Question

what is the covalency of nitrogen in N2O5 and how

Answer 1

Covalency is the number of shared electron pairs formed by an atom.

'5' is the total number of valence electrons of nitrogen.But it is not the covalency and the maximum covalency of nitrogen is '4'.

Now, N2O5 has a structure as shown below,
here each nitrogen atom has a total of 4 shared electron pairs.
1 in N--->O
1 in N----O----NO2
2 in N===O

Here, in N2O5, the covalency of nitrogen is 4 which is also its maximum covalency.
:)