how does nitrogen go to oxidation state of +5 as it does not have any vacant 2d orbital for coming into excited state

Hi,

Don't confuse Oxidation state with valency. Oxidation state is only a measure of how much an element is oxidized or reduced in a compound. Of course, under usual circumstances, Nitrogen will not want to get oxidised. But under special conditions, you can make Nitrogen accept or give electrons away, or at least force it to share a few electrons. In such cases, you cannot think of valency as the rule for formation of compounds. Valency rule only hold for simple compounds. (In case of Nitrogen, it doesn't hold even for simple compounds). Oxidation state +5 only means that in that compound, Nitrogen is in a highly oxidized condition. It is not impossible, there are mechanisms by which atoms like Nitrogen, Xenon, Argon etc. can form compounds.

Oxidation state is defined by IUPAC as the following:

"A measure of the degree of oxidation of an atom in a substance. It is defined as the charge an atom might be imagined to have when electrons are counted according to an agreed-upon set of rules: (1) the oxidation state of a free element (uncombined element) is zero; (2) for a simple (monatomic) ion, the oxidation state is equal to the net charge on the ion; (3) hydrogen has an oxidation state of 1 and oxygen has an oxidation state of -2 when they are present in most compounds. (Exceptions to this are that hydrogen has an oxidation state of -1 in hydrides of active metals, e.g. LiH, and oxygen has an oxidation state of -1 in peroxides, e.g. H₂O₂; (4) the algebraic sum of oxidation states of all atoms in a neutral molecule must be zero, while in ions the algebraic sum of the oxidation states of the constituent atoms must be equal to the charge on the ion. For example, the oxidation states of sulfur in H₂S, S₈ (elementary sulfur), SO₂, SO₃, and H₂SO₄ are, respectively: -2, 0, +4, +6 and +6. The higher the oxidation state of a given atom, the greater is its degree of oxidation; the lower the oxidation state, the greater is its degree of reduction."

You can get more info at http://en.wikipedia.org/wiki/Oxidation_state

Hi

Oxidation number is a number that has no significance to structures. Its designated by noting the charge on the atom after theoritically giving the bonded electrons in a bond to the more electronegative element.
In NO3 - we have nitrogen forming 4 covalent bonds but the oxidation state is +5.
Oxidation number or state is a mere number and nothing more.

In chemistry, the oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound. The formal oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. Oxidation states are typically represented by integers, which can be positive, negative, or zero. In some cases the average oxidation state of an element is a fraction, such as 8/3 for iron in magnetite (Fe₃O₄).