Which among the following species has a triangular planar shape?

To find the shape of the given species, we can determine the steric number (number of $\sigma$ bonds + number of lone pairs on the central atom) according to VSEPR theory:

• $\text{NO}_3^-$ (Nitrate ion): The central nitrogen atom forms $3$ $\sigma$ bonds with three oxygen atoms and has $0$ lone pairs. The steric number is $3$, which corresponds to $sp^2$ hybridisation and a trigonal planar (or triangular planar) geometry.
• $\text{N}_3^-$ (Azide ion): The central nitrogen atom forms $2$ $\sigma$ bonds with the terminal nitrogen atoms and has $0$ lone pairs. The steric number is $2$, which gives it a linear shape.
• $\text{NO}_2^-$ (Nitrite ion): The central nitrogen atom forms $2$ $\sigma$ bonds and possesses $1$ lone pair. The steric number is $3$, but due to the presence of a lone pair, its shape is bent or V-shaped.
• $\text{CO}_2$ (Carbon dioxide): The central carbon atom forms $2$ $\sigma$ bonds (double bonds) with oxygen atoms and has $0$ lone pairs. The steric number is $2$, giving it a linear shape.

Therefore, $\text{NO}_3^-$ is the species with a triangular planar shape.