Sulfate [SO4]2- ion Lewis dot structure, molecular geometry or shape, electron geometry, bond angle, formal charge, hybridization
Anyone who has even the slightest knowledge of chemistry must have at least once heard about sulfuric acid. And if you know about sulfuric acid then it is a must for you to be familiar with sulfuric acid salts containing the sulfate [SO4]2- ion. It is formed by the loss of two hydrogen ions by sulfuric acid.
In this article, we will discuss everything you need to know about the sulfate [SO4]2- ion such as how to draw its Lewis dot structure, what is its molecular geometry or shape, electron geometry, bond angle, hybridization, formal charges and/or whether it is polar or non-polar in nature.
So, for all this information and much more, continue reading!
The Lewis structure of a sulfate [SO4]2- ion consists of 1 sulfur (S) atom and 4 atoms of oxygen (O). The sulfur atom is present at the center of the Lewis structure while the oxygen atoms occupy terminal positions.
There are a total of 4 electron density regions around the central S atom in the Lewis structure of [SO4]2-. All the electron density regions are constituted of bond pairs which denotes there is no lone pair of electrons on the central S-atom in [SO4]2-.
Drawing the Lewis dot structure of sulfate [SO4]2- is quite easy. You can do so by following the guidelines given below.
Steps for drawing the Lewis dot structure of [SO4]2-
1. Count the total valence electrons in [SO4]2-
The Lewis dot structure of a molecule is referred to as a simplified representation of all the valence electrons present in it. Therefore, the very first step while drawing the Lewis structure of [SO4]2- is to count the total valence electrons present in the concerned elemental atoms.
There are two different elements present in the sulfate [SO4]2- ion i.e., the sulfur (S) and the Oxygen (O). If you look at the Periodic Table of elements, you will readily identify that both Sulfur (S) and Oxygen (O) lie in Group VI A. Thus, a total of 6 valence electrons are present in each atom of S and O.
The [SO4]2- ion consists of 1 S-atom and 4 O-atoms. Thus, the valence electrons in the Lewis dot structure of [SO4]2- = 1(6) + 4(6) = 30 valence electrons.
The twist here is that the sulfate [SO4]2- ion carries a negative (-2) charge which means 2 extra valence electrons are added in this Lewis structure.
∴ Hence, the total valence electrons available for drawing the Lewis dot structure of [SO4]2- = 30+2 = 32 valence electrons.
2. Choose the central atom
Electronegativity is defined as the ability of an atom to attract a shared pair of electrons from a covalent chemical bond. So, the atom which is least electronegative or most electropositive is placed at the center of a Lewis structure. This is because this atom is most likely to share its electrons with the more electronegative atoms surrounding it.
As Sulfur (S) is less electronegative than oxygen (O) so, an S-atom is placed at the center of the [SO4]2- Lewis structure while the four O-atoms are placed in its surroundings, as shown below.
3. Connect outer atoms with the central atom
At this step of drawing the Lewis structure of a molecule, we need to connect the outer atoms with the central atom using single straight lines. As the O-atoms are the outer atoms in the Lewis structure of the sulfate [SO4]2- ion so all 4 oxygen atoms are joined to the central S-atom using straight lines, as shown below.
Each straight line represents a single covalent bond containing 2 electrons.
Now, if we count the total valence electrons used till this step out of the 32 available initially, there are a total of 4 single bonds in the structure above. Thus, 4(2) = 8 valence electrons are used till step 3.
Total valence electrons available – electrons used till step 3 = 32-8 = 24 valence electrons.
This means we still have 24 valence electrons to be accommodated in the Lewis dot structure of [SO4]2-.
4. Complete the octet of outer atoms
There are four O-atoms present as outer atoms in the Lewis structure of [SO4]2-. Each O-atom needs a total of 8 valence electrons in order to achieve a stable octet electronic configuration.
Each S-O bond already represents 2 electrons therefore all the four O-atoms require 6 more electrons each to complete their octet. Thus, these 6 valence electrons are placed as 3 lone pairs on each O-atom, as shown below.
5. Complete the octet of the central atom
Total valence electrons used till step 4 = 4 single bonds + 4 (electrons placed around O-atom, shown as dots) = 4(2) +4(6) = 32 valence electrons.
Total valence electrons available – electrons used till step 4 = 32-32 = 0 valence electrons.
As all the valence electrons initially counted are already used up, therefore, there is no lone pair on the central S-atom in the [SO4]2- Lewis structure.
Also, the central S-atom has a total of four single bonds around it which means it already has a complete octet. So we don’t need to change anything in the Lewis structure of [SO4]2- drawn so far.
But we need to check the stability of this structure using the formal charge concept.
6. Check the stability of the SO42- Lewis structure using the formal charge concept
The less the formal charge on the atoms of a molecule, the better the stability of its Lewis structure.
The formal charge can be calculated using the formula given below.
This calculation shows that a +2 formal charge is present on the central Sulfur atom and a -1 formal charge is present on each of the four oxygen (O) atoms.
But as we already mentioned; the less the formal charges on the bonded atoms, the greater the stability of a Lewis structure.
Thus, we can reduce the formal charges present on the S and O-atoms by converting the lone pair of electrons present on terminal O-atoms into covalent bonds between the central S atom and the terminal O-atoms.
Let us see how that’s done.
7. Minimize the formal charges on atoms by converting lone pairs into covalent bonds
One lone pair from any two terminal oxygen atoms is converted into a covalent bond between the central S-atom and the respective O-atom as shown below.
Now there are a total of 2 double bonds and 2 single bonds around the central S-atom. We can again check the stability of this Lewis structure using the formal charge formula.
So here, you can see that the formal charges present on the central sulfur (S) atom and two terminal oxygen (O) atoms are reduced to zero. However, there is a -1 formal charge on each of the other two oxygen atoms.
-1 + -1 = -2 which accounts for an overall negative 2 charge on the sulfate [SO4]2- ion. This ensures that it is a correct and stable Lewis representation for the sulfate [SO4]2- ion. The [SO4]2- Lewis structure is enclosed in square brackets and a negative 2 charge is placed at the top right corner, as shown below.
If you are worried about the extra 4 electrons in the sulfur atom which means more than the octet number of electrons. Then, that’s not a problem because sulfur (S) has an expanded octet. It can accommodate more than 8 valence electrons during chemical bond formation due to the presence of 3d orbitals in its atomic structure.
Another interesting fact to keep in mind is that the actual structure of a sulfate [SO4]2- ion is a hybrid of the following resonance structures. Each resonance structure is a way of representing the Lewis structure of a molecule or an ion.
These resonance structures show that the formal charges present on [SO4]2- atoms are not stationary, rather they keep moving from one position to another. Similarly, a double bond can be formed between the central sulfur and any two oxygen atoms out of all four available.
In conclusion, all the above resonance structures contribute equally to the resonance hybrid which is the best possible Lewis structure of the sulfate [SO4]2- ion.
What are the electron and molecular geometry of SO42-?
The sulfate [SO4]2- ion has an identical electron and molecular geometry or shape i.e., tetrahedral. The four O-atoms bonded to the central S-atom lie in the same plane, in a symmetrical arrangement, along the four vertices of a tetrahedron. No lone pair of electrons is present on the central S-atom in [SO4]2- thus there is no distortion present in the geometry or shape of the molecular ion.
Molecular geometry of [SO4]2-
The sulfate [SO4]2- ion has a tetrahedral molecular geometry or shape. The four oxygen atoms lie at the four vertices of a regular tetrahedron while the sulfur atom is present at the center, refer to the figure below.
S-O and S=O bond pair-bond pair repulsions exist in the molecule which keeps the terminal O-atoms as far apart from one another as possible. However, there is no lone pair of electrons on the central S-atom therefore no lone pair-bond pair and lone pair-lone pair electronic repulsions are present in the molecule. The shape of the molecule thus stays intact, identical to its ideal electron pair geometry.
Electron geometry of [SO4]2-
According to the valence shell electron pair repulsion (VSEPR) theory of chemical bonding, the ideal electron geometry of a molecule or a molecular ion containing a total of 4 electron density regions around the central atom is tetrahedral.
In the sulfate [SO4]2- ion, there are 2 single bonds and 2 double bonds around the central sulfur atom which makes a total of 4 electron density regions. Thus, its electron geometry is also tetrahedral.
An easy way to find the shape and geometry of the molecule is to use the AXN method.
AXN is a simple formula to represent the number of atoms bonded to the central atom in a molecule and the number of lone pairs present on it. It is used to predict the geometry or shape of a molecule using the VSEPR concept. AXN notation for [SO4]2- molecular ion
A in the AXN formula represents the central atom. In the [SO4]2- ion, sulfur is present at the center so A = Sulfur.
X denotes the atoms bonded to the central atom. In [SO4]2-, four oxygen (O) atoms are bonded to the central S so X=4.
N stands for the lone pairs present on the central atom. As per the Lewis structure of [SO4]2- there is no lone pair on central sulfur so N=0.
So, the AXN generic formula for the [SO4]2- ion is AX4.
Now, you may have a look at the VSEPR chart below.
The VSEPR chart reaffirms that the ideal electron geometry and molecular geometry or shape of a molecule with AX4 generic formula are identical i.e., tetrahedral, as we already noted down for the [SO4]2- ion.
Hybridization of [SO4]2-
The sulfate [SO4]2- ion has sp3 hybridization.
The electronic configuration of a sulfur (S) atom is 1s2 2s2 2p6 3s2 3p4.
The electronic configuration of oxygen (O) atom is 1s2 2s2 2p4.
During chemical bonding, the 3s electrons of sulfur get unpaired and one of the two electrons shifts to the empty 3d atomic orbital. Similarly, the paired 3p electron gets unpaired and one of the two electrons gets excited to another empty 3d orbital of sulfur.
In consequence, the half-filled 3s atomic orbital of sulfur hybridizes with three half-filled 3p orbitals to produce four sp3 hybrid orbitals of equal energy. Each sp3 hybrid orbital has a 75% p- character and a 25% s-character and each contains a single electron only.
Two sp3 hybrid orbitals form the required sigma (σ) bonds with the p-orbitals of oxygen atoms by an sp3-p overlap in S=O bonds in the [SO4]2- molecular ion. The other two sp3 hybrid orbitals form S-O sigma (σ) bonds with sp2 hybrid orbitals of oxygen atoms by sp3-sp2 overlap.
However, the unhybridized d-orbitals of sulfur form the pi (π) bonds in S=O double bonds by overlapping with the p-orbitals of the concerned oxygen atoms.
Refer to the figure below for understanding these concepts more clearly.
A shortcut to finding the hybridization present in a molecule or a molecular ion is by using its steric number against the table given below. The steric number of central S in [SO4]2- is 4 so it has sp3 hybridization.
Steric number
Hybridization
2
sp
3
sp2
4
sp3
5
sp3d
6
sp3d2
The steric number of central Sulfur in SO42- is 4 so it has sp3 hybridization.
The SO42- bond angle
The bonded atoms in [SO4]2- ion form ideal bond angles as expected in a symmetrical tetrahedral molecule. The O-S-O bond angle is 109.5°. Each S-O bond length in the [SO4]2- ion is equivalent i.e., 149 pm.
Although a S=O double bond is expected to be stronger and shorter in length than an S-O single bond. But, it is due to the resonance present in the molecule that each S-O bond length is equal in the [SO4]2- ion.
Each S-O bond present in the [SO4]2- ion is polar due to an electronegativity difference of 0.86 units between the covalently bonded sulfur (E.N = 2.58) and oxygen (E.N = 3.44) atoms.
Oxygen more strongly attracts the shared S-O electron cloud as opposed to the sulfur atom. Thus each S-O bond has a specific dipole moment value (symbol µ).
However, the symmetrical tetrahedral shape of [SO4]2- ion cancels the dipole moment effect of S-O bonds. The dipole moment of the upwards-pointing S=O bond gets canceled with the net dipole moment of three downwards-pointing S-O and S=O bonds. Thus, the [SO4]2- ion is overall non-polar with net µ =0.
There are a total of 32 valence electrons = 32/2 = 16 electron pairs in the Lewis structure of SO42-.
Out of these 16 electron pairs, there are 6 bond pairs and 10 lone pairs.
The central S-atom is bonded to two oxygen (O) atoms via double covalent bonds while it is bonded to two other oxygen (O) atoms via single bonds.
There is no lone pair on the central S-atom. However, 3 lone pairs of electrons are present on each of the single-bonded O-atoms while 2 lone pairs are present on each double-bonded O-atom in the lewis structure of SO42-.
How many valence electrons are in SO42- lewis structure?
Total number of valence electrons in Sulfur = 6
Total number of valence electrons in Oxygen = 6
The [SO4]2- ion consists of 1 S-atom, 4 O-atoms, and a (-2) charge. Thus, the valence electrons in the Lewis dot structure of [SO4]2- = 1(6) + 4(6) + 2 = 32 valence electrons.
How do you find the molecular geometry of SO42-?
The molecular geometry of SO42- can be determined using the VSEPR concept. The AXN generic formula for SO42- ion is AX4 thus it has a tetrahedral molecular geometry and shape.
What is the bond angle of SO42-?
The bond angle of the SO42- ion is 109.5° because of its tetrahedral shape and geometry. Although a S=O double bond should be shorter than an S-O single bond so the O=S-O bond angle should be different from the O-S-O bond angle.
But it is due to the resonance present in the sulfate [SO4]2- ion that all the S-O bond lengths and O-S-O bond angles are equal i.e., 149 pm and 109.5° respectively.
The total valence electrons available for drawing sulfate [SO4]2- ion Lewis structure are 32.
The negative 2 charge present on the sulfate ion accounts for 2 extra valence electrons in its Lewis structure.
The [SO4]2- ion has an identical electron geometry and molecular geometry or shape i.e., tetrahedral.
The O-S-O bond angle is 109.5° while the S-O bond lengths are 149 pm in [SO4]2-
It is due to the resonance present in the sulfate [SO4]2- ion that each S-O bond length is equivalent as opposed to two shorter S=O bonds and two longer S-O bonds, as expected.
The central S-atom in [SO4]2- ion is sp3.
The [SO4]2- ion is non-polar in nature. The symmetry present in the molecular ion cancels the dipole moment effect of individually polar S-O bonds. Thus, the electron cloud stays uniformly distributed overall (net µ=0).
-1 formal charge is present on each singly bonded O-atom in the [SO4]2- Lewis structure which makes the molecular ion occupy an overall charge of -2.
In conclusion, [SO4]2- is a polyatomic dianion having a double negative charge.
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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S-O and S=O bond pair-bond pair repulsions exist in the molecule which keeps the terminal O-atoms as far apart from one another as possible. However, there is no lone pair of electrons on the central S-atom therefore no lone pair-bond pair and lone pair-lone pair electronic repulsions are present in the molecule. The shape of the molecule thus stays intact, identical to its ideal electron pair geometry.
An easy way to find the shape and geometry of the molecule is to use the AXN method.
AXN is a simple formula to represent the number of atoms bonded to the central atom in a molecule and the number of lone pairs present on it.
It is used to predict the geometry or shape of a molecule using the VSEPR concept.
AXN notation for [SO4]2- molecular ion
The VSEPR chart reaffirms that the ideal electron geometry and molecular geometry or shape of a molecule with AX4 generic formula are identical i.e., tetrahedral, as we already noted down for the [SO4]2- ion.
A shortcut to finding the hybridization present in a molecule or a molecular ion is by using its steric number against the table given below. The steric number of central S in [SO4]2- is 4 so it has sp3 hybridization.
The steric number of central Sulfur in SO42- is 4 so it has sp3 hybridization.
