Carbonyl sulfide (OCS) Lewis structure, molecular geometry or shape, electron geometry, bond angle, hybridization, formal charges, polar or nonpolar
OCS or COS is the chemical formula for carbonyl sulfide, a colorless gas that emits a typical, unpleasant, sulfur-like odor. It is used as an intermediate in synthesizing useful organic compounds, including herbicides.
This article is very interesting as we have taught you how to draw the Lewis dot structure of carbonyl sulfide (OCS) in it. You will also learn about molecular geometry or shape, electron geometry, bond angle, hybridization, formal charges, and polarity of OCS.
The Lewis structure of carbonyl sulfide (OCS) consists of a carbon (C) atom at the center. It is double-covalently bonded to an oxygen (O) atom and a sulfur (S) atom at the sides. The central C-atom has no lone pairs of electrons, while 2 lone pairs are present on each of the O and S atoms.
You can easily draw the Lewis dot structure of OCS if you follow the simple steps given below.
Steps for drawing the Lewis dot structure of OCS
1. Count the total valence electrons present in OCS
The three distinct elements present in OCS are carbon, oxygen and sulfur.
Carbon (C) is present in Group IV A (or 14) of the Periodic Table, containing a total of 4 valence electrons.
In contrast, both oxygen (O) and sulfur (S) lie in Group VI A (or 16), having a total of 6 valence electrons in each atom.
Total number of valence electrons in carbon = 4
Total number of valence electrons in oxygen = 6
Total number of valence electrons in sulfur = 6
The OCS molecule comprises 1 C-atom, 1 O-atom and 1 S-atom.
∴ Therefore, the total valence electrons available for drawing the Lewis dot structure of OCS = 1(4) + 1(6) + 1 (6) = 16 valence electrons.
2. Find the least electronegative atom and place it at the center
By convention, the least electronegative atom out of all those available is chosen as the central atom while drawing the Lewis structure of a molecule.
The least electronegative atom can easily form covalent bonds with other atoms by sharing its electrons.
Out of the three types of atoms present in OCS, carbon (E.N = 2.55) is less electronegative than both oxygen (E.N = 3.44) and sulfur (E.N = 2.58).
Therefore, the C-atom is placed at the center of the OCS Lewis structure while the O-atom and the S-atom occupy terminal positions, as shown below.
3. Connect the outer atoms with the central atom
In this step, the O-atom and the S-atom are joined to the central C-atom using single straight lines.
A straight line represents a single covalent bond, i.e., a bond pair containing 2 electrons.
In the above structure, there are 2 single bonds, i.e., 2(2) = 4 valence electrons are already consumed out of the 16 initially available.
Now let’s explore where we can place the remaining 12 valence electrons.
4. Complete the octet of the outer atoms
Both the outer atoms, i.e., the O-atom and the S-atom, need a total of 8 valence electrons in order to gain a full octet shell.
Both the outer atoms already have 2 valence electrons in the Lewis structure obtained till this step.
Thus, 6 more valence electrons are placed around each outer atom as 3 lone pairs, as shown below.
5. Complete the octet of the central atom and convert lone pairs into covalent bonds if necessary
Total valence electrons used till step 4 = 2 single bonds + 2 (electrons placed around each outer atom, shown as dots) = 2(2) + 2(6) = 16 valence electrons.
Total valence electrons – electrons used till step 4 = 16 – 16 = 0 valence electrons.
Hence, as all the valence electrons initially available for drawing the OCS Lewis structure are already consumed, thus there is no lone pair on the central C-atom.
But a problem here is that this C-atom still has an incomplete octet with only 4 valence electrons surrounding it.
But don’t worry because we can easily solve this problem by converting a lone pair from each outer atom into a covalent bond between the central C-atom and the corresponding outer atom.
Now that the central C-atom, as well as both the outer atoms, have a complete octet, we can proceed forward and check the stability of the OCS Lewis structure by applying the formal charge concept.
6. Check the stability of Lewis’s 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 charges can be calculated using the formula given below.
Formal charge = [valence electrons- nonbonding electrons- ½ (bonding electrons)].
Now let us use this formula and the Lewis structure obtained in step 5 to determine the formal charges present on the OCS-bonded atoms.
Zero or no formal charges present on either of the bonded atoms in OCS ensure that it is a stable Lewis structure and that we have drawn it correctly.
However, you may note that the following resonance forms are possible for representing the OCS molecule.
The pi-bonded electrons and lone pairs keep revolving from one position to another on the molecule, resulting in different resonance structures of OCS. Out of all the possible resonance forms, the structure I is the most stable as none of the atoms carry any formal charges in it.
The actual structure of OCS is a weighted average of all the above resonance forms and is known as the resonance hybrid.
What are the electron and molecular geometry of OCS?
Carbonyl sulfide (OCS) possesses an identical electron and molecular geometry or shape, i.e., linear. Two bond pairs surround the central C-atom in OCS, which has no lone pairs of electrons. Therefore, no distortion is witnessed in the shape and geometry of the molecule w.r.t the central atom.
Molecular geometry of OCS
The molecular geometry or shape of carbonyl sulfide (OCS) is linear.
The central C-atom is double-covalently bonded to an O-atom and an S-atom in OCS. There is no lone pair on the central C-atom thus, no lone pair-lone pair or lone pair-bond pair repulsions exist in the molecule, so no distortion is witnessed in its shape and/or geometry.
The bonded atoms form a linear molecular arrangement, as shown below.
Electron geometry of OCS
According to the valence shell electron pair repulsion (VSEPR) theory of chemical bonding, the ideal electron geometry of a molecule containing a total of 2 electron density regions around the central atom is linear.
In OCS, there are 2 double bonds surrounding the central carbon atom, making a total of 2 electron density regions. Hence, its electron geometry is also linear.
An easy trick to finding a molecule’s electron and molecular geometry is using the AXN method.
AXN is a simple formula representing the number of bonded atoms and lone pairs on the central atom.
It is used to predict the shape and geometry of a molecule using the VSEPR concept.
AXN notation for OCS molecule
A in the AXN formula represents the central atom. In the OCS molecule, a carbon (C) atom is present at the center, so A = C.
X denotes the atoms bonded to the central atom. In OCS, an O-atom and an S-atom are directly bonded to the central C-atom, so X= 2.
N stands for the lone pairs present on the central atom. As per the Lewis structure of OCS, the central C-atom has no lone pairs of electrons. Thus, N= 0 for OCS.
As a result, the AXN generic formula for OCS is AX2N0or simply AX2.
Now, you may have a look at the VSEPR chart below.
The VSEPR chart confirms that the molecular geometry or shape of a molecule with an AX2 generic formula is identical to its electron geometry, i.e., linear, as we already noted down for carbonyl sulfide (OCS).
Hybridization of OCS
The central C-atom is sp hybridized in OCS.
The electronic configuration of carbon is 1s2 2s2 2p2.
During chemical bonding, the 2s electrons of carbon get unpaired. One of these 2s electrons shifts to an empty 2p orbital. As a result, the 2s and one 2p atomic orbital of carbon hybridize to produce two sp hybrid orbitals.
Each sp hybrid orbital possesses a 50% s-character and a 50% p-character.
These sp hybrid orbitals form C-O and C-S sigma bonds on either side of the OCS molecule.
In contrast, the unhybridized p-orbitals of carbon form the C=O and C=S pi bonds by p-p orbital overlap.
Another shortcut to finding the hybridization present in a molecule is using its steric number against the table below. The steric number of the C-atom in OCS is 2, so it has sp hybridization.
Steric number
Hybridization
2
sp
3
sp2
4
sp3
5
sp3d
6
sp3d2
The bond angle of OCS
The carbon, oxygen, and sulfur atoms lie on a perfectly straight line in OCS, forming a mutual bond angle of 180°.
The O=C bond length is 115.78 pm, while the C=S bond length is 156.01 pm in the carbonyl sulfide (OCS) molecule.
Is OCS polar or nonpolar?
In the OCS molecule, a high electronegativity difference of 0.89 units exists between an oxygen (E.N = 3.44) and a carbon (E.N = 2.55) atom.
In contrast, an electronegativity difference of only 0.03 units is present between the carbon and sulfur (E.N = 2.58) atoms.
This implies that the O=C bond in OCS is strongly polar while the C=S bond is only weakly polar.
Oxygen, being the most electronegative element out of all three, pulls the O=C electron cloud largely towards itself, gaining a partial negative charge (δ–) while both the C-atom and the S-atom gains partial positive charges (δ+).
As the O=C and C=S dipole moments are unequal so they stay uncancelled even in the planar linear shape.
Consequently, OCS is a polar molecule overall with a non-uniformly spread electron cloud distribution (net µ = 0.71 Debye).
The Lewis dot structure for carbonyl sulfide (OCS) displays a total of 16 valence electrons, i.e., 16/2 = 8 electron pairs.
Out of the 8 electron pairs, there are 4 bond pairs and 4 lone pairs of electrons.
A C-atom at the center is double covalently bonded to an O-atom and an S-atom at the sides.
There is no lone pair of electrons on the central C-atom. However, both terminal atoms carry 2 lone pairs each.
What is the molecular geometry or shape of OCS?
The OCS molecule possesses a linear shape. The bonded atoms lie on a straight line, forming a mutual bond angle of 180°.
Is the molecular shape of OCS the same as its electron geometry?
Yes. The carbonyl sulfide (OCS) molecule possesses an identical electron and molecular geometry or shape, i.e., linear.
There is no lone pair of electrons on the central C-atom thus, no lone pair-lone pair and lone pair-bond pair electronic repulsions are present in it. As a result, the shape and geometry of OCS stay undistorted.
Is the molecular shape of OCS the same as that of CO2?
Yes. Both OCS and CO2 possess an identical linear shape. To a C-atom at the center, two atoms are attached at the terminal positions via double covalent bonds.
Why is the molecular shape of SO2 different from that of OCS?
However, the central S-atom has a lone pair of electrons as well. Lone pair-bond pair repulsions distort the molecular geometry of SO2, and it occupies a bent shape, different from its ideal electron pair geometry, i.e., trigonal planar.
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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