Is [H3O]+ polar or non-polar? - Polarity of [H3O]+

[H₃O]⁺ is the chemical formula for the hydronium ion. It is a very important factor when dealing with chemical reactions that occur in aqueous solutions and a vital entity for acid-base reactions in chemistry. It is a polyatomic ion having a molar mass of 19.02 g/mol.

To find out whether the hydronium [H₃O]⁺ ion is polar or non-polar, continue reading the article.

Is [H₃O]⁺ polar or non-polar?

Hydronium [H₃O]⁺ is a polar molecular ion. [H₃O]⁺ consists of an oxygen (O) atom and three hydrogens (H) atoms. The oxygen atom is present at the center of the molecular ion, while three atoms of hydrogen occupy terminal positions, one on each side, making a trigonal pyramidal shape.

An electronegativity difference of 1.24 units exists between an oxygen and a hydrogen atom in each of the O-H bonds present in the [H₃O]⁺ ion. Thus, all three covalent bonds are individually polar in the [H₃O]⁺ ion and possess a specific dipole moment value (symbol µ).

It is due to the asymmetric shape of the [H₃O]⁺ ion that the dipole moments of polar O-H bonds do not get canceled in the ion overall. The electron cloud stays non-uniformly distributed over the ion. Thus, [H₃O]⁺ is overall polar (net µ = 1.84 D).

Name of molecule	Hydronium [H3O]+
Bond type	Polar covalent
Molecular geometry	Trigonal pyramidal
Polar or Non-polar?	Polar
Dipole moment	1.84 D
Bond angle	107.5° and 113°

What makes a molecule polar or non-polar?

A molecule is polar if there is a non-uniform charge distribution present in it. If the charge distribution gets equally balanced in different parts, then that molecule or molecular ion is considered non-polar.

The following three factors mainly influence the polarity of a molecule or molecular ion:

• The electronegativity difference between two or more covalently bonded atoms
• Dipole moment
• Molecular geometry or shape

Now, let us discuss the effect of the above three factors one by one to prove that the hydronium [H₃O]⁺ ion is overall polar.

Factors affecting the polarity of H₃O⁺

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.

Oxygen (O) is present in group VI A (or 16) of the Periodic Table. The electronic configuration of an oxygen atom is 1s² 2s² 2p⁴. As per this electronic configuration, an O-atom has a total of 6 valence electrons. It is thus short of 2 more electrons that are required so that an O-atom can achieve a complete octet electronic configuration.

Conversely, Hydrogen (H) belongs to Group I A (or 1) of the Periodic Table. The electronic configuration of a hydrogen atom is 1s¹. Hence, each H-atom in [H₃O] ⁺ ion has a deficiency of 1 more valence electron to obtain a full outer shell.

Hence in [H3O]+ ion, each of the three H-atoms is single-covalently bonded with the central O-atom. One lone pair of electrons is present on O-atom. In this way, all three single-bonded H-atoms attain a complete duplet, while the central O-atom acquires a complete octet electronic configuration via chemical bonding in the [H3O]+ ion.

The presence of one lone pair of electrons on the central oxygen (O) atom distorts the shape of the molecular ion and makes it occupy a trigonal pyramidal molecular geometry and shape in [H₃O] ⁺.

Atom	Electronic configuration	Valence electrons
Hydrogen (1H)	1s1	1
Oxygen (8O)	1s2 2s2 2p4	6

The electronegativity of oxygen is 3.44, and that of hydrogen is 2.20. Thus there is an electronegativity difference of 1.24 units between the bonded oxygen and hydrogen atoms in each O-H bond.

The more electronegative central O-atom strongly attracts the shared electron cloud away from the terminal H-atoms in each O-H bond present in [H₃O]⁺ ion.

Thus, the oxygen atom present at the center of [H₃O]⁺ gains a partial negative (O^δ-) charge, while each terminal hydrogen atom obtains a partial positive (H^δ+) charge. As a result, each O-H bond is individually polar in the hydronium ion.

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 a polar covalent bond conventionally points from the positive center to the center of the negative charge. So in [H₃O]⁺, the dipole moment of each O-H polar bond points from H^δ+ to O^δ- (as shown below).

Molecular geometry

According to the valence shell electron pair repulsion (VSEPR) theory of chemical bonding, [H₃O]⁺ is an AX₃E₁ -type molecular ion. To one O-atom at the center (A), three O-H bond pairs (X) are attached, and there is one lone pair (E) on the central atom.

So, the molecular geometry or shape of [H₃O]⁺ is different from its ideal electron pain geometry, i.e., trigonal pyramidal versus tetrahedral.

Due to the presence of a lone pair of electrons on the central O-atom, the internal H-O-H bond angle decreases to 107.5° from an ideal 109.5°. In contrast, the external bond angle increases up to an experimentally determined value of 113°.

The lone pair of electrons present on the central O-atom in [H3O] + ensures the existence of lone pair-bond pair repulsions in the molecular ion in addition to bond pair-bond pair electronic repulsions. This strong repulsive effect influences the ion to adopt an asymmetrical shape.

It is due to this asymmetrical trigonal pyramidal shape that the dipole moments of O-H polar bonds do not get canceled in [H₃O] ⁺. The electron cloud stays non-uniformly distributed over the molecular ion. Thus, [H₃O] ⁺ is overall a polar ion (net µ = 1.84 D).

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), hydronium [H3O]+ ion, ammonia (NH3), sulfur dioxide (SO2), bromine trifluoride (BrF3), methyl chloride (CH3Cl), dichloromethane (CH2Cl2), chloroform (CHCl3), etc.	Examples include oxygen (O2), nitrogen (N2), methane (CH4), carbon disulfide (CS2), etc.

Also, check –

• H₃O⁺ lewis structure, molecular geometry, bond angle, hybridization
• How to tell if a molecule is polar or nonpolar?
• Is NCl₃ polar or nonpolar?
• Is NO₃^– polar or nonpolar?
• Is BrF₅ 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 HCN polar or nonpolar?
• Is CCl₄ polar or nonpolar?
• Is XeF₄ polar or nonpolar?
• Is CH₂O polar or nonpolar?
• Is CHCl₃ polar or nonpolar?
• Is SF₆ polar or nonpolar?
• Is BF₃ polar or nonpolar?
• Is PCl₅ polar or nonpolar?

FAQ

Why is [H3O]+ a polar molecular ion?

[H3O]+ has polar bonds present due to the electronegativity difference of 1.24 units between the bonded oxygen (E. N= 3.44) and hydrogen (E. N= 2.20) atoms in each O-H bond. Thus, each O-H bond possesses a specific dipole moment. It is due to the asymmetrical trigonal pyramidal shape of [H3O]+ that the dipole moments do not get canceled, and the electron cloud becomes non-uniformly distributed in the ion overall. Consequently, the hydronium [H3O]+ ion is overall polar with a net dipole moment µ = 1.84 D

What are the formal charges present on the bonded atoms in [H3O]+ ion?

Formal charge on an atom = [ valence electrons – non-bonding electrons- ½ (bonding electrons)] For the central O-atom Valence electrons = 6 Bonding electrons = 6 Non-bonding electrons = 1 lone pair = 2 electrons ∴ Formal charge on central O-atom = 6 – 2 – 6/2 = 6 – 2 – 3 = 6 – 5 = +1 For each H-atom Valence electrons = 1 Bonding electrons = 2 Non-bonding electrons = 0 ∴ Formal charge on each H-atom = 1 – 0 – 2/2 = 1 – 0 – 1  = 1 – 1 = 0 Zero formal charges are present on each H-atom while a +1 formal charge is present on the central O-atom; thus, the overall charge present on the hydronium ion is (0) + (+1) = +1.

Which one out of the two is more polar: [H3O] + and H2O?

The net dipole moment µ for the hydronium ion is 1.84 D while that for water is 1.85 D. Thus, both H3O+ and H2O have the same polarity. In either case, there are O-H polar bonds with high dipole moments. The O-H dipole moments do not get canceled in either the trigonal pyramidal shape of H3O+ or the bent shape of H2O. The charged electron cloud stays non-uniformly distributed thus, both are strongly polar in nature.

Summary

• Hydronium [H₃O]⁺ is a polar molecular ion.
• It consists of strongly polar O-H bonds due to a high electronegativity difference of 1.24 units between an oxygen and a hydrogen atom.
• Hydronium [H₃O]⁺ has an asymmetrical trigonal pyramidal shape with internal H-O-H 107.5^° while external 113^° bond angles.
• The electron cloud stays non-uniformly distributed in the asymmetrical [H₃O]⁺ trigonal pyramidal shape.
• Due to the presence of one lone pair of electrons on the central O-atom, the dipole moment of polar O-H bonds does not get canceled.
• The net dipole moment in [H₃O]⁺ is 1.84 Debye, so it is overall polar.