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What's the oxidation state of nitrogen in $\ce{NO3-}$ ion. Simple calculation depicts that it's $+5$. But second period elements never show variable oxidation states. And it's structure is so confusing to me. Some people show dative/co-ordinate bonds, whereas others show a positive charge on nitrogen. Which should be considered as correct one?

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You are correct that the oxidation state is +5. The second period elements, particularly carbon, nitrogen and oxygen, do show variable oxidation states regularly.

The $\ce{NO3-}$ ion cannot be depicted using a single Lewis structure. However, its resonance structures are shown below: enter image description here

What this means is that the actual structure of the nitrate ion is somewhere in between the three structures shown below. It is sometimes also depicted like this to show the symmetrical nature of the ion and the distribution of charges across it.

enter image description here

http://en.wikipedia.org/wiki/List_of_oxidation_states_of_the_elements http://en.wikipedia.org/wiki/Nitrate#Structure

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  • $\begingroup$ but if Nitrogen can show +5 oxidation state here, then why it cann't form NCl5 (whereas PCl5 is possible). As per my knowledge is concerned, 2nd period elements cann't go for octet expansion due to the abscence of vaccant d-orbital... please enlighten me.. $\endgroup$ – user27811 Jan 29 '15 at 20:59
  • $\begingroup$ in nitrate the nitrogen uses its p orbitals to bond with the oxygens. however, in a hypthetical $\ce{NCl5}$ molecule the orbital overlap between the chlorine and nitrogen orbitals would be poor due to energy mismatching and steric hindrance between the chlorines and so this molecule is not stable $\endgroup$ – bon Jan 29 '15 at 21:50

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