[HusamEddin]

Hi there
my question is short , and may be simple , but it really made me confused

now ,, Nitrogen has valence = 3 ,, Oxygen has valence = 2
definition of valence = max. number of bonds the atom can make
then , Nitrogen must have 3 bonds as a maximum , and a pair of unshared electrons
also , Oxygen must have 2 bonds as a maximum , and 2 pairs of unshared electrons

but the problem is that I see many compunds that have Oxygen with 3 bonds or sometimes 4 !!!
also Nitrogen with 4 bonds !!!


how it could be just like that ?

[Borek]

definition of valence = max. number of bonds the atom can make

This is an incorrect definition, no wonder it yields wrong conclusions.

but the problem is that I see many compunds that have Oxygen with 3 bonds or sometimes 4 !!!

Many such compounds? List one.

[Corribus]

A protonated carbonyl obviously qualifies as an oxygen bonded three ways, but equilibrium doesn't favor it except in very acidic solutions.  Of course, though, you are right - without proper definitions, there is no easy route to understanding.   HusamEddin, you should seek a better definition of what "valence" means. 

http://en.wikipedia.org/wiki/Valence_(chemistry)

If you look at the table, the IUPAC accepted valence of nitrogen is FIVE and the valence of oxygen is TWO.  But valence in any case is an ambiguous term and generally refers to stable molecules, not transient species.  I also regard it as rather antequated - good for learning basic chemistry but like all things you learn in basic general chemistry, it's a gross simplification of the truth.

[HusamEddin]

I'm sorry friends ,,, here is my book:
Organic chemistry (hart)
The valence of an element is simply the number of bonds that an atom of the element can form.
under this definition there is a table ,, and is writtin under the Nitrogen its valence ,,,, it is 3
this is what I ment ... with the previous definition ,,, it means that N will form only 3 bonds
but look at this compound (HONO2) or (HNO3) in the next pic:



Nitrogen in this one forms 4 bonds !!!
how could it be ??

[Corribus]

I would regard that as a guide, not a rule.  In chemistry there are exceptions to just about anything.  Organic chemistry in particular is full of rules that aren't really rules but rather tendencies. 

True, nitrogen USUALLY has a valency of 3.  Quite often it has a valency of four (ammonium hydroxide) and CAN have a valency of five.  Carbon almost always has a valency of 4, but valencies of 2 and 3 are not unheard of and carbons with five bonds have been observed. 

Common valencies like oxygen = 2, nitrogen = 3, carbon = 4, etc., represent the most energetically stable conformations for atoms.  But short-lived (and even long-lived, under some circumstances) deviations from these valencies are not unusual.  And that's not even broaching the subject of transition states.

[Borek]

1. None of the structures you listed contains oxygen with valency different than 2.

2. As Corribus wrote - 3 for nitrogen is only one of possible valencies. If you will look at nitrogen oxides, you will find all possible valencies from 1 to 5.

[HusamEddin]

I would regard that as a guide, not a rule.  In chemistry there are exceptions to just about anything.  Organic chemistry in particular is full of rules that aren't really rules but rather tendencies. 

True, nitrogen USUALLY has a valency of 3.  Quite often it has a valency of four (ammonium hydroxide) and CAN have a valency of five.  Carbon almost always has a valency of 4, but valencies of 2 and 3 are not unheard of and carbons with five bonds have been observed. 

Common valencies like oxygen = 2, nitrogen = 3, carbon = 4, etc., represent the most energetically stable conformations for atoms.  But short-lived (and even long-lived, under some circumstances) deviations from these valencies are not unusual.  And that's not even broaching the subject of transition states.

really ,,, Thank you very much

1. None of the structures you listed contains oxygen with valency different than 2.

2. As Corribus wrote - 3 for nitrogen is only one of possible valencies. If you will look at nitrogen oxides, you will find all possible valencies from 1 to 5.

Calm down ma friend  ,, the last pic I viewed , I wasn't talking about Oxygen ,, but Nitrogen with 4 bonds

anyways ,, thank you too

but this lead me to another question
when resonance is required to be drawn , am I free to change a single-bonded oxygen to make it double bonded (as one contributor of the resonance) ? -surely I mean without disturbing the structure of the compound- ... or there is a rule could prevent me to make a double-bonded oxygen in some conditions

[HusamEddin]

ok you asked me to bring one of triple-bonded oxygens ,,, and here we are



I took a look on the table here:
http://en.wikipedia.org/wiki/Valence_%28chemistry%29

I realised that "N" could make up to 5 bonds ...
but oxygen as listed ,,, has a maximum of only 2 bonds !!!!


but in the resonance in the image upward ,,, there is one contributor with 3 bonds on the oxygen !!!

can you explain the reason to me ,,, please