Phosphorous trichloride (PCl3) lewis dot structure, molecular geometry, polar or nonpolar, hybridization, Bond angle
Phosphorous trichloride appears as colorless to fuming liquid having chemical formula PCl3. It is an inorganic compound and exists in a liquid state.
In this tutorial, we will discuss Phosphorous trichloride (PCl3) lewis structure, molecular geometry, Bond angle, hybridization, polar or nonpolar, etc.
Phosphorus trichloride is a toxic and very reactive compound. It sometimes also known as Trichlorophosphane.
PCl3 lewis structure is made up of three P-Cl bonds, with a phosphorus (P) atom in a central position and all three chlorine (Cl) as outer atoms in the lewis diagram. The lewis structure of PCl3 contains a total of 3 bond pairs and 10 lone pairs(3 lone pairs on each chlorine atom and 1 on the central atom).
The drawing of the PCl3 lewis’s structure is very easy and simple. Let’s see how to do it.
Steps for drawing the Lewis dot structure for PCl3
1. Count total valence electron in PCl3
First of all, determine the valence electron that is available for drawing the lewis structure of PCl3 because the lewis diagram is all about the representation of valence electrons around atoms.
So, an easy way to find the valence electron of atoms in the PCl3 molecule is, just to look at the periodic group of phosphorous and chlorine atoms.
As the phosphorous atom belongs to the 5A group in the periodic table and chlorine is situated in the 7A group, hence, the valence electron for the phosphorous is 5, and for the chlorine atom, it is 7.
∴ Total number of valence electron available for the PCl3 Lewis structure = 5 + 7×3 = 26 valence electrons [∴ PCl3 molecule has one phosphorous and three chlorine atoms]
2. Find the least electronegative atom and place it at center
An atom with a less electronegative value is preferable for the central position in the lewis diagram because they are more prone to share the electrons with surrounding atoms.
In the case of the PCl3 molecule, the phosphorous atom is less electronegative than the chlorine atom.
Hence, put the phosphorous atom at the central position of the lewis diagram and all three chlorine atoms outside to it.
3. Connect outer atoms to central atom with a single bond
In this step, join all outer atoms to the central atom with the help of a single bond.
In, the PCl3 molecule, chlorine is the outer atom, and phosphorous is the central atom. Hence, joined them as shown in the figure given below.
Count the number of valence electrons used in the above structure. There are 3 single bonds used in the above structure, and one single bond means 2 electrons.
Hence, in the above structure, (3 × 2) = 6 valence electrons are used from a total of 26 valence electrons available for drawing the PCl3 Lewis structure.
∴ (26 – 6) = 20 valence electrons
So, we are left with 20 valence electrons more.
4. Place remaining electrons on the outer atom first and complete their octet
Let’s start putting the remaining valence electrons on outer atoms first. In the case of the PCl3 molecule, chlorine is the outer atom and each of them needs 8 electrons to have a full octet.
Start putting the remaining electrons on chlorine atoms as dots till they complete their octet.
So, all chlorine atoms in the above structure completed their octet, because all of them have 8 electrons(6 electrons represented as dots + 2 electrons in every single bond) in their outer shell.
Now again count the valence electron in the above structure.
In the above structure, there is 18 electrons are represented as dots + three single bonds that contain 6 electrons means a total of 24 valence electrons is used in the above structure.
We have a total of 26 valence electrons available for drawing the lewis structure of PCl3. And we used 24 valence electrons in the above structure.
∴ (26 – 24) = 2 valence electrons
So, we are left with just 2 valence electrons.
5. Complete the octet of the central atom
Since we have already completed the octet for outer atoms in PCl3, now, check the octet of the central atom as well.
In PCl3, the central atom is phosphorous and it requires a total of 8 electrons to have a full valence shell.
If you look at the 4th step structure, the phosphorous atom is attached to three single bonds that means it have 6 electrons, so, it just short of 2 electrons.
We already have the remaining 2 valence electrons, hence, put these two electrons on the phosphorous atom to complete its octet as well.
In the above structure, we see, each atom completed its octet comfortably, now, Let’s check the formal charge for the above structure to verify it’s stable or not.
6. Check the stability with the help of a formal charge concept
The lesser the formal charge on atoms, the better is the stability of the lewis diagram.
To calculate the formal charge on an atom. Use the formula given below-
What are the electron and molecular geometry of PCl3?
The molecular geometry of PCl3 is a Trigonal pyramid. The trigonal pyramid geometry is formed when the central atom is attached to three atoms and contains one lone pair. So, In PCl3, the phosphorous (P) is a central atom that has one lone pair on it and it is also attached to the three chlorine (Cl) atoms.
The molecular geometry or shape of PCl3 is a Trigonal pyramid, because, the lone pair present on the central Phosphorous (P) atom greatly repel the adjacent bonded pairs, therefore, the three bonds(P-Cl) are pushed down even further away from their respective position, and the final shape of PCl3 appears like Trigonal pyramid.
It should be noted that, Molecular geometry only consider bonded atom while determining the shape of molecule, it doesn’t count lone pair, but the influence of lone pair(repelling effect) will be counted on overall shape of molecule.
So, while determining the molecular geometry of PCl3, we will consider the repelling effect of the lone pair present on the phosphorous central atom, but the lone pair will be invisible when we look at the actual molecular geometry of PCl3.
PCl3 Molecular geometry
Now, What is the electron geometry of PCl3?
The electron geometry consider bond pair as well lone pair while determining the geometry of any molecule.
The electron geometry of PCl3 is Tetrahedral, because, the phosphorous central atom has one lone pair and it is attached to three bonded pairs as well. So, there are 4 regions of electron density(3 bond pair + 1 lone pair) around the central atom.
According to the VSEPR theory, the central atom with four regions of electron density adopts a tetrahedral electron geometry. Because repulsion is minimum in electron pairs at this position.
“A region of electron density means the group of bonding or nonbonding electrons that present around the atom. “
According to the VSEPR theory, “the maximum distance four regions of electron density can get away from affords a geometry called Tetrahedral.”
Now, a very simple way to determine the electron and molecular geometry of PCl3 is the AXN method.
AXN is a simple formula that represents the number of the bonded atom and lone pair on the central atom to predict the shape or geometry of the molecule using the VSEPR chart.
AXN notation for PCl3 molecule:
A denotes the central atom, so, Phosphorous (P) is the central atom in PCl3 molecule A = Phosphorous
X denotes the bonded atoms to the central atom, Phosphorous (P) is bonded with three chlorine (Cl) atoms. Therefore, X = 3
N represents the lone pair on the central atom, as per PCl3 Lewis structure, the Phosphorous central atom has one lone pair. Hence, N = 1
So, the AXN generic formula for the PCl3 molecule becomes AX3N1.
As per the VSEPR chart, if a molecule gets AX3N1 generic formula then its molecular geometry or shape will be a trigonal pyramid, and electron geometry will also be tetrahedral.
Therefore, the molecular geometry for PCl3 is a trigonal pyramid and its electron geometry is Tetrahedral.
Hybridization of PCl3
The hybridization of PCl3 is Sp3. Because the steric number of the phosphorous central atom is four.
The formula for calculating the steric number is-
Steric number = (Number of bonded atoms attached to central atom + Lone pair on central atom)
In the case of the PCl3 molecule, phosphorous is the central atom that is attached to the three bonded atoms(chlorine) and it has one lone pair as well.
Hence, (3 + 1) = 4 is the steric number of central atom phosphorous in the PCl3 molecule that gives Sp3 hybridization.
Steric number
Hybridization
1
S
2
Sp
3
Sp²
4
Sp³
5
Sp³d
6
Sp³d²
The bond angle of PCl3
The bond angle of PCl3 will be less than 109º. Because the repelling effect of the lone pair on bonded atoms (Cl-P-Cl) will reduce the ideal bond angle(from 109º to some approx 103º) since the lone pair take more room than bonded pairs.
In PCl3, the Cl-P-Cl bond angle will be less than 109,º because of lone pair-bond pair repulsion.
So, Is PCl3 polar or nonpolar? PCl3 is a polar molecule. The chlorine (Cl) atom is more electronegative than the phosphorous (P) atom, therefore, the net shift in electron density towards the chlorine atom, hence, it develops a negative charge, whereas, the phosphorous gets a positive charge.
The separation of positive and negative charges leads to a dipole moment in PCl3.
Also, the molecular geometry of PCl3 is Trigonal pyramidal which is not symmetrical since there is one lone pair present on the central atom that causes distortion in a molecule, so, it results in unequal sharing of electrons, which generates permanent dipole moment, and, makes, PCl3 is a polar molecule.
How many lone pairs are present in the lewis structure of PCl3?
Lone pairs are those represented as dots in the lewis diagram that do not take part in the formation of bonds and are also called nonbonding electrons.
By looking at the PCl3 Lewis structure, we see, there are 20 dot electrons means 10 lone pairs present. [∴ 2 dot electrons means one lone pair).
So, in the PCl3 Lewis structure, a total of 10 lone pairs are present. (3 lone pairs on each chlorine atom and 1 on the phosphorous atom)
The total number of bond pairs present in the lewis structure of PCl3?
Bonding pairs are the pair of electrons that are in a bond. A single bond has one bond pair means 2 bonding electrons.
Two bonding electron between the atoms forms a single covalent bond.
Now, as per the PCl3 Lewis structure, the central atom phosphorous is attached with three single covalent bonds, and one single covalent bond means 2 bonding electrons.
Hence, the total bonding electrons is (3 × 2) = 6 bonding electrons that make 3 bond pairs.
∴ In the PCl3 Lewis structure, a total of 3 bond pairs is present.
Why the molecular geometry of PCl3 is Trigonal pyramid and electron geometry is Tetrahedral?
Two types of geometry can be predicted with the help of VSEPR theory- (a). Electron geometry (b). Molecular geometry
Electron geometry considers all electrons(Bonding and Lone pair electrons) whereas molecular geometry considers only Bonding atoms to determine the shape of any molecule.
∴ The molecular geometry of PCl3 is a Trigonal pyramid. Since the lone pair on the phosphorous (P) atom pushes down the bonded atoms because of repelling effect according to VSEPR. The final molecular shape of PCl3 appears like a Trigonal pyramid, with phosphorous (P) at the apex and three chlorine (Cl) atoms at the corners of the trigonal base.
However, the electron geometry of PCl3 will be Tetrahedral, because, the phosphorus (P) central atom is surrounded by 4 regions of electron density which implies, its electron domain geometry will be Tetrahedral, according to VSEPR.
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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