Question

Why is `N(OH)_3` unknown?

Answer 1

Refer to the explanation.

Explanation:

The nitrate ion, `"NO"_3"^(-)`. It is a polyatomic anion that has an overall charge of `"1"^(-)"`. The nitrate anion is considered a single ion. The hydrogen cation, `"H"^+"` has a charge of `"1"^+"`. Since nitric acid is a neutral compound, it is composed of one hydrogen cation and one nitrate anion.

`"NO"_3^(-)"+H"^+"` `rarr` `"HNO"_3"`

Note: carbonic acid is composed of the carbonate anion, `"CO"_3"^(2-)`, and the hydrogen cation. As with the nitrate anion, the carbonate anion is a polyatomic anion that has an overall charge of `"2"^(-)"`, and is also considered a single ion. Since the carbonate anion has a charge of `"2"^(-)"`, it will need to be combined with two `"H"^(+)"` cations to make the neutral compound carbonic acid.

`"CO"_3^(2-)"+ 2H"^(+)"` `rarr` `"H"_2"NO"_3"`

The following is a list of polyatomic ions with their names, formulas, and charges.
http://www.chalkbored.com/lessons/common-polyatomic-ions.pdf
More information can be found here: http://antoine.frostburg.edu/chem/senese/101/compounds/polyatomic.shtml

Answer 2

To address your question, I think you ask why a molecule such as `N(OH)_3` is not observed?

Explanation:

Trihydroxylamine, `N(OH)_3`, is probably observed spectroscopically. However, this species is probably unstable with respect to hydroxylamine, which further disproportionates to nitrous and nitric oxide:

`N(OH)_3 rarr HO-N=O + H_2O`

`2HO-N=O rarr NO_2 + NO + H_2O`

Just to add that for the Lewis structure of nitrate anion, `O=N^+(−O)_2^(-)`, every atom is associated with 8 electrons. However, for each mesomer, there are 3 formal charges assigned. Two of these are negative charges and are formally oxygen based, and the nitrogen is quaternized and bears a formal positive charge. Of course the overall charge is still `−1`, as required.

For the neutral parent acid, `O=N^+(−O^−)(−OH)`, there are thus 2 formal charges assigned.