[xemist]

1.I do not understand why HNO₂ cannot have this structure?

2. Is molecule NH₃ coordinate (dative covalent)??

[Ligander]

 1. How many electrons around nitrogen are there in your structure? Where in periodic table it is situated?

 2. A co-ordinate bond (also called a dative covalent bond) is a covalent bond (a shared pair of electrons) in which both electrons come from the same atom.
 http://www.chemguide.co.uk/atoms/bonding/dative.html


 

[xemist]

There are 5 e^- around N.

[Ligander]

There are 5 e^- around N.

Even in your incorrect structure N possesses(alone or shares with others) much more than 5 electrons. What do the lines represent ?

 What is octet rule?

 And it might be useful, but not nesessary, to understand what are free radicals, how do they differ from regular molecules and what are their basic properties.