Phosphite ion (PO33-) Lewis structure, molecular geometry
or shape, electron geometry, bond angles, formal charges,
hybridization, polar or nonpolar
PO33- represents the chemical formula for the phosphite ion, an inorganic polyatomic anion. It is the conjugate base of phosphoric acid (H3PO4).
The phosphite ion is primarily used as a raw material for fertilizer manufacturing. It protects the plants against bacterial and fungal attacks. PO33- is also widely used in water treatment processes to control algae growth.
In this article, we will teach you how to draw the Lewis dot structure of the phosphite (PO33-) ion, what is its molecular geometry or shape, electron geometry, bond angles, hybridization, formal charges, polarity, etc.
As very little information is available on this topic on the net, so we encourage you to continue reading till the end and learn all about the phosphite ion.
How to draw lewis structure of PO33- (Phosphite ion)?
The Lewis structure of the phosphite (PO33-) ion contains a phosphorus (P) atom at the center. The P-atom is surrounded by three oxygen (O) atoms via single covalent bonds. A lone pair of electrons is present on the central P-atom, while each terminal O-atom carries 3 lone pairs, respectively.
Drawing PO33- Lewis structure is quite easy. You can easily learn to do so by following the simple steps given below.
Steps for drawing the Lewis dot structure of PO33-
1. Count the total valence electrons present in PO33-
The two distinct elements present in PO33- are phosphorus and oxygen.
Phosphorus (P) is located in Group VA (or 15) of the Periodic Table, containing a total of 5 valence electrons in each P-atom.
In contrast, oxygen (O) is present in Group VI A (or 16) of the Periodic Table of Elements. Thus, it has a total of 6 valence electrons in each atom.
Total number of valence electrons in phosphorus = 5
Total number of valence electrons in oxygen = 6
The PO33- ion comprises 1 P-atom and 3 O-atoms.
An important point to remember is that the PO33- ion carries a negative three (-3) charge, which means 3 extra valence electrons are added in this Lewis structure.
∴ Therefore, the total valence electrons available for drawing the Lewis dot structure of PO33- = 1(5) + 3(6) = 23 + 3 = 26 valence electrons.
2. Find the least electronegative atom and place it at the center
By convention, the least electronegative atom out of all those available is chosen as the central atom while drawing the Lewis structure of a molecule or molecular ion.
The least electronegative atom can easily form covalent bonds with other atoms by sharing its electrons.
Phosphorus (E.N = 2.19) is invariably less electronegative than oxygen (E.N = 3.44).
Therefore, the P-atom is placed as the central atom in PO33- Lewis structure, while the three O-atoms are spread around it, as shown below.
3. Connect the outer atoms with the central atom
In this step, the outer atoms, i.e., 3 O-atoms, are joined to the central P-atom using single straight lines.
A straight line represents a single covalent bond, i.e., a bond pair containing 2 electrons.
In the above structure, there are a total of 3 P-O single bonds, i.e., 3(2) = 6 valence electrons are already consumed out of the 26 initially available.
Now let’s see in the next steps where to place the remaining 20 valence electrons in the PO33- Lewis dot structure.
4. Complete the octet of the outer atoms
An O-atom needs a total of 8 valence electrons in order to achieve a stable octet electronic configuration.
A P-O bond represents 2 valence electrons already present around each of the three oxygen atoms.
Therefore, to complete its octet, the remaining 6 valence electrons are placed as 3 lone pairs on each O-atom, as shown below.
5. Complete the octet of the central atom
Total valence electrons used till step 4 = 3 single bonds + 3(electrons placed around each O-atom, shown as dots) = 3(2) + 3(6) = 24 valence electrons.
Total valence electrons – electrons used till step 4 = 26 – 24 = 2 valence electrons.
Thus these 2 valence electrons are placed as a lone pair on the central P-atom.
This completes the octet of the central P-atom in the PO33- Lewis structure.
Now let’s check its stability by applying the formal charge concept.
6. Check the stability of Lewis’s structure using the formal charge concept
The less the formal charge on the atoms of a molecule or molecular ion, the better the stability of its Lewis structure.
The formal charges can be calculated using the formula given below.
Formal charge = [valence electrons- nonbonding electrons- ½ (bonding electrons)].
Now let us use this formula and the Lewis structure obtained in step 5 to determine the formal charges present on the PO33- bonded atoms.
As per the above calculation, zero or no formal charge is present on the central P-atom in PO33- Lewis structure. However, each O-atom carries a -1 formal charge; thus, the overall charge present on the phosphite ion = -1 +(-1) + (-1) = -3.
Now that we have obtained the correct Lewis representation for PO33-, let’s discuss its shape and geometry.
What are the electron and molecular geometry of PO33-?
The molecular geometry or shape of the phosphite (PO33-) ion is trigonal pyramidal, while its ideal electronic geometry is tetrahedral. The presence of a lone pair of electrons on the central P-atom leads to strong lone pair-bond pair repulsions, thus distorting the overall molecular shape of the ion.
Molecular geometry of PO33-
The molecular geometry or shape of the phosphite (PO33-) ion is trigonal pyramidal.
The presence of a lone pair of electrons on the central P-atom in PO33- leads to strong lone pair-bond pair electronic repulsions. The bonded O-atoms tilt away from the lone pair at phosphorus to minimize this strong repulsive effect. Thus, the bonded atoms form a triangular base, while the lone pair forms a pyramid at the top.
The shape of PO33-is thus called trigonal pyramidal, as shown in the figure drawn below.
Electron geometry of PO33-
In PO33-, the P-atom at the center is surrounded by 3 bond pairs and 1 lone pair of electrons, making a total of 4 electron density regions. Hence, the ideal electron pair geometry of the PO33- ion w.r.t the central atom is tetrahedral.
An easy trick to finding a molecule’s electron and molecular geometry is using the AXN method.
AXN is a simple formula representing the number of bonded atoms and lone pairs present on the central atom.
It is used to predict the shape and geometry of a molecule or molecular ion using the VSEPR concept.
AXN notation for PO33-
A in the AXN formula represents the central atom. In PO33-, a phosphorus (P) atom is present at the center, so A = P.
X denotes the atoms bonded to the central atom. In PO33-, 3 O-atoms are directly bonded to the central P-atom. So X = 3 for PO33-.
N stands for the lone pairs present on the central atom. As per the Lewis structure of PO33-, the central P-atom has 1 lone pair of electrons. Thus, N = 1 for PO33-.
As a result, the AXN generic formula for PO33- is AX3N1 or simply AX3N.
Now, you may have a look at the VSEPR chart below.
The VSEPR chart confirms that the molecular geometry or shape of a molecule or molecular ion with an AX3N generic formula is trigonal pyramidal while its electron geometry is tetrahedral, as we already noted down for the phosphite (PO33-) ion.
Hybridization of PO33-
The central P-atom is sp3 hybridized in PO33-.
The electronic configuration of phosphorus (P) is 1s2 2s2 2p6 3s2 3p3.
During chemical bonding in PO33-, the 3s atomic orbital of phosphorus hybridizes with its 3p orbitals to produce four sp3 hybrid orbitals.
One of these hybrid orbitals possesses paired electrons which are situated as a lone pair on the central P-atom in PO33-.
In contrast, the other three sp3 hybrid orbitals of phosphorus-containing a single, unpaired electron form P-O sigma bonds by overlapping with the half-filled p orbitals of adjacent oxygen atoms.
Refer to the figure drawn below.
Another shortcut to finding the hybridization present in a molecule or molecular ion is using its steric number against the table below.
The steric number of the central P-atom in PO33- is 4, so it has sp3 hybridization.
Steric number
Hybridization
2
sp
3
sp2
4
sp3
5
sp3d
6
sp3d2
The bond angles of PO33-
The ideal bond angle in a tetrahedral molecule or molecular ion is 109.5°. However, it is due to the distortion present in the molecular shape of the phosphite ion that each O-P-O bond angle reduces below the ideal value.
As per Pauling’s electronegativity scale, a polar covalent bond is formed between two dissimilar atoms with an electronegativity difference between 0.4 to 1.6 units.
A high electronegativity difference of 1.25 units is present between the covalently bonded phosphorus (E.N = 2.19) and oxygen (E.N = 3.44) atoms in each P-O bond.
Therefore, all three P-O bonds present in the phosphite (PO33-) ion are strongly polar in nature.
It is due to the asymmetric trigonal pyramidal shape of PO33- that the P-O dipole moments stay uncancelled.
This results in an overall polar phosphite ion with a non-uniformly distributed electron cloud spread over it (net µ> 0).
The Lewis dot structure of the phosphite (PO33-) ion displays a total of 26 valence electrons, i.e., 26/2 = 13 electron pairs.
Out of the 13 electron pairs, there are 3 bond pairs and 10 lone pairs of electrons.
A P-atom is present at the center of the PO33- Lewis structure, single covalently bonded to three O-atoms at the sides.
There is 1 lone pair of electrons on the central P-atom, while the terminal O-atoms carry 3 lone pairs each.
A -1 formal charge is present on each terminal O-atom leading to an overall formal charge of -3 on the phosphite ion.
In the Lewis structure of PO33-, how many bonds and non-bonding pairs are around the central atom?
The central phosphorus atom is surrounded by three P-O bonds and 1 non-bonding electron pair in the PO33- Lewis dot structure.
What is the charge present on the PO33- ion?
Zero or no formal charge is present on the central P-atom in PO33-.
-1 formal charge is present on each O-atom.
The overall charge present on PO33-= 0 + (-1) + (-1) + (-1) = -3
What is the VSEPR shape of PO33-?
The AXN generic formula for PO33- is AX3N1. To one P-atom at the center, three O-atoms are covalently bonded, and there is a lone pair of electrons on the central P-atom.
Lone pair-bond pair repulsions make the phosphite ion adopt a trigonal pyramidal VSEPR shape and molecular geometry.
Are the electron geometry and molecular shape of PO33- the same?
No. The ideal electron pair geometry of an AX3N1-type molecule or molecular ion such as PO33- is tetrahedral.
However, it adopts a different molecular shape, i.e., trigonal pyramidal, to reduce the lone pair-bond pair repulsive effect present in it, owing to a lone pair of electrons on the central P-atom.
How are the molecular shapes of the following ions the same or different?
Nitrate, nitrite, phosphite, and phosphate
The shape of nitrate (NO3–) ion is trigonal planar. An N-atom at the center is surrounded by three O-atoms. There is no lone pair of electrons on the central N-atom thus, no lone pair-lone pair or lone pair-bond pair electronic repulsions exist in the molecular ion.
This leads to an identical, ideal trigonal planar shape and electron geometry.
Nitrite (NO2–) ion is bent, angular, or V-shaped. The presence of a lone pair of electrons on the central N-atom leads to strong lone pair-bond pair repulsions, thus distorting the overall molecular shape.
The shape of the phosphite (PO33-) ion is trigonal pyramidal. The central P-atom is bonded to three O-atoms and it has a lone pair of electrons.
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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A straight line represents a single covalent bond, i.e., a bond pair containing 2 electrons.
