrons that are not involved in bonding. These two electrons are located in a non-bonding orbital and are referred to as a "lone pair." The lone pair is not shared with any other atom and is attracted primarily to the nitrogen nucleus. This gives nitrogen a total of eight electrons around it (two in a sigma bond and six in a lone pair), satisfying the octet rule.
The presence of a lone pair makes nitrogen more reactive compared to elements with a complete octet. The lone pair can participate in various chemical reactions, such as coordination bonding, hydrogen bonding, and Lewis acid-base reactions.