Question

Nitrogen is a gas at room temperature while phosphorous is a solid .Explain

Answer 1

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The size of nitrogen is quite small. As a result two nitrogen atoms can be linked to each other by three covalent bonds in order to complete the octets of both the nitrogen atoms. This diatomic molecule N2 is very stable and has very negligible attraction towards other diatomic nitrogen molecules. Thus there is weak interaction between nitrogen molecules. So nitrogen exist as gas.

While phosphorus being large in size has less tendency to form three bonds. Therefore, P atoms completes octets by sharing its valence electrons with three other P atoms. As a result, it exist as tetratomic P4 molecule. In white phosphorous there is network which holds the phosphorous atoms together. This catenation tendency of phosphorus is reason why it exist as solid.

Answer 2

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.

Also refer the attachment above for your better understanding........

Hope it helps you.........