OCS normally written as COS is the chemical formula of Carbonyl Sulfide, a well-known and most abundant stratospheric gas. It plays a significant role in the sulfur cycle to affect life on land, air, and water. However, sulfur is a toxic element for human beings and animals and can cause sudden death due to acute toxicity.
Do you know carbonyl sulfide is a common gas on planet Venus? Due to this, scientists believe in life on Venus, though it is not a clear sign.
Back on earth, as carbonyl sulfide is the main source of sulfur here, it becomes crucial to understand the molecular behavior of this chemical compound.
The Lewis structure is a pictorial representation of electrons participating in the bond formation to form new compounds with new chemical properties altogether.
Hence, it is the best place to begin studying the behavioral properties of carbonyl sulfide.
Lewis Structure of Carbonyl Sulfide (COS)
The Lewis structure is drawn by keeping the symbol of an element at the center and drawing valence electrons in pairs around it.
The valence electrons are present in the outermost shell of the atom and participate in the bond formation.
The explanation of this lies in the structure of an atom where the nucleus is present at the center and electrons revolve around it in their orbits.
The nucleus exerts a force of attraction which helps electrons with showcasing their natural properties without jumping in other orbits.
By this logic, the farther the orbit is weaker will be the force of attraction by the nucleus. Due to this, electrons in the outermost shell do not feel the nucleus’s force of attraction and readily bond with another element available nearby.
The maximum number of valence electrons an atom can have is eight.
To begin with the Lewis structure of carbonyl sulfide, first, we need to study the same for all the participating elements.
Carbon has four valence electrons and the atomic number six.
Oxygen has six valence electrons and atomic number eight.
Sulfur has 6 valence electrons and the atomic number 16.
Steps to Draw the Lewis structure of carbonyl sulfide
Step 1: Determine the valence electrons for each participating atom: It is four for carbon and six for both oxygen as well as sulfur.
Step 2: Determine the total number of valence electrons available to draw the Lewis structure of carbonyl sulfide: It is 16 for one OCS molecule.
Step 3: Determine the number of valence electrons further needed to stabilize one carbonyl sulfide molecule: It is 8 as the total number of valence electrons required is 24.
Step 4: Determine the bond formation taking place between the participating atoms: Double bond will form as oxygen and sulfur will bond with carbon only for two valence electrons each.
Step 5: Search for the central atom: It will be carbon as it has the lowest electronegativity value out of all the three participating atoms.
Step 6: Collate all the aforementioned points and draw the Lewis structure of carbonyl sulfide:
Why are double bonds formed in carbonyl sulfide?
It is clear from the structure that both oxygen and sulfur atoms need two valence electrons to complete their octet. As carbonyl sulfide is a covalent compound, donation of valence electrons is not possible.
Hence, sharing of valence electrons takes place among all the participating atoms to achieve a stable condition.
Both single bond and triple bond shares the odd number of valence electrons where its only double bond with which the molecule has achieved a stable condition.
Why must the central atom in a molecule have the lowest electronegativity?
It is so because the lower the electronegativity value, the higher will be the tendency of sharing the electrons.
Low electronegativity is essential for the central atom as it only then will share most of its valence electrons.
Otherwise, the central atom will not share its valence electrons and the formation of a new molecule will not take place.
Molecular Geometry of Carbonyl Sulfide (OCS)
From the Lewis structure, it is clear that the molecular geometry of carbonyl sulfide is linear as all three participating elements are arranged at 180° from one another.
Moreover, the molecular geometry of this molecule can be studied in detail with the help of the Valence Shell Electron Pair Repulsion (VSEPR) Theory.
The bond length between oxygen-carbon is 115.78 pm and sulfur-carbon is 156.01pm.
The double bonds between oxygen-carbon and sulfur-carbon as well as the equal number of lone pairs of electrons on both sulfur and oxygen atoms exert an almost similar force of repulsion to give a symmetrical structure to the carbonyl sulfide molecule.
The molecular geometry can further be studied through the below-mentioned structural representation:
From the above-mentioned, it may confuse you that even after having a lone pair of electrons, the molecular structure of carbonyl sulfide is linear.
It is because of the fact that there is an equal number of lone pairs of valence electrons on oxygen and sulfur which cancel out the overall effect making the structure symmetric.
Hence, the carbonyl sulfide starts behaving as a linear molecule. If there would have been unequal lone pairs of valence electrons, the structure of carbonyl sulfide would be trigonal planar.
Hybridization in Carbonyl Sulfide (OCS)
Being a linear molecule, carbonyl sulfide has sp hybridization. It may sound like an anomaly to many because the carbonyl group is sp2 hybridized.
Here, it is important to realize that usually, the carbonyl group forms a tetrahedral shape with a bond angle of 120° which is not the case with carbonyl sulfide as it has linear molecular geometry.
In carbonyl sulfide, sp hybridization occurs because three participating elements are arranged in linear fashion due to which one s and one p orbital of the same shell within an atom mixes to produce two new orbitals of equal energy.
A double bond consists of one sigma bond and one pi bond which makes it a stronger bond in comparison to others as both sigma and pi bonds exert their effect on the bond equally.
Though we know, sigma bonds are much stronger than pi, it is because of them the molecule exerts different chemical properties.
Hybridization is a mathematical procedure of determining the reason behind bond formation taking place among the participating atoms with a molecule.
It studies intermixing of different atomic orbitals with an atom that leads to the formation of new atomic orbitals of similar energy.
It goes a further step in the form of a molecular orbital diagram where participating atomic orbitals can be studied in detail.
Polarity in Carbonyl Sulfide (OCS)
Polarity is the separation of electrical charges that makes cationic and anionic ends within an atom. It is this property that makes an atom either accept or share its valence electrons.
To determine the polarity in any element, first, it is essential to find the electronegativity values of all the participating atoms.
Electronegativity values of:
The difference between the central atom and other participating atoms must be greater than 0.4 to become polar in nature.
In case, if the difference is lower than 0.4 the bond becomes nonpolar in nature.
Now, let’s calculate for carbonyl sulfide (COS)
Oxygen – Carbon = 0.89
As the electronegativity difference is greater than 0.4, the C=O bond becomes polar making the entire carbonyl sulfide molecule polar in nature.
Another explanation for the polar nature of carbonyl sulfide lies in the fact that the net dipole moment on this molecule is not zero.
The carbon-sulfur and carbon-oxygen bonds do not cancel each other’s effect because of the large difference in their electronegativity values. Due to this, a dipole cloud exists on the carbonyl sulfide molecule making it polar.
Carbonyl sulfide (OCS or COS) is a linear molecule with an anomaly of not showing the true carbonyl group nature. Being the simplest of all carbonyl groups it prefers to be simple and possess chemical properties with no exceptions involved.
Carbonyl sulfide is made of three elements arranged in linear formation attached through the double bond with the central atom. The overall structure is symmetrical hence the equal force of repulsion is exerted by the elements present on both ends.
Even though linear molecule tends to show nonpolar characteristics, carbonyl sulfide is polar because of a larger difference in the electronegativity values of the participating atoms that creates a dipole cloud on the molecule. Due to the symmetrical linear structure, carbonyl sulfide is sp hybridized and has equal s and p characteristics.