Question

why nitrogen exist as a gas but phosphorus as solid

Answer 1

Nitrogen is smaller in size hence it forms multiple bonds with other nitrogen and exists as diatomic molecule. Whereas phosphorus atomic size is more so it can form single bond with other phosphorus atoms. Hence it exists as tetra atomic molecule.

Answer 2

N2 is exist as diatomic molecule in which two nitrogen atoms are attached to each other by triple bond this triple bond is due to p(pie)-p(pie) multiple bonding (sidewise overlapping) and such bonding is not possible in phosphorus due to it's large size but possible in nitrogen due to it's small size. <br> <br>In case of phosphorus four atoms are associated so it have high molecular mass ,large magnitude of venderwall forces so phosphorus is solid and nitrogen is gas.

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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