Hydrogen iodide (HI) is a diatomic molecule belonging to the hydrogen halide family. It exists as a colorless gas with a pungent odor at room temperature. The aqueous solution of HI is a strong acid known as hydroiodic acid.
HI is toxic by inhalation. Long-term exposure to this gas can result in adverse health effects.
In this article, we will discuss the chemical nature of hydrogen iodide (HI), i.e., is it polar or non-polar? Continue reading to find out.
Is HI polar or non-polar?
Hydrogen iodide (HI) is a weakly polar molecule. Hydrogen and iodine share an electron pair to form a single covalent bond in a linear-shaped molecule.
The electronegativity of the iodine (I) atom is greater than the hydrogen (H) atom. The iodine atom thus strongly attracts the shared electron pair in the H-I bond.
The electronegativity difference causes a partial positive charge (δ+) on the H atom and a partial negative (δ–) charge on the I atom. However, the poles generated are weak, as there is only a slight electronegativity difference between the bonded atoms.
As a result, the H-I bond in the HI molecule is polar and exhibits a dipole moment value. Thus, hydrogen iodide (HI) is a slightly polar molecule with a dipole moment equal to 0.38 D.
A molecule is polar if there is an unequal charge distribution between different centers of bonded atoms.
It is formed by the covalent bond between two different atoms leading to an asymmetric electron density.
In this case, the atoms acquire partial positive (δ+) and partial negative (δ–) charges.
If the dipole moments of individually polar bonds are not canceled, the molecule will be polar, such as HI.
Hence a polar molecule has an unequal distribution of the electronic charge. Contrarily, if the electronic charge is evenly distributed over the molecule, in that case, it will be a non-polar molecule overall.
The following three factors control the polarity of any covalently bonded molecule:
Electronegativity
Dipole moment
Molecular geometry
In the next section, we will discuss how these factors control the polarity of the HI molecule.
Factors affecting the polarity of HI
Electronegativity
It is defined as the ability of an atom to attract a shared pair of electrons from a covalent chemical bond.
Electronegativity decreases down the group in the Periodic Table of elements while it increases across a period.
Greater the electronegativity difference between bonded atoms in a molecule, the higher the bond polarity.
Hydrogen belongs to group 1-A (or 1) of the Periodic Table. The electronic configuration of hydrogen is 1s1, indicating that it only has 1 valence electron.
On the other hand, iodine belongs to group VII-A (or 17) of the Periodic Table. The electronic configuration of iodine is [Kr] 4d10 5s2 5p5, so it has 7 valence electrons available for bonding.
The hydrogen and iodine atoms share an electron pair to form a single covalent bond.
Atom
Electronic configuration
Valence electrons
Iodine (53I)
1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2 5p5
7
Hydrogen (1H)
1s1
1
Iodine (E.N = 2.66) is more electronegative than hydrogen (E.N = 2.2). There is an electronegativity difference of 0.46 units between these two atoms.
Due to this electronegativity difference, the iodine atom strongly attracts the shared electron cloud from the H-I bond. The bonded electrons are held close to the iodine atom in the HI molecule.
The I-atom thus gains a partial negative (Iδ-) charge, while the hydrogen atom, less electronegative, obtains a partial positive (Hδ+) charge.
In this way, oppositely charged poles develop in the HI molecule. However, the poles generated are weak as there is only a little electronegativity difference between the bonded atoms.
Dipole Moment
The dipole moment is the product of electrical charge (Q) and bond length (r) between two bonded atoms. It is a vector quantity expressed in Debye (D) units.
It is represented by a Greek symbol µ and measures the polarity of a bond.
The dipole moment of any molecule depends on the difference in electronegativity between the bonded atoms. The greater the electronegativity difference, the higher the bond polarity, resulting in a high dipole moment value.
It points from the partial positive (δ+) center to the partial negative (δ–) center of a bond or molecule.
The dipole moment of the H-I bond points from Hδ+ to Iδ- due to the electronegativity difference between the H and I atoms.
However, the dipole moment is weaker in the HI molecule as there is only a slight difference in the electronegativity of the atoms.
Thus, the H-I bond in the HI molecule is polar, with a dipole moment value of 0.38 D.
Molecular geometry
As discussed earlier, a hydrogen iodide (HI) molecule consists of a single H-I covalent bond. The overall molecule contains a total of 8 valence electrons. The central iodine atoms contain three lone pairs of electrons.
The molecule adopts a linear shape to minimize the repulsion effects due to the lone pairs on the I atom. There is a bond angle of 180°.
Due to the electronegativity difference between the bonded atoms, there is an unequal electronic charge distribution over the molecule in the only H-I bond.
The induced charges do not oppose each other in the linear molecule. Asa result, the HI molecule has a specific dipole moment ( µ = 0.38 D).
Thus, HI is a polar molecule.
Difference between polar and nonpolar?
Polar molecule
Non-polar molecule
Atoms must have a difference in
electronegativity
Atoms may have the same or different electronegativity values
Unequal charge distribution overall
Equal charge distribution overall
Net dipole moment greater than zero
Net dipole moment equals to zero
Examples include water (H2O), ethanol (CH3CH2OH), ammonia (NH3), sulfur dioxide (SO2), hydrogen iodide (Hl), nitric oxide (NO), bromine pentafluoride (BrF5), etc.
Examples include oxygen (O2), nitrogen (N2), methane (CH4), carbon disulfide (CS2), etc.
Why is hydrogen iodide (HI) a slightly polar molecule?
HI has a weakly polar covalent bond because of a slightelectronegativity difference between bonded H and I atoms.
The H-atom gains a partial positive (Hδ+) charge, while the iodine atom carries a partial negative (Iδ-) charge. The linear molecule has a dipole moment value equal to 0.38 D.
Thus, HI is a slightly polar molecule.
Compare the polarity of HF and HI.
The greater the electronegativity difference, the higher the bond polarity, resulting in a high dipole moment value.
HF has a strong polar covalent bond of a great electronegativity difference (1.78 units) between the bonded H and F atoms. The dipole moment of the HF molecule is 1.91 D.
HI has a relatively weaker polar double covalent bond because of a little electronegativity difference (0.46 units) between the bonded H and I atoms. The dipole moment of the HI molecule is 0.38 D.
Thus, HF is more polar than HI because of a greatelectronegativity difference leading to a higher dipole moment value.
Is there a formal charge on the bonded atoms in the HImolecule?
Formal charge of an atom = [ valence electrons – non-bonding electrons- ½ (bonding electrons)]
For hydrogen atom
Valence electrons = 1
Bonding electrons = 2
Non-bonding electrons = 0
∴ The formal charge on the hydrogen atom = 1-0-2/2 = 1-1 = 0
For iodine atom
Valence electrons = 7
Bonding electrons = 2
Non-bonding electrons = 6
∴ The formal charge on the iodine atom = 7-6-2/2 = 1-1 = 0
Thus, no formal charge is present on the hydrogen iodide (HI) molecule.
Summary
Hydrogen iodide (HI) is a slightly polar molecule.
It consists of a polar H-I covalent bond due to an electronegativity difference of 46 units between the bonded atoms.
The iodine atom is more electronegative and strongly attracts the shared electron cloud from the H-I bond.
The HI molecule forms a linear structure to minimize the repulsion effects of the lone pairs of the I atom.
Due to the slight electronegativity difference, the H-I bond has a dipole moment value, and poles are formed in the molecule. Thus, HI is weakly polar.
The dipole moment value of the H-I bond is 0.38 D, pointing from Hδ+ to Iδ-.
Vishal Goyal is the founder of Topblogtenz, a comprehensive resource for students seeking guidance and support in their chemistry studies. He holds a degree in B.Tech (Chemical Engineering) and has four years of experience as a chemistry tutor. The team at Topblogtenz includes experts like experienced researchers, professors, and educators, with the goal of making complex subjects like chemistry accessible and understandable for all. A passion for sharing knowledge and a love for chemistry and science drives the team behind the website. Let's connect through LinkedIn: https://www.linkedin.com/in/vishal-goyal-2926a122b/
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