Question

why n exist as gas ??

Answer 1

Nitrogen is a small atom. Due to its electronic structure, it forms bonds with another atom to form a molecule N2.  The bonds are very strong. It is a triple bond. So the molecule is small.

Due to its stable electronic configuration, N2 molecule does not interact with other molecules. So intermolecular forces are very little. Since the molecular weight does not seem to be high, VanderWaal's forces are also quite less. Gravity is also less.

So Nitrogen molecules require very little energy to gain kinetic energy and become gaseous. They move rapidly. 

Thus Nitrogen is a gas at room temperature.

Answer 2

it is because of the covalency of nitrogen which is limited to 4. Therefore, nitrogen only exists in the diatonic form with triple boun between the two atoms. It's also inert at room temperature nitrogen is a stable gaseous Molecule.