PBr3 lewis dot structure, molecular geometry, polar or nonpolar, hybridization, bond angle
Phosphorous tribromide appears as a colorless liquid having a penetrating odor. It has the chemical formula PBr3. It has a molar mass of 270.69 g/mol.
In this tutorial, we will discuss Phosphorous tribromide (PBr3) lewis structure, molecular geometry, polar or nonpolar, Bond angle, hybridization, etc.
Phosphorus tribromide act as liquid fumes in moist air due to hydrolysis.
PBr3 lewis structure is made up of three P-Br bonds, with a phosphorus (P) atom in a central position and all three bromine (Br) as outer atoms in the lewis diagram. The lewis structure of PBr3 contains a total of 3 bond pairs and 10 lone pairs(3 lone pairs on each bromine atom and 1 on the central atom).
The drawing of the PBr3 lewis’s structure is very easy and simple. Let’s see how to do it.
Steps for drawing the Lewis dot structure for PBr3
1. Count total valence electron in PBr3
First of all, determine the valence electron that is available for drawing the lewis structure of PBr3.
So, an easy way to find the valence electron of atoms in the PBr3 molecule is, just to look at the periodic group of phosphorous and bromine atoms.
As the phosphorous atom belongs to the 5A group in the periodic table and bromine is situated in the 7A group, hence, the valence electron for the phosphorous is 5, and for the bromine atom, it is 7.
⇒ Total number of the valence electrons in bromine = 7
∴ Total number of valence electrons available for the PBr3 Lewis structure = 5 + 7×3 = 26 valence electrons [∴ PBr3 molecule has one phosphorous and three bromine 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 PBr3 molecule, the phosphorous atom is less electronegative than the bromine atom.
Hence, put the phosphorous atom at the central position of the lewis diagram and all three bromine 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 PBr3 molecule, bromine 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 PBr3 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 PBr3 molecule, bromine is the outer atom and each of them needs 8 electrons to have a full octet.
Start putting the remaining electrons on bromine atoms as dots till they complete their octet.
So, all bromine 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 PBr3. 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 PBr3, now, check the octet of the central atom as well.
In PBr3, 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 PBr3?
The molecular geometry or shape of PBr3 is a Trigonal pyramid. This geometry contains a phosphorous (P) central atom at the apex position, and the three bromine (Br) atoms at the corners of the trigonal base. A trigonal pyramid geometry is different than a regular tetrahedral geometry.
The molecular geometry or shape of PBr3 is a Trigonal pyramid, because, the lone pair present on the central Phosphorous (P) atom greatly repels the adjacent bonded pairs, therefore, the three bonds(P-Br) are pushed down even further away from their respective position, and the final shape of PBr3 appears like Trigonal pyramid.
PBr3 Molecular geometry
Now, What is the electron geometry of PBr3?
The electron geometry of PBr3 is Tetrahedral, because, the phosphorous (P) 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.
Also, the PBr3 molecule has an AX3N1 generic formula, which shows, its electron geometry will be tetrahedral, and its molecular geometry will be a trigonal pyramid.
Therefore, the molecular geometry for PBr3 is a trigonal pyramid and its electron geometry is Tetrahedral.
Hybridization of PBr3
The hybridization of PBr3 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 PBr3 molecule, phosphorous is the central atom that is attached to the three bonded atoms(bromine) and it has one lone pair as well.
Hence, (3 + 1) = 4 is the steric number of central atom phosphorous in the PBr3 molecule that gives Sp3 hybridization.
Steric number
Hybridization
1
S
2
Sp
3
Sp²
4
Sp³
5
Sp³d
6
Sp³d²
The bond angle of PBr3
The bond angle of PBr3 will be less than 109º. Because the repelling effect of the lone pair on bonded atoms (Br-P-Br) will reduce the ideal bond angle(from 109º to some approx 101º) since the lone pair take more room than bonded pairs.
In PBr3, the Br-P-Br bond angle will be less than 109,º because of lone pair-bond pair repulsion.
So, Is PBr3 polar or nonpolar? PBr3 is a polar molecule. One main reason is, PBr3 has a distorted shape which causes a non-zero dipole moment in this molecule.
Also, The bromine (Br) atom is more electronegative than the phosphorous (P) atom, therefore, the net shift in electron density towards the bromine 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 PBr3.
Since the molecular geometry of PBr3 is Trigonal pyramidal which is not symmetrical, therefore, it results in unequal charge distribution, which generates a permanent dipole moment, and, makes, PBr3 is a polar molecule.
How many lone pairs and bond pairs are present in the lewis structure of PBr3?
Lone pairs are those represented as dots in the lewis diagram. 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 PBr3 Lewis structure, we see, there are a total of 10 lone pairs are present(3 lone pairs on each bromine atom and 1 lone pair on the phosphorous atom).
Also, in the PBr3 lewis structure, a total of 3 bond pairs are present as well.
Why the molecular geometry of the PBr3 is Trigonal pyramid and the 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 PBr3 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 PBr3 appears like a Trigonal pyramid, with phosphorous (P) at the apex and three bromine (Br) atoms at the corners of the trigonal base.
However, the electron geometry of PBr3 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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