Question

nitrogen is a gas while phosphorous is solid

Answer 1

Physical state of any substance depends on the attractive force between its two molecules. Higher the force of attraction closer will be the molecules i.e. the state of the substance will start becoming solid.

N belongs to second period. Hence it can effectively form pi bond with itself. So, in N2 molecule N forms triple bond with itself.

P belongs to third period. Due to larger atomic size it does not form pi bonds with itself. Hence, in P4 molecule P forms sigma bond with other phosphorous atoms.

Due to the above point, force of attraction between N2 molecules is Van der waal force of attraction which is very weak. While the force of attraction between P4 molecules is a sigma bond which is strong, like this:

So, since the force of attraction between Phosphorous molecules is high and that between Nitrogen molecules is low, Phosphorous is a solid while Nitrogen is a gas.

Note: We are not talking about intramolecular force of attraction i.e. the bond between two N atoms has no effect on physical state. We are talking about force of attraction between two N2 molecules.




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Answer 2

Hey !!

Nitrogen due to small size and high electronegativity forms pπ - pπ multiple bonds and exist as a diatomic molecule. These molecules are held together by waek van der Waal forces and hence N₂ exist as a gas at room temperature. In constrast due to larger size and lower electronegativity P does not form pπ - pπ bond with itself rather it prefer to form single P-P bonds and exist as tetrahedral P₄ molecules. Due to bigger size, the force of attraction holding P₄ molecules are quite stronger and cannot broken by the collision of molecules at room temperature and hence solid at room temperature.

Hope it helps you !!