Hydrogen Phosphate, H3PO4 is an important chemical compound. Phosphoric acid, also known as orthophosphoric acid is a colorless and odorless weak acid with an 85% aqueous solution. It is also available in a transparent solid-state with a density of 1.834 g/cc.
Phosphoric acid has a lot of uses in daily life. It has its application in dentistry during fillings and roughening of teeth surfaces. Other than this, it has a wide range of applications in the world of fertilizers.
It is also used in anti-rust treatment, microfabrication, sanitizers, and compound semiconductor industry.
Health hazards include severe skin irritation and damages to the eyes.
H3PO4 Lewis Structure
To learn the nature of bonding inside any chemical molecule, the first and foremost step is to have a basic idea about its structure.
Lewis Structure diagram gives us a simple representation of any given molecular composition with the help of electron dot methodology.
The above diagram shows the Lewis Structure of a common molecule NO or nitric oxide.
Here, as we can see, we have got a 2D diagrammatic sketch that can help us understand the electronic arrangement in an undetailed yet crisp manner.
We have used dot notations to represent the valence electrons and lone pairs surrounding the constituent atoms. We have used atomic symbols and straight lines to indicate the type of bond formed.
Let us see how we can find out the most perfect Lewis Structure sketch for H3PO4:
Steps to draw Lewis Structure for H3PO4
Step 1: Find out the total valence electron number.
A single molecule of hydrogen phosphate contains three atoms of hydrogen, one phosphorus, and four oxygen atoms.
P belongs to group 5 of the periodic table, therefore has 5 valence electrons. O belongs to the chalcogen group or group 6, hence has 6 valence electrons.
A hydrogen atom has only an atomic number of 1, therefore the valency. Total number of valence electrons in H3PO4
= 1*3 + 5 + 6*4 = 32.
Step 2: Identify the central atom.
The above-mentioned chart gives us the electronegativity values of all main group elements in the periodic table.
During the formation of any molecule, usually, the least electronegative element takes up the central position barring a few exceptions.
This is probably due to the fact that electropositive elements are more likely to be conducive to sharing electrons and forming bonds helping in maintaining stability.
Here, we have P as the least electronegative element and it will become the central atom. Since we have a triatomic molecule, we have placed the atoms in the following manner:
Step 3: Sketch the molecular structure with electron-dots.
We have now surrounded the molecular atoms with the help of dot notations.
Step 4: Check Octet Rule.
The main group elements of the periodic table have a tendency to attain octet configuration in their outermost shells i.e. they tend to follow the valence shell configurations of the noble gases of the same period.
For E.g. Carbon tends to attain the octet configuration of Neon (Atomic No:10).
So, let us now check the valence shells around the atoms in the Lewis Structure.
As we can see quite vividly, all the atoms have attained octet fulfillment like P and O each of the atoms having eight electrons.
However, hydrogen has only two electrons surrounding it but it also has achieved noble gas configuration
( in this case, it is Helium, Atomic No: 2 ).
Step 5: Type of Bond formation.
As octet fulfillment has been done, let us check the type of bond formation inside a molecule of hydrogen phosphate.
Since two electrons have been shared between each of the atomic pairs, we will stick to single bonding.
Step 6: Check Formal Charge.
We cannot verify whether we have got the most suitable Lewis Structure configuration unless and until we have checked the formal charge values.
A charge is assigned to a bonded atom assuming that it is shared equally among all the bonded atoms inside the given molecule. This is known as the formal charge.
Formal charge of P = 5 – 0.5*8 – 0 = +1.
Formal charge of O at the top = 6 – 0.5*2 – 6 = -1.
Formal charge of other O atoms each = 6 – 0.5*4 – 4 = 0.
Formal charge of each H atom = 1 – 0.5*2 – 0 = 0.
Since P has a formal charge of +1, we can do some variations to make it of a lesser value.
An Exception to Octet Rule
In the Lewis Structure, we will shift the top electrons from the O atom at the top towards the P atom to form double bonds.
This will make P have a valence electron number of 10 thus an exception to the octet rule.
The formal charge of the O at the top is now 0. The formal charge of P = 0.
The most suitable lewis structure for H3PO4 is:
H3PO4 Molecular Geometry
Electrons are like-charged particles that form a negatively charged cloud surrounding the atomic nuclei inside a polyatomic molecule.
This causes repulsion which is needed to be minimized for stability and balance. This is known as the VSEPR theory or Valence Shell Electron Repulsion Pair Theory.
Now, for H3PO4, we have to find out the number of electron density regions surrounding the central atom of the molecule.
Here, the central atom is Phosphorus.
If we look at the Lewis structure, we can find out that there are four electron density regions around P: three OH and one O region.
There is zero lone pair on P.
So, according to VSEPR theory, we get a tetrahedral arrangement.
Orbital Hybridization: A Brief Intro
An orbital can be defined as the mathematical probability of the presence of electrons in any given regional space.
We have several atomic orbitals ( AOs ) of different shapes and energy levels like s, p, d, f.
When AOs combine and fuse to form hybrid orbitals like sp, sp2, sp3d, and so on, the process is known as orbital hybridization. This is an important concept of chemical bonding.
Now, let us look at the molecule and understand the concept of sigma and pi bonds. A single bond consists of one sigma bond i.e. a sigma pair.
A double bond however consists of one sigma and one pi pair. The pi pair does not take part in the hybridization process.
Let us look at the sigmas in H3PO4.
The central atom P is bonded singly with three O atoms ( forming the O-H or hydroxyl bonds) and doubly bonded with one O atom.
So, the three single bonds and one double bond consist of four sigmas.
The hybridization of the H3PO4 molecule is sp3.
What do you mean by Polarity?
During formal charge calculation, we assume that electrons are shared between bonding atoms in equal proportions, but this is not what happens in reality in all cases.
If we consider a molecule like H2 or Cl2, since both of the molecules being similar have the same electronegativity values, we can say that the electron pairs are shared equally here.
But if we consider HCl, since H and Cl have different electronegativity values, there will be a presence of partial charges.
The atom having less electronegativity value will have a partial positive charge ( δ+) and the more electronegative atom will possess a partial negative charge ( δ- ).
This is known as polarity. HCl is a polar molecule whereas Cl2 and H2 are nonpolar.
Polarity of H3PO4
Let us look at the Pauling Electronegativity chart:
Electronegativity value of P: 2.19
Electronegativity value of O: 3.44
Electronegativity value of H: 2.20
H3PO4 is an asymmetrical tetrahedral molecule with differences in electronegativity values, hence, it is polar.
The net dipole moment here is not equal to zero.
In this article, we learned a lot about the nature of chemical bonding occurring inside H3PO4. We have discussed the steps to draw the perfect Lewis Structure and the molecular geometry via VSEPR theory. We have also talked about hybridization and polarity.