The compound Phosphorous Trihydride (PH3), also known as phosphine consists of phosphorus and hydrogen atoms. It is an inflammable and toxic gas without any color.
Phosphine does not have any odor when it is pure, but most samples of the gas have the unpleasant odor of rotten garlic or decaying fish.
This chemical is used as a pesticide, and for fumigation of cereals. This compound is also used in semiconductors and the plastic industry.
The molecular formula of phosphene is PH3, which means it has one phosphorous and three hydrogen atoms.
It will be interesting to study the molecular structure, geometry, and hybridization of this compound.
Lewis Structure of Phosphene
The molecular formula of phosphene is PH3 which indicates the compound has one phosphorous atom bonding with three hydrogen atoms.
To understand the structure of PH3, we should know the electronic configuration of the atoms and how many valence electrons are there in the atoms.
Hydrogen has an electronic configuration of 1S1 as it has only one electron.
Phosphorous has an electronic configuration of 1S2 2S2 2P6 3S2 3P3.
If you look at the periodic table, you will see hydrogen is placed in the first column while phosphorous in the 5th column.
It means hydrogen has one valence electron while phosphorous has three. In Phosphene, three hydrogen atoms combine with phosphorous.
The electronic configuration of the atoms let us know how many atoms can participate in the bonding.
Since hydrogen has one valence electron, and phosphorous has three, so P is the central atom in the molecular structure of this compound.
Three valence electrons of phosphorous forms pairs with three valence electrons from the hydrogen atoms. The remaining two unpaired electrons of phosphorous are placed on the 4th side that forms a lone pair.
Now, if you check the surrounding electrons of both the compounds, you can see each Hydrogen atom has two surrounding atoms, while the phosphorous atom has eight electrons around it.
So, the combination has a stable structure now.
The bond angle in PH3 is 93 degrees. The geometric structure of this compound is a trigonal pyramid in shape. In the following section, we will learn in detail about the geometric shape of this compound.
Molecular Geometry of Phosphene
The molecular geometry of a compound is determined by two factors; the Lewis structure and the VSEPR (valence shell electron pair repulsion) theory.
From the Lewis molecular structure of PH3, we have seen the phosphorous atom has five valence electrons.
During the bonding process, Phosphorous is surrounded by three hydrogen atoms, and each is connected by a single bond.
The two remaining electrons form a lone pair. The shape of a molecule is defined by how many lone pairs and the number of covalent bonds it has.
If you have studied the VSEPR theory, you know that each pair of electrons tend to stay at the maximum possible distance from one another.
It reduces the repulsion between the valence electrons, thus helping the molecule get a stable structure. The number of lone pairs and bonds affect the shape of a molecule.
Every kind of electron pair repulse the other pairs; the force of repulsion is maximum between the two lone pairs.
This force is lower between a lone pair and a bond pair, whereas it is the lowest between two bond pairs of electrons.
Here is the increasing order of repulsion forces:
Bond pair – bond pair < bond pair – lone pair < lone pair – lone pair
The PH3 molecule has one lone pair and three bond pairs. So, the lone pair remains at a maximum distance from the three bond pairs.
As a result, the PH3 molecule attains the shape of a trigonal pyramid wherein the three bond pairs form the shape like the base of a pyramid, while the lone pair remains at the top, maintaining a larger distance from all the three bond pairs.
The three electron pairs and the larger repulsive force between the lone pair and three bond pairs is responsible for this shape (trigonal pyramid) of the molecule.
Hybridization of The PH3 Molecule
What is Hybridization?
Orbital hybridization or hybridization is the concept of combining two or more atomic orbitals with the same level of energy to form a new type of orbitals.
If we take the example of carbon, the atoms form a bond by combining the s and p orbitals. Carbon forms different compounds through different hybridization.
Learning how the atoms are arranged within a molecule helps you get a better understanding of the shape of the molecule.
To understand different chemical substances around us, it is essential to learn and visualize the structure of the molecules in three dimensions.
It gives you a better understanding of the shape, physical and chemical properties of a substance.
Reason for Hybridization
Hybridization takes place when an atom participates in a chemical bonding by sharing its electrons from s and p orbitals.
During such chemical bonding, an imbalance in the energy levels is created, and to attain a balance in the energy levels, the orbitals combine, which results in a hybrid orbital.
Since you have a clear idea of hybridization now, it will be easier for you to understand the hybridization of PH3.
Hybridization of PH3
It is quite surprising that hybridization does not take place in Phosphine. The reason is this compound has a distinct orbital structure and distribution of electrons.
Let us look at why it happens with the phosphene molecule.
If we analyze the structure of the PH3 molecule, we can see that valence electrons in the p orbitals participate in bond formation. It prevents the p orbital to get hybridized.
In PH3, phosphorous has a lone pair and three bond pairs. Drago’s rule explains the hybridization of PH3 in a better way. In the next section, we will look into Drago’s rule in the hybridization process of phosphine.
Drago’s Rule and Hybridization of Phosphine
According to Drago’s rule, hybridization in a molecule will not take place in some special conditions. The followings are the conditions.
- The central atom’s electronegativity 2.5 or less than that.
- If you add the number of lone pairs and sigma bonds, the total is 4.
- The central atom is placed in any of the groups between 13 to 17 or in the 3 to 7 Period.
- There is a minimum of one lone pair in the central atom.
If we talk about the phosphine molecule, the central atom is phosphorous. It belongs to the 3rd period and 15th group in the modern periodic table.
Phosphorous has an electronegativity of 2.9. Moreover, there is a lone pair in phosphine.
So, it meets three conditions of Drago’s rule, and we know if any compound meets just one of Drago’s rules, hybridization does not take place in its case.
Hence, hybridization does not take place in the PH3 molecule.
Polarity in PH3
Ph3 is considered as a polar molecule because it has a lone pair and due to which the shape of the molecule is formed as trigonal pyramidal.
As a result, the charge distribution is non-uniform across the whole molecule.
For more detailed information, you must also read out the article on the polarity of PH3.
Fun Facts About Phosphine
- Orbital hybridization does not take place in PH3 molecules.
- The pure ‘p’ orbitals participate in the formation of the P-H bond in the PH3 molecule.
- The bond angle in Ph3 is 93.5 degrees.
This article is an overview of the Lewis structure, molecular geometry, and hybridization of the phosphene (PH3) molecule.
We have tried to cover everything related to this topic including Drago’s rule that explains clearly why this compound does not have hybridization.
Apart from this, you have learned some interesting facts about this molecule. Hope you enjoyed reading it.