Selenium oxychloride is an inorganic compound. It is a colorless or light-yellow colored liquid and is denser than water.
SeOCl2 is a Selenium analog of thionyl chloride.
It is a corrosive chemical and can be used as chlorinating agent.
In this article, we will draw Lewis Structure, predict geometry, hybridization, and polarity of a given compound and understand the basic concepts required for the same.
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Lewis Structure
Many attempts were made to explain the formation of the chemical bond. Lewis was among the first ones to explain. He postulated that only valence electrons take part in bonding, and hence in his representation, only valence electrons are considered.
Lewis structure is a structural formula for representing the arrangement of valence shell electrons around each atom in a molecule. These electrons are represented by dots around the chemical symbol of the respective elements.
Some compounds can have more than one possible lewis dot structure. In such cases, the one that satisfies the octet rule and has the lowest number of formal charges is the most stable.
Octet Rule
In the main group, group 18 elements are considered inert and stable. They have 8 electrons in their valence shell, except He.
All other elements prefer to have a valence shell configuration similar to the nearest member of group 18 to attain stability. All elements form bonds to have a stable configuration. The preference of elements to have 8 electrons in their valence shell gives rise to the octet rule.
It is one of the primitive concepts. Some compounds can never follow the octet rule and are still stable. Exceptions to octet rule are-
• Odd electron species can never complete their octet because the sum of electrons is always odd. For example, NO, NO2, etc.
• Hypo valent Species– some species have less than 8 electrons in their valence shell and are still stable. For example, B in BH3, Al in AlCl3, etc.
• Hypervalent Species– some species have more than 8 electrons in their valence shell and are still stable. For example, S in SF6, etc.
Formal Charge
It compares the number of valence electrons in an isolated neutral atom with the number of valence electrons in the bonded form in a molecule. It is calculated using the given formula
Formal charge= (number of electrons in an isolated neutral atom)- (number of nonbonding electrons on an atom in the compound)- 0.5*(number of electrons shared in bonds by atom)
The most stable Lewis structure from the aspect of formal charges is when formal charges are zero or close to zero.
If a formal charge is present, a negative formal charge should be on more electronegative elements, while a positive formal charge should be on the less electronegative element.
If opposite formal charges are present on adjacent bonds, a bond can be formed by replacing the formal charges.
Steps to Draw Lewis Structure of SeOCl2
1. Count the total number of valence shell electrons on the compound
Before drawing the structure, we need to know the number of valence shell electrons on all constituent atoms and their sum.
| Atom | Atomic Number | Group Number | Valence electrons according to group number | Electronic configuration (E.C.) | Valence shell from E.C. | Valence electrons from E.C. |
|---|---|---|---|---|---|---|
| One Se | 34 | 16 | 6 | 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p4 | n=4 | 6 |
| Two Cl | 17 | 17 | 7 | 1s2 2s2 2p6 3s2 3p5 | n=3 | 7 |
| One O | 8 | 16 | 6 | 1s2 2s2 2p4 | n=2 | 6 |
| Total number of valence shell electrons= 6 + (7*2) +6= 26 | ||||||
2. Draw the lewis dot structure for elements.
We draw the Lewis structure of elements by arranging the valence shell electrons around the elements’ respective chemical symbols.
The chemical symbols for Selenium, chlorine and oxygen Se, Cl and O, respectively. The Lewis dot structure for Se, Cl, and O are as follows-
3. Choose a suitable central atom for the compound.
The central atom is supposed to be the least electronegative one out of the constituent atoms, as the central atom is supposed to share its electron density with all other atoms.
If the central atom is electronegative like Cl or O, it will keep the electron density towards itself.
Thus, Se is the central atom for this compound.
4. Draw a skeletal diagram.
In this step, we have to arrange the side atoms and central atoms suitably.
5. Arrange the valence electrons around the elemental symbols.
The total valence shell electrons (calculated in step 1) are placed around the symbol of chemical elements.
6. Complete the octet of atoms by forming bonds.
All side atoms form a single bond with the central atom to complete their octet. There are two possible structures
Each Cl has seven valence electrons in the isolated state. They share one electron with Se to have a fully filled valence shell configuration.
O had six valence electrons in the isolated state. It got two electrons from Se, and the octet is complete.
Se had six valence electrons in the isolated state. It shared electrons with Cl and gave to O, and the octet is complete
7. Calculate the formal charge on all atoms.
The net charge on this compound is zero. Therefore, the sum of formal charge on seven atoms should come out to be zero.
| Atom | Total number of valence electrons in a free atom | Total number of nonbonding electrons | (Total number of bonding electrons)*0.5 | Formal Charge |
|---|---|---|---|---|
| Se | 6 | 2 | 6*0.5=3 | 6-2-3=+1 |
| Cl1 | 7 | 6 | 2*0.5=1 | 7-6-1=0 |
| Cl2 | 7 | 6 | 2*0.5=1 | 7-6-1=0 |
| O | 6 | 6 | 2*0.5=1 | 6-6-1=-1 |
In the Lewis structure, opposite charges are present on the adjacent atom. A double bond can replace these charges, and the formal charge would become zero.
This requires an expansion of the octet on the central atom. Se can expand because it has vacant d orbitals.
SeOCl2 Geometry
Molecular geometry is the 3D arrangement of atoms of bonds.
Lewis structure is a 2D representation and cannot tell us about the 3D arrangement of atoms. Thus, we needed another theory for predicting geometry.
VSEPR theory (Valence shell electron pair repulsion theory) predicts geometry.
According to VSEPR theory-
• The valence electron pairs repel each other, and this leads to instability.
• To make the arrangement of the electrons stable, the repulsions between them have to be decreased.
• As a result, electrons align themselves so that the repulsion is the least, and the distance between them is maximum.
• The stable arrangement of the valence electron pairs of atoms helps determine the molecular geometry.
Predict Geometry of SeOCl2: VSEPR Chart
We know from the Lewis structure that there are 3 sigma bonds, 1 pi bond, and one lone pair on the central atom.
For predicting geometry, we need to know the central atom (A), number of side atoms(X), and number of lone pairs(E).
Central Atom is Se.
X=3
E=1
The compound is of the type AX3E1
From the table, we can find the geometry and shape using this empirical formula.
Electron geometry is tetrahedral and molecular shape is trigonal pyramidal. Geometry is not the same as the shape of a compound with lone pairs.
For more understanding of VSEPR theory. I have attached a video below, Have a look.
SeOCl2 Hybridization
Hybridization is one of the fundamental concepts used to explain bond formation when VBT fails.
Hybridization involves mixing two or more atomic orbitals with similar energy, size, and shape to form equivalent orbitals. The hybridized orbitals form bonds, and the extent of overlap is better than unhybridized orbitals.
It does not involve literal mixing. Just mixing of wavefunctions takes place.
For example, one 4s and one 4p can form two sp hybrid orbitals, but 1s and 5p cannot. 
Below is the image of orbital overlapping in the SeOCl2 molecule.
In SeF6, Se is the central atom.
In the ground state, Se has 2 unpaired electrons. It can only form two bonds.
Promotion of electrons occurs, and out of 6 valence electrons,4 become unpaired along with one lone pair in 4s.
These 4 electrons are present in different orbitals.
The 4s and three 4p orbitals undergo hybridization to form sp3 hybrid orbitals.
One of the hybrid orbitals has a lone pair. Two orbitals form a sigma bond with Cl and one with O. The unpaired electron in d orbital forms a pi bond with O.
Another method for predicting hybridization
The table below can predict hybridization using total electron pairs or steric numbers. Steric number = number of (sigma bonds +lone pair on central atom)
Steric number for SeOCl2= (3+1)=4
Hybridization comes out to be sp3from the table.
Also, Take a look at the below topics.
NH3 Lewis Structure, Geometry, Hybridization, and Polarity
PCl3 Lewis Structure, Geometry, Hybridization, and Polarity
Cl2 Lewis Structure, Geometry, Hybridization, and Polarity
XeO4 Lewis Structure, Geometry, Hybridization, and Polarity
XeOF4 Lewis Structure, Geometry, Hybridization, and Polarity
SeOCl2 Polarity
The polarity of a compound depends on the presence or absence of net dipole moment. The net dipole moment of a compound depends on
• Dipole moment of the bond
• Difference in electronegativity
• Geometry
There are two types of bonds in the compound, S=O, and S-Cl. No two elements have the same electronegativity. This implies that all the bonds are polar and the bond dipole moment is non-zero.
In trigonal pyramidal geometry, the dipole moment of bonds will not be canceled, so SeOCl2 is a polar compound.
NH3 (Ammonia) also has a similar structure and shape. You can read the article on the polarity of NH3.
Conclusion
The Lewis structure predicted for SeOCl2 has a minimum formal charge.
The geometry is tetrahedral, and the shape is trigonal pyramidal.
The hybridization of the central atom is sp3.
It is a non-polar compound.
Happy Reading!
