PH3 lewis structure is made up of one phosphorus (P) atom and three hydrogens (H) atoms. In, lewis structure of PH3, three bond pairs, and one lone pair are present. The phosphorous (P) atom is situated at a central position in the lewis diagram.
The drawing of the PH3 Lewis structure is very easy and simple. Let’s see how to do it.
Steps for drawing the Lewis dot structure for PH3
1. Count total valence electron in PH3
First of all, determine the valence electron that is available for drawing the lewis structure of PH3 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 PH3 molecule is, just to look at the periodic group of phosphorous and hydrogen atoms.
As the phosphorous atom belongs to the 5A group in the periodic table and hydrogen is situated in the 1A group, hence, the valence electron for the phosphorous is 5, and for the hydrogen atom, it is only 1.
⇒ Total number of the valence electrons in hydrogen = 1
∴ Total number of valence electrons available for the PH3 Lewis structure = 5 + 1×3 = 8 valence electrons [∴ PH3 molecule has one phosphorous and three hydrogen 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.
It should be noted that “Hydrogen always go outside in lewis diagram” Because, Hydrogen atom can form only one single bond.
Hence, put the phosphorous atom at the central position of the lewis diagram and all three hydrogen 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 PH3 molecule, hydrogen 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 8 valence electrons available for drawing the PH3 Lewis structure.
∴ (8 – 6) = 2 valence electrons
So, we are left with only 2 valence electrons.
4. Complete the octet of all atoms
In the 3rd step structure, the hydrogen atoms completed their octet since they have 2 electrons(one single bond means 2 electrons) in their outer shell.
Hydrogen atom only need 2 electrons to fulfill the outer shell.
Now the Phosphorous central atom, in the PH3 molecule, requires a total of 8 electrons to have a full outer shell.
If you look at the 3rd 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, that each atom completed its octet comfortably, now, 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-
What are the electron and molecular geometry of PH3?
The molecular geometry of PH3 is a Trigonal pyramid. The trigonal pyramid geometry is formed when the central atom is attached to three atoms and contains one lone pair. So, In PH3, the phosphorous (P) is a central atom that has one lone pair on it and it is also attached to the three hydrogens (H) atoms.
The molecular geometry or shape of PH3 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-H) are pushed down even further away from their respective position, and the final shape of PH3 appears like Trigonal pyramid.
PH3 Molecular geometry
Now, What is the electron geometry of PH3?
The electron geometry of PH3 is Tetrahedral, because, the phosphorous 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.
Now, a very simple way to determine the electron and molecular geometry of PH3 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 PH3 molecule:
A denotes the central atom, so, Phosphorous (P) is the central atom in PH3 molecule A = Phosphorous
X denotes the bonded atoms to the central atom, Phosphorous (P) is bonded with three hydrogens (H) atoms. Therefore, X = 3
N represents the lone pair on the central atom, as per PH3 Lewis structure, the Phosphorous central atom has one lone pair. Hence, N = 1
So, the AXN generic formula for the PH3 molecule becomes AX3N1.
As per the VSEPR chart, if a molecule gets AX3N1 generic formula then its molecular geometry or shape will be a trigonal pyramid, and electron geometry will also be tetrahedral.
Therefore, the molecular geometry for PH3 is a trigonal pyramid and its electron geometry is Tetrahedral.
Hybridization of PH3
It might surprise you, that, PH3 will not form any hybridization. This can be explained by using Drago’s rule.
The molecule that comes under Drago’s rule has no hybridization and has the least bond angle.
According to Drago’s rule, the molecule will form “no hybridization” in some conditions. The conditions are-
The central atom is placed in any of the groups between 3 to 7 Period.
At least one lone pair on the central atom.
The electronegativity of the terminal atom is less than approx 2.5.
So, in the case of PH3, the central atom Phosphorous(P) has one lone pair and it belongs to the 3rd period in the periodic table. Also, the terminal atom, hydrogen(H) has 2.2 electronegativity which is less than the electronegativity of carbon(2.55).
∴ PH3 meets all the conditions of Drago’s rule, hence, there is no hybridization takes place in the PH3 molecule.
PH3 does not have any hybridisation as it forms all bonds using it’s pure p orbitals.
The bond angle of PH3
The H-P-H bond angle in PH3 is 93.5º. The bond angle in PH3 is lower than the ideal value because of the large repulsive force exerted by the lone pair on 3 bonding orbitals.
So, Is PH3 polar or nonpolar? PH3 is a polar molecule. Because its molecular geometry is Trigonal pyramidal which is not symmetrical since there is one lone pair present on the central atom Phosphorous(P) that causes distortion in a molecule, so, it results in unequal sharing of electrons, which generates permanent dipole moment, and, makes, PH3 is a polar molecule.
How many lone pairs and bond pairs are present in the lewis structure of PH3?
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 PH3 Lewis structure, we see, that there is only 1 lone pair present(2 dot electrons on the central Phosphorous (P) atom).
Also, in the PH3 lewis structure, a total of 3 bond pairs are present as well.
Why the molecular geometry of PH3 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 PH3 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 PH3 appears like a Trigonal pyramid, with phosphorous (P) at the apex and three hydrogens (H) atoms at the corners of the trigonal base.
However, the electron geometry of PH3 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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