Stability of oxides of higher oxidation state decreases with increasing atomic number Then why P2O5 is thermally stable while N2O5 not. Infact N2O5 is the strongest oxidising agent among pnictogen oxides because N2O3 is most stable. Please Offer a lucid explaination to understand this...
A very easy half-answer would base this on electronegativity.
Nitrogen’s electronegativity is $\approx 3$ while phosphorus’ is $2.19$. Oxygen’s is, of course, $3.5$. So the electronegativity difference between nitrogen and oxygen is much lower, meaning that oxygen is much less able to stabilise nitrogen’s high oxidation state.
Contrarily, phosphorus’ low electronegativity means oxygen can stabilise its high oxidation state rather well and the compound is much less likely to act as an oxidating agent by itself.
For the same reason you have Na3PO4 (not NaPO3) and NaNO3 (never Na3NO4). Nitrogen doesn't have an orbital to form five bonds, while phosphorus has. This explains why PF5 exists, but NF5 doesn't. In N2O5 n=o bonds have character similar to bonds in NH4+. But in NH4+ nitrogen is surrounded by electron donating H atoms, so it is happy to donate it's lone pair of electrons. In N2O5 it is surrounded with electronwithdrawing O atoms and is not to happy to give away its lone pair. thus it is happy to transfer one oxygen atom elsewhere (in other words - to oxidize something).
tldr N is the only element in the group that doesn't have p orbital.
"NaPO3" is actually a trimer Na3P3O9