[Dolphinsiu]

Deduce the structure...

MS: 207(10), 205(30), .

...

According to nitrogen rule, as M+. is 205 (odd), then the compound must
contain odd number of nitrogen atoms.(at least one nitrogen atom)

Are above concept really correct or not??

[kiwi]

what about odd numbers of H? or atoms of I, F etc which also have odd weights?

[FeLiXe]

nitrogen is the only common element which has an even atomic mass but an uneven valence. that's why the nitrogen rule works.

[Custos]

That's right. The only trap is to be sure you are dealing with the molecular weight as in molecular ion, not the molecular weight as in molar mass.

The molecular weight of bromobenzene is 157 but it has no nitrogens. Why? Because bromine has two isotopes (79 and 81) in a roughly 50/50 ratio. So in the "molecular ion" region of the MS of bromobenzene shows two equal peaks of 156 and 158. The average molecular weight is 157.

But if you are dealing with actual molecular ion peaks in a MS, yes an odd numbered molecular ion would usually indicate an odd number of nitrogens.