Phosphorous pentachloride (PCl5) lewis dot structure, molecular geometry, hybridization, Bond angle
Phosphorous pentachloride appears like yellowish-white crystals with a pungent odor. It has the chemical formula of PCl5. It is non-flammable in nature.
In this tutorial, we will discuss Phosphorous pentachloride (PCl5) lewis structure, molecular geometry, Bond angle, hybridization, polar or nonpolar, etc.
Phosphorus pentachloride is soluble in water and is mostly used as a chlorinating reagent.
PCl5 lewis structure is made up of five P-Cl bonds, with a phosphorus (P) atom in a central position and all five chlorine (Cl) as outer atoms in the lewis diagram. The lewis structure of PCl5 contains a total of 5 bond pairs and 15 lone pairs(3 lone pairs on each chlorine atom).
The drawing of the PCl5 lewis’s structure is very easy and simple. Let’s see how to do it.
Steps for drawing the Lewis dot structure for PCl5
1. Count total valence electron in PCl5
First of all, determine the valence electron that is available for drawing the lewis structure of PCl5 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 PCl5 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 PCl5 Lewis structure = 5 + 7×5 = 40 valence electrons [∴ PCl5 molecule has one phosphorous and five 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 PCl5 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 five chlorine atoms outside it.
3. Connect outer atoms to the 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 PCl5 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 5 single bonds used in the above structure, and one single bond means 2 electrons.
Hence, in the above structure, (5 × 2) = 10 valence electrons are used from a total of 40 valence electrons available for drawing the PCl5 Lewis structure.
∴ (40 – 10) = 30 valence electrons
So, we are left with 30 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 PCl5 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 30 electrons are represented as dots + five single bonds that contain 10 electrons means a total of 40 valence electrons is used in the above structure.
Remember, we have a total of 40 valence electrons available for drawing the lewis structure of PCl5. And we used all 40 valence electrons in the above structure.
∴ (40 – 40) = 0 valence electrons
So, we don’t have any remaining valence electrons.
Also, if we look at the above structure, the phosphorous atom is attached to five single bonds that means it have 10 electrons.
Note:- A atom that have more than 8 electrons in their valence shell called Expanded octet.
“The octet rule can be ‘expanded’ by some elements by utilizing the d-orbitals found in the third principal energy level and beyond. Sulfur, phosphorus, silicon, and chlorine are common examples of elements that form an expanded octet.”
Let’s check the formal charge for the above structure to verify whether it’s stable or not.
5. 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 PCl5?
The molecular geometry of PCl5 is a Trigonal Bipyramidal. In PCl5, the central atom Phosphorous (P) has no lone pair and is attached to five chlorine (Cl) atoms, which means, there are 5 regions of electron density and all the regions are bonding.
According to the VSEPR theory, “The maximum distance five regions of electron density can get away from affords a geometry called trigonal bipyramidal”.
The molecular geometry or shape of PCl5 is a Trigonal bipyramidal. In PCl5, the Phosphorous (P) atom is present at the central position whereas three chlorine atoms are present at the equatorial position and two chlorine atoms at the Axial position. The final arrangement of this molecule (PCl5) appears like a Trigonal bipyramidal shape.
“A trigonal bipyramid simply means two pyramid, If you look at above picture, you should be able to see the pyramids relative to the top half and the bottom half of the central atom.”
Now, What is the electron geometry of PCl5?
The electron geometry consider bond pair as well lone pair while determining the geometry of any molecule. And molecular goometry only look at bond pairs.
If there is no lone pair on central atom of molecule, then –
Molecular geometry = Electron geometry for that molecule
The electron geometry of PCl5 is also Trigonal bipyramidal, because, the phosphorous (P) central atom has zero lone pair, so, only bonded pairs are considered while evaluating its electron geometry.
According to the VSEPR theory, the central atom with five regions of electron density adopts a trigonal bipyramidal electron geometry. Because repulsion is minimum in electron pairs at this position.
Now, a very simple way to determine the electron and molecular geometry of PCl5 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 PCl5 molecule:
A denotes the central atom, so, Phosphorous (P) is the central atom in PCl5 molecule A = Phosphorous
X denotes the bonded atoms to the central atom, Phosphorous (P) is bonded with five chlorine (Cl) atoms. Therefore, X = 5
N represents the lone pair on the central atom, as per PCl5 Lewis structure, the Phosphorous central atom has zero lone pair. Hence, N = 0
So, the AXN generic formula for the PCl5 molecule becomes AX5N0 or AX5.
As per the VSEPR chart, if a molecule gets AX5 generic formula then its molecular geometry or shape will be a trigonal bipyramidal, and electron geometry will also be trigonal bipyramidal.
Therefore, the molecular geometry for PCl5 is trigonal bipyramidal and its electron geometry is also trigonal bipyramidal.
Hybridization of PCl5
The hybridization of PCl5 is Sp3d. Because the steric number of the phosphorous central atom is five.
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 PCl5 molecule, phosphorous is the central atom that is attached to the five bonded atoms(chlorine) and it has zero lone pair.
Hence, (5 + 0) = 5 is the steric number of central atom phosphorous in the PCl5 molecule that gives Sp3d hybridization.
Steric number
Hybridization
1
S
2
Sp
3
Sp²
4
Sp³
5
Sp³d
6
Sp³d²
The bond angle of PCl5
There are two types of bonds formed in PCl5, the Axial bond and the Equatorial bond.
The axial bonds is slightly longer than equitorial bond since they bear more repulsive interaction force.
In PCl5, two P-Cl bonds are situated at the Axial position where one lies above the equatorial plane and the other below the plane, they make the angle with the plane is 90º.
Also, three P-Cl bonds are situated at the equatorial position, and all lie in the same plane, they make an angle with each other, and the angle made between them is 120º.
So, Is PCl5 polar or nonpolar? The bond in PCl5 is polar because the chlorine (Cl) atom is more electronegative than the phosphorous (P) atom but the overall structure of PCl5 is nonpolar because of its symmetrical structure due to which the polarity of P-Cl bonds gets canceled by each other.
How many lone pairs and bond pairs are present in the lewis structure of PCl5?
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. Bonding pairs are the pair of electrons that are in a bond. A single bond has one bond pair means 2 bonding electrons.
By looking at the PCl5 Lewis structure, we see, that there are 30 dot electrons means 15 lone pairs present. [∴ 2 dot electrons means one lone pair). And, there are 5 bonded atoms present which contain 10 bonding electrons.
Why the molecular geometry of PCl5 is Trigonal bipyramidal?
VSEPR theory says that “pairs of electrons(bond and lone pair) repels each other and these electrons occupy such position in space so that repulsion should between them minimum”.
In the case of PCl5, there is no lone pair on the central Phosphorous atom, so, what we have, is 5 bonded pairs. These bonded pairs will repel each other and going to push far away from each other.
According to VSEPR theory, the five bonded pairs will have minimum repulsion between them when they are stretched at the corners of a triangular bipyramid.
These five bonded pairs will be divided into two positions in which, two atoms will be in the Axial position, and three atoms in the Equatorial position.
The original molecular geometry or shape of PCl5 will be Trigonal bipyramidal with two chlorine atoms at the Axial position and three chlorine atoms at the Equatorial position.
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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