HNO Lewis Structure, Characteristics : 11 Facts You Should Know

Through this article, we shall learn about HNO facts, how to construct HNO lewis structure, its detailed characteristics and uses.

HNO, which is commonly named as Nitroxyl or azanone which is its IUPAC name can be identified as the conjugate base of nitroxide anion, NO. It is also the reduced form of NO , or the one-electron reduced form of nitric oxide. It is most commonly found in gas phase and found in solution phase as an intermediate with a very short lifetime.

Some properties of HNO :

  1. Molar mass is 31 g/mol .
  2. H-NO has bond dissociation energy of 49.5 kcal/mol .
  3. Has heat capacity of 33.9 JKmol.

It has been found that HNO could be produced from Angeli’s salt and Piloty’s acid . However, HNO is not a fully fledged stable compound, it is rather viewed as an intermediate produced during various steps of a chemical reaction.

How to draw HNO lewis structure ?

HNO Lewis structure or simply lewis dot structure is a simple representation of the skeletal structure of a molecule describing how the atoms are connected and through how many bonds maintaining the octet rule.

Octet rule states that every atom tries to accommodate eight electrons in their valence orbitals to attain a stable and noble gas configuration.

A molecule that has completely filled orbitals tends to be most stable in that state as it behaves similarly to that of a noble gas element.

HNO Lewis structure
Blue ball = Nitrogen, Red ball = Oxygen and Grey ball = Hydrogen . HNO ball stick representation from Wikipedia

Steps to draw HNO Lewis structure :

  1. The first step involves counting the total number of valence electrons available. Nitrogen belongs to the group 15 and therefore, it has 5 valence electrons available ( Ground state electronic configuration of N : [Ne]2s22p3 ) . Oxygen belongs to group 16 so it has a total of 6 valence electrons available ( Ground state electronic configuration of O : [Ne]2s22p4 ) and H has only one electron in its valence shell which is the 1s orbital . So, it makes a total of 12 valence electrons available to construct the Lewis dot structure of HNO .
  2. Next the central atom is chosen based on their electronegativity, however, H cannot be the central atom as its valency is one so it can cannot form more than one bonds. Magnitude of electronegativity difference ( X ) of O, N and H are XO = 3.44 , XN = 3.04  , XH = 2.2 . The electronegativity difference reveals that N is the second least electronegative element after hydrogen.
  3. Now, the atoms are drawn with two electrons between them to form a bond. In doing so, we have to keep in mind the octet rule.
  4. As oxygen is generally a divalent element and nitrogen is a trivalent element, one more bond can be formed between O and N by sharing one electron each from the valence orbitals. This completes the octet of nitrogen , oxygen and hydrogen as well which is an electron deficient element. Therefore, a single bond is formed between H-N and a double bond is formed between O and N .
  5. As a result, one lone pair of electrons resides on each N and O atom. Observing the final skeletal structure makes it clear that the octet rule is well obeyed by N and O. H, however, being an electron deficient element, violates octet rule.

The schematic representation of step-wise construction of HNO lewis structure is shown below that can put an insight of the above detailed explanation :

HNO lewis structure
HNO lewis structure

HNO lewis structure resonance :

HNO lewis structure involves 3 resonance structures where two of them involve charge separation. In one of the resonance structures, N acquires a positive charge and oxygen acquires a negative charge. In the other, nitrogen acquires a negative charge and oxygen acquires a positive charge.

The most stable resonance structure is the one that do not involve any charge separation, followed by the one where O has a negative charge and nitrogen has a positive charge. The least stable resonance structure is the one that has a positive charge on oxygen and positive charge on nitrogen atom.

This is because O is more electronegative than nitrogen, as such a positive charge on nitrogen is more stable than on an oxygen atom.

Resonance structure of HNO lewis structure

HNO valence electrons :

HNO lewis structure contains a total of 12 valence electrons. Valence electrons are the outermost electrons in the subshells that can participate in a chemical bond formation by either sharing or transferring the electrons.

Ground state electronic configuration of N : [Ne]2s22p3

Ground state electronic configuration of O : [Ne]2s22p4

Ground state electronic configuration of H : 1s1

The outermost electrons in 2s and 2p subshells and 1s subshell are the total valence electrons of HNO lewis structure.

HNO lewis structure lone pairs :

HNO lewis structure has a total of 3 lone pairs of electrons. One resides on N atom and the other two reside on O atom .

Lone pairs of electrons are those that do not participate in a chemical bond formation and resides on the respective atom, but it can be delocalized if required or possible.

HNO solubility :

Since it can be viewed as an H+ NO, it is soluble in water and other protic and polar solvents.

Below are the few solvents where HNO solubility has been tested :

Mineral acidsSoluble
Table : It shows solubility in different solvents.

HNO hybridization :

N undergoes sp2 hybridisation and the lone pair resides on one of 2p subshells that doesn’t undergo hybridization. O undergoes sp2 hybridisation with N and H undergoes sp3 hybridisation with N.

Ground state electronic configuration of N : [Ne]2s22p3

Ground state electronic configuration of O : [Ne]2s22p4

Ground state electronic configuration of H : 1s1

N has 3 unpaired electrons in the valence subshells and one lone pair of electrons in the 2s shell.

Oxygen has 2 unpaired electrons in the 2p subshells and two lone pairs of electrons, one in 2s and the other in 2p subshells.

HNO lewis strucrure shape :

HNO lewis strucrure adopts a bent shape or dihedral shape.

This is in accord with VSEPR theory according to which a sp2 hybridisation with lone a pairs of electrons is best when it adopts a bent shaped. This makes the lone pairs at maximum distance from each other for minimum repulsion and mimimum bond pair repulsion.

HNO lewis structure showing its shape, angle and hybridisation.

HNO lewis structure angle :

HNO lewis structure has a bond angle close to 1200 HNO as the structure adopts a bent-shape structure with sp2 hybridisation. The bond angle is between H-N-O.

Bond angle is defined as the angle between the participating orbitals in bond formation with the central atom.

HNO lewis structure formal charge :

HNO lewis structure formal charge is the hypothetical charge assigned to the constituent atoms in a molecule if the bonding electrons are shared equally and evenly. It is not the net charge of the molecule but rather an electronic charge assumed by the atoms in the lewis dot structure.

               HNO formal charge can be calculated mathematically by the formula :

Formal charge = (Number of valence electrons in a free atom of the element) – (Number of unshared electrons on the atom) – (Number of bonds to the atom)

In addition, Charge on the molecule= sum of all the formal charges.

Formal charge of H = 1- 0- 1 = 0

Formal charge of N = 5 – 2 – 3 = 0

Formal charge of O = 6 – 4 – 2 = 0

HNO lewis structure octet rule :

HNO lewis structure follows octet rule in its lewis dot structure. Octet meaning completely filled subshells is well followed by molecule.

But, H atom, being electron deficient, doesn’t accommodate 8 electrons thereby violating octet rule. Thus, considering the atoms individyally, O and N follow octet rule whereas, H doesn’t due to its less electron in its subshells or low valency.

Is HNO soluble in water ?

HNO lewis structure is soluble in water as it is one electron reduced species of NO.

It is the conjugate acid of the NOgroup which. They exist as polar ions and are well dissolved in water  by charge separation. However, its existence in solution phase is only short-lived.

Is HNO an acid or base ?

HNO lewis structure is an impressing acid. The proton is easily lost when it is dissolved in a basic solution.

However, studies are still going on its acidic and other chemical properties. Because it exists as an intermediate, investigations to be carried out are difficult and complicated.

Conclusion :

HNO lewis structure is an intermediate species with a very short lifetime that adopts a bent shape structure with very low bond dissociation energies indicating its low stability.

Nandita Biswas

Hello. I am doing my Masters in Chemistry with specialisation in organic and physical chemistry. Also, I have done two projects in chemistry- One dealing with colorimetric estimation and determination of ions in solutions. Other in Solvatochromism study of fluorophores and their uses in the field of chemistry alongside their stacking properties on emission. I have also specialised in Resin chemicals.

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