POCl3 Lewis dot structure, molecular geometry or shape, electron geometry, bond angle, formal charges, hybridization, polar vs nonpolar
POCl3 is the chemical formula for phosphorus oxychloride. Its IUPAC name is phosphoryl trichloride. It exists as a colorless, fuming liquid at room temperature and pressure. It is corrosive in nature and its inhalation can prove lethal to human body tissues. POCl3 is an important chemical compound as it is used in hydraulic fuels and as a gasoline additive.
In this article, you will learn how to draw the Lewis structure of POCl3. In addition to that, you will also find interesting facts about POCl3 chemistry such as what is its molecular geometry or shape, electron geometry, bond angle, hybridization, formal charges, polarity, etc.
If you are curious to know all this and much more about a phosphorus oxychloride molecule, then continue reading till the end.
The Lewis structure of POCl3 consists of three different elemental atoms. The phosphorus (P) atom is present at the center of the molecule. It is bonded to an oxygen (O) atom and three atoms of chlorine (Cl), one on each side.
There are a total of 4 electron density regions around the central P-atom in the Lewis structure of POCl3. All four electron density regions are constituted of bond pairs which mean there is no lone pair on the central P-atom in POCl3.
Drawing the Lewis structure of POCl3 is not a difficult task at all. So, grab a paper and pencil and draw this Lewis structure with us following the simple steps given below.
Steps for drawing the Lewis dot structure of POCl3
There are three different types of atoms present in a phosphorus oxychloride (POCl3) molecule. We can find the valence electrons present in each element by identifying them in the Periodic Table.
Phosphorus (P) is present in group V-A of the Periodic Table, so it has a total of 5 valence electrons. Oxygen (O) is situated in group VI A, so it has a total of 6 valence electrons. Conversely, chlorine (Cl) is a halogen present in group VII-A which denotes it has a total of 7 valence electrons.
∴ The POCl3 molecule is made up of 1 phosphorus atom, 1 oxygen atom, and 3 atoms of chlorine. So, the total valence electrons available for drawing the Lewis dot structure of POCl3 = 5+6+3(7) = 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. Thus, the least electronegative atom is most likely to share its electrons with other atoms in its surroundings.
Phosphorus (P) is less electronegative as compared to both oxygen (O) and chlorine (Cl) in the phosphorus oxychloride molecule. Therefore, phosphorus is placed at the center of the Lewis structure of POCl3 while the O and three Cl atoms are spread around it, as shown in the figure below.
3. Connect outer atoms with the central atom
In this step, we need to connect the outer atoms with the central atom of the Lewis structure using single straight lines. As O and Cl atoms are the outer atoms in POCl3 so the O atom as well as the three Cl atoms are joined to the central P atom using straight lines, as shown below.
Each straight line represents a single covalent bond i.e., a bond pair containing 2 electrons. There are a total of 4 straight lines in the above structure which means 4(2) = 8 valence electrons are used so far, out of the 32 initially available.
Total valence electrons available – electrons used till step 3 = 32-8 = 24 valence electrons.
This means we still have 24 valence electrons available to be accommodated in the Lewis dot structure of POCl3.
4. Complete the octet of the outer atoms
One oxygen (O) and three chlorine (Cl) are the outer atoms in POCl3.
All these atoms require a total of 8 valence electrons in order to achieve a stable octet electronic configuration.
The P-O bond at the center represents 2 electrons. Thus, oxygen requires 6 more electrons to complete its octet. These 6 valence electrons are placed as 3 lone pairs around the oxygen atom.
Similarly, the three P-Cl bonds at the sides of the molecule represent 2 electrons each which mean each Cl atom requires 6 more electrons to achieve a stable octet configuration. These 6 electrons are placed as 3 lone pairs around each Cl atom as shown below.
5. Complete the octet of the central atom
Total valence electrons used till step 4 = 4 single bonds + electrons placed around O-atom + 3 (electrons placed around each Cl-atom) = 4(2) + 6 + 3(6) = 32 valence electrons.
Total valence electrons available – electrons used till step 4 = 32 – 32 = 0 valence electrons.
All the valence electrons available are now used so there is no lone pair of electrons on the central P-atom in the POCl3 Lewis structure. Also, this phosphorus atom already has a complete octet with a total of 4 single bonds around it.
But is this structure stable? Let’s check that using the formal charge concept.
6. Check the stability of the 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.
The above calculation shows that although no formal charges are present on any of the three chlorine atoms. However, a +1 and a -1 formal charge is present on the phosphorus and the oxygen atoms respectively in the Lewis structure drawn so far.
But as we told you earlier that the less the formal charge, the greater the stability of a Lewis structure.
So is there any way we can reduce the formal charges present on the bonded P and O atoms in the POCl3 Lewis structure?
Well, there certainly is and that is to convert a lone pair from the outer O-atom into a covalent bond between the central P and outer O-atom.
7. Reduce the formal charges by converting a lone pair into a bond pair and again check the stability of the POCl3 Lewis structure
A lone pair present on the O-atom is converted into a covalent bond between the P and O atoms, as shown below.
In this way, each of the outer O and Cl atoms attains a complete octet with 1 double bond and 2 lone pairs on oxygen and 1 single bond and 3 lone pairs on each chlorine atom respectively.
The central P-atom has a total of 10 valence electrons (1 double bond + 3 single bonds) in this structure. This situation falls under the expanded octet concept. After completely filling the 3p orbitals, phosphorus can accommodate the extra electrons in its 3d orbitals during chemical bonding. In this way, more than 8 valence electrons can be held by the phosphorus atom.
As a final step, we just need to check the formal charges on each atom in the POCl3 Lewis structure again.
For phosphorus atom
Valence electrons of phosphorus = 5
Bonding electrons = 3 single bonds +1 double bond = 3 (2) + 2(2) = 10 electrons
Non-bonding electrons = no lone pair = 0 electrons
Zero formal charges present on each atom in the structure now obtained ensure that it is a stable and correct Lewis representation of the phosphorus oxychloride (POCl3) molecule.
What are the electron and molecular geometry of POCl3?
The POCl3 molecule has an identical electron pair geometry and molecular geometry or shape i.e., tetrahedral. The four atoms bonded to the central phosphorus atom lie in a planar arrangement. There is no lone pair of electrons on the P-atom thus no distortion is present in the geometry and shape of the molecule.
Molecular geometry of POCl3
POCl3 has a tetrahedral molecular geometry or shape. The four bonded atoms lie at the four vertices of a regular tetrahedron while the phosphorus atom is present at the center, refer to the figure below.
P=O and P-Cl bond pair-bond pair electronic repulsions exist in the molecule which keeps the bonded atoms as far apart from one another as possible. However, there is no lone pair of electrons on the central P-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 POCl3
According to the valence shell electron pair repulsion (VSEPR) theory of chemical bonding, the ideal electron geometry of a molecule containing a total of 4 electron density regions around the central atom is tetrahedral.
In POCl3, there are 3 single bonds and 1 double bond around the central phosphorus atom which makes a total of 4 electron density regions. Thus, its electron geometry is also tetrahedral.
A simpler way of finding the electron and the molecular geometry of a molecule or a molecular ion is by using 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 POCl3 molecule
A in the AXN formula represents the central atom. In POCl3, phosphorus is present at the center so A = P.
X denotes the atoms bonded to the central atom. In POCl3, one oxygen (O) atom and three chlorine (Cl) atoms are bonded to the central P so X=4.
N stands for the lone pairs present on the central atom. As per the Lewis structure of POCl3, there is no lone pair on central phosphorus so N=0.
So, the AXN generic formula for POCl3 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 phosphorus oxychloride (POCl3) molecule.
Hybridization of POCl3
The central phosphorus atom has sp3 hybridization in POCl3.
The electronic configuration of a phosphorus (P) atom is 1s22s22p63s23p3.
During chemical bonding, the paired 3s electrons of phosphorus get unpaired and one of these electrons shifts to an empty 3d atomic orbital. Consequently, the 3s orbital hybridizes with three half-filled 3p orbitals to yield four sp3 hybrid orbitals. Each sp3 hybrid orbital has a 25 % s-character and a 75% p-character and contains a single electron only.
One sp3 hybrid orbital forms the required sigma (σ) bond with the sp2 hybrid orbital of an oxygen atom by an sp3-sp2 orbital overlap in the P=O bond in POCl3. The other three sp3 hybrid orbitals form P-Cl sigma (σ) bonds with p orbitals of the three chlorine atoms by sp3-p overlap.
However, the unhybridized d-orbital of phosphorus forms the P=O pi (π) bond by overlapping with the unhybridized p-orbital of the double-bonded oxygen atom.
A shortcut to finding the hybridization present in a molecule is by using its steric number against the table given below. The steric number of central P in POCl3 is 4 so it has sp3 hybridization.
Steric number
Hybridization
2
sp
3
sp2
4
sp3
5
sp3d
6
sp3d2
The bond angles of POCl3
The bonded atoms in POCl3 form ideal bond angles as expected in a symmetrical tetrahedral molecule. The O=P-Cl bond angle is thus 109.8°. However, the Cl-P-Cl bond angle is slightly smaller i.e., 103°.
The P=O bond length is 146 pm while the P-Cl bond length is 198 pm in the POCl3 molecule. The P=O double bond is stronger and thus shorter in length as compared to the P-Cl single bond.
An electronegativity difference of 1.25 units exists between the covalently bonded phosphorus (E.N = 2.19) and oxygen (E.N = 3.44) atoms in the P=O bond. Thus the P=O bond is polar in the POCl3 molecule.
Similarly, each P-Cl bond is also polar with an electronegativity difference of 0.97 units between the bonded phosphorus (E.N = 2.19) and chlorine (E.N = 3.16) atoms respectively.
Although the POCl3 molecule has a planar tetrahedral shape. But the P=O bond is different from three the P-Cl bonds.
The net dipole moment of three downwards-pointing P-Cl bonds is not equal to the dipole moment of the one upwards-pointing P=O bond. Thus these dipole moments do not get canceled equally. Rather, the electron cloud stays non-uniformly distributed in the molecule overall. Hence POCl3 is polar (net µ > 0).
The Lewis structure of POCl3 displays a total of 32 valence electrons i.e., 32/2 =16 electron pairs.
Out of the 16 electron pairs, there are 5 bond pairs and 11 lone pairs of electrons.
The central P-atom is double-bonded to an O-atom and single-bonded to three Cl-atoms respectively.
There is no lone pair of electrons present on the central P-atom. However, 3 lone pairs are present on each Cl-atom while 2 lone pairs are present on the O-atom in the lewis structure of POCl3.
How is the molecular geometry or shape of POCl3 different from its electron geometry?
There is no difference in the electron geometry and molecular geometry or shape of POCl3. This is because there is no lone pair of electrons present on the central P-atom in POCl3.
So, no lone pair-bond pair and lone pair-lone pair electronic repulsions are present in the molecule. Thus, (POCl3) occupies an identical electron and molecular geometry or shapes i.e., tetrahedral.
What kinds of bonds exist between P, O, and Cl in POCl3?
A double covalent bond is present between the P and O atoms while a single covalent bond is present between the P and Cl atoms in POCl3.
What is the bond angle of POCl3?
The ∠O=P-Cl bond angle is 109.8° while the ∠Cl-P-Cl bond angle is 103° in the regular tetrahedral shape of POCl3.
The total number of valence electrons available for drawing the POCl3 Lewis structure is 32.
The POCl3 molecule has an identical electron geometry and molecular geometry or shape i.e., tetrahedral.
The O=P-Cl bond angle is 109.8° while the Cl-P-Cl bond angle is 103° in POCl3.
The P=O bond length is 146 pm while the P-Cl bond length is 198 pm in POCl3.
The POCl3 molecule has sp3 hybridization.
The POCl3 molecule is polar in nature. The electron cloud stays non-uniformly distributed, strongly attracted towards the O-atom in the POCl3 molecule.
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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