Question

covalency of nitrogen is N2O5 IS 4 or 5 . please explain 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'.

let us take an example;

In the formation of ammonium ion(NH4+), first we have ammonia- NH3

NH3 has 3 shared electron pairs and 1 lone pair.
During the formation of NH4+, the lone pair is donated towards a proton (H+)
thus making a coordinate covalent bond(dative bond) with it.

Therefore, there are now 4 covalent bonds made by nitrogen and it cannot form more as it does not have any electron in its valence shell nor does it have a vacant orbital to accept donated lone pairs to make another bond.
hence 4 is the maximum covalency of nitrogen.
 
Now, N2O5 has a structure as shown below,

here each nitrogen atom has a total of 4 shared pair of electrons.
1 in N--->O
1 in N----O----NO2
2 in N===O

Here the covalency of nitrogen is 4 which is also its maximum covalency.

:)

Answer 2

Answer:covalency is the sharing of valence electrons in a chemical compound. The number of electrons that N2O5 shares are 4. this is its covalency