PBr5 lewis dot structure, molecular geometry, polar or nonpolar, hybridization, Bond angle

Home PBr5 lewis structure and its molecular geometry/shape

PBr5 lewis structure molecular geometry

Phosphorous pentabromide appears as a yellow solid, it has the chemical formula of PBr5. It is reactive in nature.

In this article, we will discuss Phosphorous pentabromide (PBr5) lewis structure, molecular geometry, Bond angle, hybridization, polar or nonpolar, etc.

Phosphorus pentabromide is irritating to the skin and eyes. It decomposes in water.

Name of MoleculePhosphorus pentabromide
Chemical formulaPBr5
Molecular geometry of PBr5Trigonal bipyramidal
Electron geometry of PBr5Trigonal bipyramidal
HybridizationSp3d
Bond angle 90º and 120º
NatureNonpolar molecule
Total Valence electron in PBr540
Overall Formal charge in PBr5Zero

How to draw lewis structure of PBr5?

PBr5 lewis structure is made up of five P-Br bonds, with a phosphorus (P) atom in a central position and all five bromine (Br) as outer atoms in the lewis diagram. The lewis structure of PBr5 contains a total of 5 bond pairs and 15 lone pairs(3 lone pairs on each bromine atom).

The drawing of the PBr5 lewis’s structure is very easy and simple. Let’s see how to do it.

Steps for drawing the Lewis dot structure for PBr5

1. Count total valence electron in PBr5

First of all, determine the valence electron that is available for drawing the lewis structure of PBr5.

So, an easy way to find the valence electron of atoms in the PBr5 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 electron in Phosphorous = 5

⇒ Total number of the valence electrons in bromine = 7

∴ Total number of valence electrons available for the PBr5 Lewis structure = 5 + 7×5 = 40 valence electrons         [∴ PBr5 molecule has one phosphorous and five bromine atoms]

valence electrons in pbr5 lewis structure

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 PBr5 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 five bromine atoms outside it.

central atom in pbr5

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 PBr5 molecule, bromine is the outer atom, and phosphorous is the central atom. Hence, joined them as shown in the figure given below.

pbr5 skeletal structure

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 PBr5 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 PBr5 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.

pbr5 lewis dot structure

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 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 PBr5. 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.

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 the stability of the lewis diagram.

To calculate the formal charge on an atom. Use the formula given below-

formal charge formula for lewis diagram

⇒ Formal charge = (valence electrons – nonbonding electrons –   1/2 bonding electrons)

Let’s count the formal charge for the 4th step structure.

 For bromine atom

⇒ Valence electrons of bromine = 7

⇒ Nonbonding electrons on bromine = 6

⇒ Bonding electrons around bromine (1 single bond) = 2

∴ (7 – 6 – 2/2) = 0 formal charge on the bromine atoms.

 For phosphorous atom

⇒ Valence electrons of phosphorous = 5

⇒ Nonbonding electrons on phosphorous = 0

⇒ Bonding electrons around phosphorous (5 single bonds) = 10

∴ (5 – 0 – 10/2) = 0 formal charge on the phosphorous central atom.

pbr5 lewis structure

Phosphorous pentabromide (PBr5) Lewis structure

Hence, in the above PBr5 lewis structure, all atoms get a formal charge equal to zero.

Therefore, the above lewis dot structure of PBr5 is most stable and appropriate in nature.

What are the electron and molecular geometry of PBr5?

The molecular geometry of PBr5 is a Trigonal Bipyramidal. In PBr5, the central atom Phosphorous (P) has no lone pair and is attached to five bromine (Br) 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”.

PBr5 molecular geometry or shape

The molecular geometry or shape of PBr5 is a Trigonal bipyramidal. In PBr5, the  Phosphorous (P) atom is present at the central position whereas three bromine atoms are present at the equatorial position and two bromine atoms at the Axial position. The final arrangement of this molecule (PBr5) appears like a Trigonal bipyramidal shape.

Now, What is the electron geometry of PBr5?

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 PBr5 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 PBr5 is the AXN method.

AXN method to determine shape and geometry

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 PBr5 molecule:

  • A denotes the central atom, so, Phosphorous (P) is the central atom in PBr5 molecule A = Phosphorous
  • X denotes the bonded atoms to the central atom, Phosphorous (P) is bonded with five bromine (Br) atoms. Therefore, X = 5
  • N represents the lone pair on the central atom, as per PBr5 Lewis structure, the Phosphorous central atom has zero lone pair. Hence, N = 0

So, the AXN generic formula for the PBr5 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.

Electron geometry and molecular geometry of PBr5 as per VSEPR

Therefore, the molecular geometry for PBr5 is trigonal bipyramidal and its electron geometry is also trigonal bipyramidal.

Hybridization of PBr5

The hybridization of PBr5 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 PBr5 molecule, phosphorous is the central atom that is attached to the five bonded atoms(bromine) and it has zero lone pair.

Hence, (5 + 0) = 5 is the steric number of central atom phosphorous in the PBr5 molecule that gives Sp3d hybridization.

Steric numberHybridization
1S
2Sp
3Sp²
4Sp³
5Sp³d
6Sp³d²

The bond angle of PBr5

There are two types of bonds formed in PBr5, the Axial bond and the Equatorial bond.

The axial bonds is slightly longer than equatorial bond since they bear more repulsive interaction force.

In PBr5, two P-Br 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-Br 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.

Is PBr5 polar or nonpolar?

So, Is PBr5 polar or nonpolar? The bond in PBr5 is polar because the bromine (Br) atom is more electronegative than the phosphorous (P) atom but the overall structure of PBr5 is nonpolar because of its symmetrical structure due to which the polarity of P-Br bonds gets canceled by each other.

FAQ

How many lone pairs and bond pairs are present in the lewis structure of PBr5?

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 PBr5 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.

lone pair and bond pair in pbr5 lewis structure

Why the molecular geometry of PBr5 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 PBr5, 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.

explain pbr5 geometry

The original molecular geometry or shape of PBr5 will be Trigonal bipyramidal with two bromine atoms at the Axial position and three bromine atoms at the Equatorial position.

Also read:

Properties and uses of Phosphorous pentabromide

  • It appears as a yellow solid.
  • It has a boiling point of 106 °C.
  • It decomposes in ethanol and water.
  • It is soluble in CCl4 and CS2.
  • It is reactive and highly corrosive in nature.
  • It is used to convert carboxylic acids to acyl bromides.

Reactions of Phosphorous pentabromide

Phosphorous pentabromide starts to decompose above 100ºC into phosphorus tribromide and bromine.

⇒ PBr5 → PBr3 + Br2

Summary

  • The total valence electron is available for drawing the Phosphorous pentabromide (PBr5) Lewis structure is 40.
  • The molecular geometry or shape of PBr5 is a Trigonal bipyramidal.
  • In the PBr5 Lewis dot structure, a total of 15 lone pairs and 5 bond pairs are present.
  • The electron geometry of PBr5 is also Trigonal bipyramidal.
  • The hybridization of phosphorous in PBr5 is sp3d. Since its steric number is 5.
  • In PBr5, axial atoms(2 P-Br bonds) make a 90º angle with the plane, and equatorial atoms(3 P-Br bonds) make a 120º angle with each other. 
  • PBr5 is nonpolar in nature.
  • The overall formal charge in PBr5 is zero.
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