PBr3 is a colorless fuming chemical compound with a strong pungent odor and exists in a liquid state.
It exhibits a unique property of acting both as a Lewis acid and a Lewis base.
This is how PBr3 is prepared:
2P + 3Br3 ——> 2PBr3
It has a boiling point of 1750 C and a freezing point of 400 C.
It has several applications. It can be used as fire suppressants, in the process of manufacturing pharmaceuticals, as catalysts, and for the analysis of sugar.
But it can cause severe damage to the skin and irritate the respiratory tract and other internal organs of the human body.
Other than this, it is highly toxic and can even cause explosions.
Phosphorus tribromide has a major role to play in the bromination of acids and conversion of alcohols, hence it is important that we get to know deeply about this molecule.
We have presented an extensive article on the chemical bonding of PBr3.
PBr3 Lewis Structure
Before we start discussing the Lewis Structure of PBr3, we should have an idea about the concept and why we need to learn it in the first place?
Definition and Introduction
Lewis Structure is a systematic approach towards deciphering the nature and position of atoms for chemical bonding inside a molecule.
This was formulated by Gilbert N Lewis and stands for a diagrammatic representation of bonds and valence electrons of a chemical molecule.
Here, we work towards sketching a skeleton diagram of the molecule with atoms represented by their symbols, valence electrons represented by dots, and bonds represented by straight lines.
This is a two-dimensional approach and therefore, one of the first and foremost steps of chemical bonding for any given ion or molecule.
Phosphorus tribromide or Pbr3 molecule consists of a phosphorus atom and three atoms of bromine. Phosphorus has an atomic number of 15 and therefore has a valency of 5.
In the case of Br, it belongs to the family of halogens and consists of seven valence electrons.
Total valence electrons in a single molecule of PBr3 = 5 + 7*3
= 5 + 21
Now, that we have calculated the valence electron number, we have to identify the central atom( the atom with the least electronegativity value).
Here, phosphorus, being less electronegative, will act as the central atom with three bromine atoms surrounding it.
So, here’s how we have put the four atoms inside the molecule.
Now, we will put the outermost shell electrons as dots around each atom so that we can check the bond formation and the lone pairs.
Here, we will follow the octet rule.
By the octet rule, we try to establish the tendency of every main group atomic element to achieve its octet electronic configuration as per the periodic table.
We can see that each and every atom including the central P and the surrounding Br has achieved octet fulfillment.
This is supposedly the most suitable lewis structure of PBr3 but we can never be 100% sure.
Henceforth, we will follow the formal charge rule so that we can make sure whether all the constituent atoms are present in their least possible formal charge values.
This is how we calculate the formal charge.
For P, the formal charge = 5 – 0.5*6 – 2 = 0
For Br, the formal charge= 7 – 0.5*2 – 6 = 0 ( holds true for all the three bromine atoms) So, now we put the single bonds to get our perfect Lewis Structure.
PBr3 Molecular Geometry
Learning about the lewis structure leads us to our next concept: Molecular Bonding.
Definition and Introduction
Well, the entire universe including our planet is composed of millions and billions of atoms forming several different molecular compositions leading to new compounds. Each of these molecules inside a compound has a definite shape which can be described better in a three-dimensional format.
Lewis Structure has its own drawbacks, it cannot help us determine the 3D shape of any molecule. So, here comes the VSEPR theory, short for Valence Shell Electron Pair Repulsion Model theory.
VSEPR theory is crucial in chemical bonding to comprehend the molecular geometry of a molecule along with its bond angles and bond lengths.
Electrons being likely charged (negative) form a cloud over the nuclei that makes them repel each other to maintain balance. This repulsion is minimized to form a stable molecule which in turn makes it attain its required possible shape.
VSEPR theory to determine PBr3 molecular shape
In the case of Phosphorous Tribromide, to determine the molecular geometry, we need to take the help of the VSEPR model.
In the VSEPR theory, we have the notation AXN. Here, A stands for Phosphorous, the central atom. X stands for 3 Bromine atoms surrounding the central P.
And, N stands for 1 lone pair of electrons on the central P.
So, it is AX3N.
Look at the above diagram.
We can see that for PBr3 there are four electron-dense areas out of which three are bonded zones and one is lone pair around the central atom.
Hence, the required shape is trigonal bipyramidal. The bond angle is around 109.5 degrees approx.
Let us go through an important topic of discussion: Polarity.
Definition and Introduction
Drawing the electron dot structure and determining the 3D geometry of a given molecule paves the way for us to find out about the polarity of the molecule.
What is polarity? Why do we need to understand whether a molecule is polar or nonpolar?
To describe the term polarity, we need to introduce another concept known as dipole moment.
Do you know that when atoms combine to form a structure, there are bonds formed, sometimes between elements with different electronegativity values?
This leads to a separation of electric charges and a presence of partial positive and negative poles which in turn leads to polarity with the formation of a dipole moment.
When there is no electric charge separation i.e difference in electronegativity or the charges cancel out each other inside a molecule, we get a net-zero dipole and the molecules are termed non-polar.
Is PBr3 polar or nonpolar?
PBr3 has one P atom with an electronegativity value of 2.19 and three Br atoms each with a 2.96 value of electronegativity. With such a high difference, we have polar bonds between P and Br where there is a + partial charge near P and a – partial charge near Br in each of the bonds.
Do the bonds cancel each other to get a net-zero dipole moment?
The answer is, NO.
We have already found out the trigonal bipyramidal shape of PBr3 which turns out to be asymmetrical. So, the dipole moments do not happen to cancel each other out and we get a resultant polar molecule.
I have also written a detailed article on the polarity of PBr3.
Hybridization is an important concept of chemical bonding which is dependent on mathematical probability functions called atomic orbitals on a given x-y-z plane.
Linus Pauling, the famous chemist, proposed the hybridization idea. We work with different orbitals like s,p,d,f, etc.
The mixing of these orbitals leads to the formation of hybridized orbitals like sp, sp2, sp3, sp3d, and so on.
This is explained by quantum mechanics where we talk about the superimposition of these several AOs of comparable energies with varying proportions.
What is the hybridization of PBr3?
Before we can discuss the hybridization type of a molecule of phosphorus tribromide, let us see what we already know about the bonding nature here.
We have found out the most appropriate Lewis Structure where we checked the single bonds formed between P and each Br atom. ( P-Br)
We have found out that PBr3 is polar due to asymmetrical property and has a trigonal bipyramidal shape.
Coming to electronic configurations,
P:1s2 2s2 2p6 3s2 3p3
Br:1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p5
In PBr3, there are a total of 3 sigma bonds formed due to the three single bonding between P and Br. There is one lone pair present on the central atom P.
Here, a tetrahedral arrangement happens therefore resulting in sp3 hybridization. The phosphorus atom here forms the sp3 hybridization with the three bond pairs and the one lone pair. Here, one ‘s’ electron goes to the d orbital in excited form.
Therefore, the one 3s electron and the other three electrons in 3p orbitals combine together to result in sp3.
Here, in this article, we discussed in detail one of the most prominent halides of phosphorus. PBr3 or phosphorous tribromide is an important molecular composition and we have learned about its Lewis Structure, hybridization, molecular geometry, and polarity.