Is Sulfate ion (SO42-) polar or nonpolar? - Polarity of SO42-

SO₄^2- is the chemical formula for the sulphate or sulfate ion.  It is formed by the loss of two hydrogen ions (H⁺) by sulfuric acid (H₂SO₄). Sulfate ion and its slats are used in detergents, emulsifiers, and foaming agents. SO₄^2- ion has a molar mass of 96.06 g/mol.

This article will discuss a very important property, i.e., the polarity, of the super important industrial ion, sulfate (SO₄^2-).

To know further, continue reading this article.

Is SO₄^2- polar or non-polar?

Sulfate (SO₄^2-) is a non-polar molecular ion. It consists of one sulfur (S) atom and four oxygen (O) atoms. The sulfur atom is present at the center of the molecular ion, while oxygen atoms occupy terminal positions, one on each side, making two S=O and two S-O bonds, adopting a symmetrical tetrahedral molecular shape.

An electronegativity difference of 0.86 units exists between a sulfur and an oxygen atom.

Hence, each S-O and S=O bond is individually polar in SO₄^2- and thus possesses a specific dipole moment value (symbol µ).

However, it is due to the symmetrical shape and geometry of the SO₄^2- ion that the charged electron cloud stays uniformly distributed, and the dipole moments of S=O bonds get canceled by the dipole moments of S-O bonds.

Thus, sulfate (SO₄^2-) is overall a non-polar molecular ion (net µ = 0).

Name of molecule	Sulfate (SO42-)
Bond type	Polar covalent
Molecular geometry	Tetrahedral
Polar or Non-polar?	Overall non-polar
Dipole moment	Zero
Bond angle	∠(O-S=O) = 109.5°

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:

• 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 sulfate (SO₄^2-) ion is overall non-polar.

Factors affecting the polarity of SO₄^2-

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 or molecular ion, the higher the bond polarity.

Sulfur (S) belongs to Group VI A (or 16) of the Periodic Table. The electronic configuration of a sulfur atom is 1s² 2s² 2p⁶ 3s² 3p⁴. Hence, each S-atom in SO₄^2- has a deficiency of 2 more valence electrons to obtain a complete octet electronic configuration.

Similarly, oxygen (O) also belongs to Group VI A (or 16) of the Periodic Table. The electronic configuration of an oxygen atom is 1s² 2s² 2p⁴. So each O-atom has a deficiency of 2 more valence electrons for it to complete its octet.

Hence, in the SO42- ion, the central sulfur atom is double-covalently bonded to two oxygen atoms and single-covalently bonded to another two oxygen atoms. All 6 valence electrons of sulfur get consumed in covalent bonding thus, there is no lone pair of electrons present on the central S-atom in SO42-. Contrarily, each of the terminal single and double-covalently bonded O-atoms carry 3 and 2 lone pairs of electrons, respectively.

In this way, all the bonded atoms in the molecular ion attain a completely stable octet electronic configuration via chemical bonding and lone pairs in the SO₄^2- ion.

Due to the absence of any lone pair of electrons on the central sulfur (S) atom, there is no distortion in the molecular shape of the ion, and SO₄^2- thus attains a symmetrical tetrahedral molecular shape and geometry.

Atom	Electronic configuration	Valence electrons
Sulfur (16S)	1s22s22p63s23p4	6
Oxygen (8O)	1s22s22p4	6

An electronegativity difference of 0.86 units exists between the bonded S-atom (E.N = 2.58) and O-atom (E.N = 3.44) in each of the S=O and S-O bonds.

Therefore, in SO₄^2-, each of the terminal oxygen atoms gains a partial negative (O^δ-) charge while the central sulfur atom obtains a partial positive (S^δ+) charge.

As a result, each S-O and S=O bond is individually polar in the sulfate (SO₄^2-) ion.

Dipole Moment

Dipole moment (μ) is a vector quantity that points from the positive pole to the negative pole of a bond or a molecule.

It is mathematically calculated as a product of the magnitude of charge (Q) and charges separation (r). The dipole moment is expressed in a unit called Debye (D).

The dipole moment of a polar covalent bond conventionally points from the positive center to the center of the negative charge.

So, in SO₄^2-, the dipole moment of each S-O and S=O bond points from S^δ+ to O^δ- (as shown below).

Molecular geometry

According to the valence shell electron pair repulsion (VSEPR) theory of chemical bonding, SO₄^2- is an AX₄E₀ or simply AX₄-type molecular ion. To one S-atom at the center (A), four (two S-O and two S=O) bond pairs (X) are attached, and there is no lone pair of electrons (E) on the central atom.

So, the molecular geometry or shape of SO₄^2- is tetrahedral, which is identical to its ideal electron pair geometry.

Due to the absence of any lone pair of electrons on the central S-atom, there are no lone pair-lone pairs or lone pair-bond pair electronic repulsions present in the molecular ion. The bonded atoms thus obtain a mutual bond angle ∠(O-S=O) of 109.5°.

In the symmetrical tetrahedral shape of the SO₄^2- ion, the net dipole moment of three downwards-pointing S-O and S=O bonds get neatly canceled with the dipole moment of an upwards-pointing S=O bond.

The charged electron cloud stays uniformly spread over the molecular ion. Consequently, the sulfate (SO₄^2-) ion is a non-polar molecular ion overall (net µ = 0).

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), bromine trifluoride (BrF3), nitric oxide (NO), carbonyl sulfide (OCS or COS), etc.	Examples include oxygen (O2), nitrogen (N2), methane (CH4), ethane (C2H6), propane (C3H8), ethyne (C2H2), silicon dioxide (SiO2), carbonate ion (CO32-), sulfate (SO42-), etc.

Also, check –

• SO₄^2- lewis structure, molecular geometry, bond angle, hybridization
• How to determine if a molecule is polar or nonpolar?
• Is CH₂Cl₂ polar or nonpolar?
• Is CH₃F 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?
• Is CH₃Cl polar or nonpolar?

FAQ

Why is SO42- non-polar?

SO42- has polar bonds (two S-O and two S=O) due to an electronegativity difference of 0.86 units between the bonded S-atom (E.N = 2.58) and O-atom (E.N = 3.44). However, the SO42- ion attains a symmetrical tetrahedral shape due to which the dipole moment of the upwards-pointing polar bond gets canceled with that of the downward-pointing polar bonds, resulting in an overall non-polar molecular ion. Consequently, the sulfate (SO42-) ion is a non-polar molecular ion, with a net dipole moment µ = 0.

Which out of the two, SO42- and PO43- is polar in nature?

None. Both sulfate (SO42-) and phosphate (PO43-) are non-polar molecular ions. In SO42-, there are two polar S-O bonds and two polar S=O bonds. It is due to the symmetrical tetrahedral shape of the molecular ion that the individual S-O and S=O dipole moments get canceled equally in opposite directions, leading the molecular ion an overall non-polar nature (net µ = 0). The phosphate (PO43-) ion also possesses a symmetrical tetrahedral shape and geometry. The individual dipole moments of polar P-O and P=O bonds get canceled uniformly thus, PO43- is also overall non-polar (net µ = 0).

Which of the following molecular ions is polar in nature? a) NH4+   b) H3O+   c) SO42-   d) SO32-

Both hydronium (H3O+) and sulfite (SO32-) ions are polar in nature. Contrarily, ammonium (NH4+) and sulfate (SO42-) are non-polar molecular ions. NH4+ consists of four polar N-H bonds, having an electronegativity difference of 0.84 units between the bonded atoms. The individual dipole moments of polar N-H bonds get canceled in the symmetrical tetrahedral shape of the NH4+ ion. It is thus overall non-polar (net µ = 0). H3O+ comprises three strongly polar O-H bonds, owing to an electronegativity difference of 1.24 units between the bonded atoms. The individual O-H dipole moments do not get canceled in the asymmetrical trigonal pyramidal shape of the H3O+ ion. Thus, it is overall polar (net µ = 1.84 D). The polar S-O and S=O dipole moments get canceled uniformly in the symmetrical tetrahedral shape of SO42- so it is overall non-polar (net µ = 0). SO32- is also made up of polar S-O and S=O bonds, but their dipole moments do not get canceled in the trigonal pyramidal shape of the molecular ion; it is thus overall polar (net µ > 0).

Which of the following is more stable, BeSO4, MgSO4, CaSO4, or BaSO4?

The stability increases as the basic nature of the metal increases. In the case of alkaline earth metals (Group II A), the size of metal cations increases down the group. Therefore, its polarization effect on the big SO42- ion increases, making the sulfate salt more stable overall. Hence, BaSO4 is the most stable sulfate salt out of all four mentioned above.

Is there a formal charge on the bonded atoms in the SO42- molecule?

Formal charge on an atom = [ valence electrons – non-bonding electrons- ½ (bonding electrons)] For the central S-atom Valence electrons = 6 Bonding electrons = 12 Non-bonding electrons = 0 ∴ Formal charge on the central S-atom = 6 – 0 – 12/2 = 6 – 0 – 6 = 6 – 6 = 0 For double-bonded O-atoms Valence electrons = 6 Bonding electrons = 4 Non-bonding electrons = 4 ∴ Formal charge on each double bonded O-atom = 6 – 4 – 4/2 = 6 – 4 – 2 = 6 – 6 = 0 For single-bonded O-atoms Valence electrons = 6 Bonding electrons = 2 Non-bonding electrons = 6 ∴ Formal charge on each single bonded O-atom = 6 – 6 – 2/2 = 6 – 6 – 1 = 6 – 7 = -1 Zero formal charges are present on the central S-atom and on each of the two double-bonded O-atoms. While a -1 formal charge is present on each single bonded O-atom. Hence, the overall charge present on the sulfate (SO42-) ion is (0) + (0) + (0) + 2 (-1) = -2.

Summary

• Sulfate (SO₄^2-) is a non-polar molecular ion.
• It consists of polar bonds, including two S=O and two S-O bonds, due to an electronegativity difference of 0.86 units between the bonded S-atom (E.N = 2.58) and O-atom (E.N = 3.44).
• SO₄^2- has a symmetrical tetrahedral molecular and electron pair geometry with an ideal bond angle of ∠(O-S=O) = 109.5°.
• The charged electron cloud stays uniformly distributed in the symmetrical tetrahedral shape of the SO₄^2-.
• Due to equal dipole moment values of S-O and S=O bonds, the dipole moment of an upwards-pointing S=O bond gets canceled equally with the net dipole of three downwards-pointing polar bonds (two S-O bonds and S=O bonds).
• The net dipole moment in sulfate (SO₄^2-) is thus zero (net µ = 0); hence it is overall a non-polar molecular ion.