Is HCN polar or nonpolar? - Polarity of HCN

Hydrogen cyanide (HCN) is a colorless, extremely poisonous chemical substance that smells like bitter almond oil.

It was used as a warfare agent in World War I. Since then, it found many different uses in explosives, mining, and the industrial sector.

HCN is also used as a fumigant in pest control. Now the question that interests us is what is the chemical nature of HCN. Is it polar or nonpolar?

Is HCN polar or nonpolar? 

Hydrogen cyanide (HCN) is a strongly polar molecule. It consists of three different types of atoms i.e., hydrogen (H), carbon (C), and nitrogen (N). Nitrogen is the most electronegative atom out of all three. It strongly attracts the shared electron cloud from a C≡N bond and also from the C-H bond.

Charged poles develop in the molecule. The electron cloud density stays unbalanced in the molecule overall. Thus, HCN is a polar molecule with a net dipole moment µ= 2.98 D.

Name of molecule	Hydrogen cyanide (HCN)
Bond type	Polar covalent
Molecular geometry	Linear
Polar or Non-polar?	Polar molecule
Dipole moment	2.98
Bond angle	180º

You must have inferred by the information given above that polar substances have an unbalanced electron cloud distribution in their molecules.

How do oppositely charged poles develop in a molecule and what factors contribute to an uneven charge distribution overall?

We have discussed all this and more interesting information about the polarity of HCN in the next section. So, continue reading!

Factors affecting the polarity of HCN

Electronegativity 

Electronegativity is the ability of an elemental atom to attract a shared pair of electrons from a covalent bond.

Greater the electronegativity difference between bonded atoms, the higher the bond polarity.

An HCN molecule is made up of three different atoms. There is a carbon (C) atom present at the center. Carbon is from group IV A (or group 14) of the Periodic Table. It has 4 valence electrons available for bonding.

In the HCN molecule, carbon uses 1 of its valence electrons to form a single
covalent bond with a hydrogen (H) atom on one side. On the other side, carbon uses the remaining 3 valence electrons to form a triple covalent bond with a nitrogen (N) atom.

Atom	Electronic configuration	Valence electrons
Carbon (6C)	1s2 2s2 2p2	4
Hydrogen (1H)	1s1	1
Nitrogen (7N)	1s2 2s2 2p3	5

In this way, H obtains a stable duplet electronic configuration while each of the C and N atoms attains a stable octet configuration.

The nitrogen atom is highly electronegative. N (E.N = 3.04) is more electronegative than C (E.N= 2.55). There is an electronegativity difference of 3.04 -2.55 = 0.49 units between the two atoms. So, nitrogen strongly attracts the shared electron cloud from the CN bond.

Conversely, there is a relatively small electronegativity difference between C and H (E.N= 2.20) atoms i.e., 2.55-2.20= 0.35 units. Carbon also attracts the shared electron cloud from a C-H bond, but this is a weak attraction.

As a consequence, nitrogen pulls the electron cloud of a C-H bond in addition to attracting the CN electrons. Densely charged negative (N^δ-) and positive ( C^δ+ and H^δ++ ) poles develop in the HCN molecule. This is called the bond polarity of HCN.

Dipole moment

The dipole moment is a vector quantity. It is mathematically defined as the product of electrical charge (Q) and charge separation (r).

The dipole moment is measured from the center of the positively charged pole to the center of the negative pole. Its unit is Debye (D).

The charge separation is measured from the center of the positive pole to the
center of the negative pole, technically the bond length.

As there are oppositely charged poles present in an HCN molecule so the molecule has a specific dipole moment value. The dipole moment arrow points from C^δ+ to N^δ-. The net dipole moment of the HCN molecule is 2.98 D.

Molecular geometry

Hydrogen cyanide (HCN) is a polyatomic molecule. It has a linear shape or molecular geometry. All the bonded atoms lie in one plane, along a line, forming a mutual bond angle of 180°.

But it is due to a lone pair present on nitrogen that the charge distribution is not balanced in the molecule overall.

The dipole moment effect of individual C-H and CN bonds is enhanced by this uneven charge distribution. In conclusion, HCN is a polar molecule with a net dipole moment greater than zero.

Why is HCN a polar covalent molecule and not an ionic compound?

It is due to the high dipole moment value of HCN (µ=2.68 D) that it is sometimes doubted to be an ionic compound.

Ionic substances are produced by the complete transference of electrons from the positive end to the negative end.

HCN dissociates in water to give hydrogen (H⁺ ) ion and a cyanide (CN^–) ion.

But this does not make hydrogen cyanide an ionic compound because there is no complete transference of electrons while its bond formation.

HCN is indeed a covalent molecule, it interacts with water using its polar ends and in that way, it shows some ionic behavior.

You must remember while studying chemistry that no covalent compound is purely covalent unless it is made up of two identical atoms such as H₂, Cl₂, O₂, etc. with no electronegativity difference.

All polar covalent compounds will have intermediate characteristics.

Difference between polar, non-polar covalent molecules and ionic compounds

Covalent		Ionic
Non-polar molecule	Polar molecule
Made up of identical or different atoms	Made up of dissimilar atoms	Made up of dissimilar atoms
Atoms may have the same or different electronegativity values	Atoms have an electronegativity difference	Atoms have a very high electronegativity difference
The electron cloud is equally shared in the molecule overall	The shared electron cloud is non-uniformly dispersed over the molecule	Formed by complete transference of electrons
Net dipole moment =0	Net dipole moment > 0	Net dipole moment >>> 0
Examples: H2, O2, BF3, etc.	Examples: H2O, NH3, HCN, etc.	Examples: NaCl, MgO etc.

Also check –

• HCN lewis structure, molecular geometry, bond angle, hybridization
• How to tell if a molecule is polar or nonpolar?
• Is SF4 polar or nonpolar?
• Is CO2 polar or nonpolar?
• Is NH3 polar or nonpolar?
• Is SO2 polar or nonpolar?
• Is SO3 polar or nonpolar?
• Is H2O polar or nonpolar?
• Is H2S polar or nonpolar?
• Is CCl4 polar or nonpolar?
• Is XeF4 polar or nonpolar?
• Is CH2O polar or nonpolar?
• Is CHCl3 polar or nonpolar?
• Is BrF5 polar or nonpolar?
• Is SF6 polar or nonpolar?
• Is BF3 polar or nonpolar?
• Is PCl5 polar or nonpolar?

FAQ

Why is HCN a polar molecule?

Nitrogen is more electronegative than carbon while carbon in turn is more electronegative than hydrogen. N strongly attracts all the shared electrons from the HCN molecule. The molecule (HCN) overall has a non-uniform charge distribution, thus, it is polar (µ=2.98 D). “In the HCN molecule, there is a partial, positive charge on the hydrogen atom and there is a partial negative charge on the nitrogen atom because of the electronegativity difference. Therefore, it is a polar molecule.”

Is HCN planar or nonplanar?

HCN is a planar molecule. It has a linear shape with a 180° bond angle. All the atoms lie in the same plane.

How many valence electrons does the HCN lewis structure have?

There are a total of 10 valence electrons in an HCN lewis structure.

What is the bonding type of CH4, HCN, and CaO, polar or nonpolar covalent?

CH4 is a non-polar covalent molecule. Each C-H bond is slightly polar due to a small electronegativity difference between the bonded C and H atoms, but the molecule overall is non-polar due to its symmetric, tetrahedral shape and geometry. HCN is a polar covalent molecule. CaO is not a covalent but an ionic compound that is formed by the complete transference of two valence electrons of Ca to an O atom. Oppositely charged Ca2+ and O2- ions develop an electrostatic force of attraction.

Are CCl4, HCN, CO2, CCl2F2, and H2O all polar?

CCl4 is a non-polar molecule. The individual C-Cl bonds have an electronegativity difference between bonded atoms and are polar, but the dipole moments get canceled in the overall symmetric tetrahedral shape of the molecule. µ=0. HCN is a polar molecule. µ > 0. CO2 is a non-polar molecule. The C=O dipole moments get canceled in the linear shape of the molecule. µ=0. CCl2F2 is a polar molecule. It has a tetrahedral shape but as a C-F bond is more polar than a C-Cl bond thus the dipole moments do not get canceled overall. µ > 0. H2O is a polar molecule. There are two lone pairs of electrons present on central oxygen. Lone pair-lone pair and lone pair-bond pair repulsions decrease the bond angle, making H2O occupy a bent shape. µ > 0.

Explain hybridization in HCN?

The C and N atoms in HCN are sp hybridized. The electronic configuration of carbon is 1s22s22p2. While bonding, one 2s electron shifts to the empty 2p orbital. This excitation results in half-filled 2s, 2px, 2py, and 2pz orbitals. The 2s orbital mix with a 2p orbital to form two hybrid sp orbitals. Each sp forms a sigma bond on each side of the molecule. The unhybridized p orbitals of carbon are used for pi bonding with the unhybridized p orbitals of N. The electronic configuration of Nitrogen is 1s22s22p3. The 2s orbital hybridizes with 2p to produce two sp hybrid orbitals. One sp contains one electron only and it forms a sigma bond with the sp of C. The other sp contains 2 electrons and is situated as a lone pair on N.

Summary

• Hydrogen cyanide (HCN) is a polar molecule.
• The three covalently bonded atoms in the HCN molecule have a specific electronegativity difference.
• The electronegativity difference between C and N atoms (0.49 units) is higher than the electronegativity difference between a C and an H atom (0.35 units).
• Nitrogen strongly attracts the shared electron cloud from both the C≡N and the C-H bonds. Oppositely charged poles develop in the molecule
• HCN is a linear molecule with a 180° bond angle but due to a non-uniform charge distribution in the molecule, the molecule overall is polar.
• It has a net dipole moment value of 2.98 D.