both the elements belong to the fifth group of the periodic table,atomic number of nitrogen is 7 and phosphorus is 15. Nitrogen is in second period and phosphorus in third of Mendeleev's periodic table. as one moves from top to bottom in a group the atomic radii increases nad as nitrogen is diatomic ,two atoms to form a molecule and phosphorus tetra atomic , the later becomes a solid because of lesser space available in between the molecules as compared to nitrogen.
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