Formation of Coordinate Bond in HNO3

Introduction:
In order to understand how oxygen forms a coordinate bond in HNO3, it is necessary to first understand what a coordinate bond is. A coordinate bond is a type of covalent bond in which both electrons forming the bond are donated by one atom, known as the donor atom, to another atom, known as the acceptor atom. This type of bond is also known as a dative bond or a coordinate covalent bond.

Explanation:
In HNO3, the oxygen atom does not have any vacant orbitals in its valence shell. However, it still forms a coordinate bond by utilizing one of its lone pairs of electrons to donate to the nitrogen atom.

Step 1: Formation of Nitronium Ion (NO2+)
Before the formation of the coordinate bond, HNO3 undergoes ionization to form the nitronium ion (NO2+). This process involves the loss of a proton (H+) from the nitric acid molecule. The resulting nitronium ion consists of a nitrogen atom bonded to two oxygen atoms, with a positive charge on the nitrogen atom.

Step 2: Donation of Lone Pair
In the nitronium ion (NO2+), the nitrogen atom has a positive charge and is electron-deficient. On the other hand, the oxygen atom has two lone pairs of electrons, one of which can be used for donation. The lone pair of electrons on the oxygen atom forms a coordinate bond with the nitrogen atom by donating both electrons.

Step 3: Formation of Coordinate Bond
As a result of the donation of the lone pair of electrons, the nitrogen atom in the nitronium ion (NO2+) gains a full octet by sharing the electron pair from the oxygen atom. This forms a coordinate bond between the oxygen atom and the nitrogen atom. The oxygen atom acts as the donor atom, providing both electrons to the nitrogen atom, which acts as the acceptor atom.

Conclusion:
In summary, even though the oxygen atom in HNO3 does not have any vacant orbitals, it can still form a coordinate bond by utilizing one of its lone pairs of electrons. This allows the oxygen atom to donate both electrons to the nitrogen atom, resulting in the formation of a coordinate bond. The formation of the coordinate bond is facilitated by the ionization of HNO3, which leads to the formation of the nitronium ion (NO2+).