The phosphate ion is an anion derived from phosphoric acid (H3PO4). The chemical formula of phosphate ion is PO43-.
PO43- ions are important for developing biological structural materials such as bones and teeth. They are also essential for plant growth, so phosphate salts are widely used in fertilizer manufacturing.
The phosphate (PO43-) ion is often confused with being a polar specie due to its negative charge. However, it is a non-polar ion. Continue reading this article to learn how.
Is PO43- polar or non-polar?
Although it has a negative charge on the oxygen atoms, the phosphate ion (PO43-) is non-polar. Each P-O and P=O bond in the PO43- ion is polar due to an electronegativity difference between the bonded P and O atoms.
The electronegativity of the oxygen (O) atom is more than the phosphorus (P) atom. The O atoms attract the shared electron pair in each P-O and P=O bond with more influence.
Thus, each bond in the PO43- ion is polar and possesses a dipole moment value.
However, the dipole moments of all the individual bonds get canceled in the symmetric, tetrahedral PO43-ion. Thus, PO43- is a non-polar ion with a net dipole moment equal to 0.
In chemistry, a molecule with an equal charge distribution between different centers of bonded atoms is a non-polar molecule,
It is formed by the covalent bond between two same atoms leading to a symmetric electron density in the molecule overall.
In the case of symmetric molecules, If the dipole moments of individually polar bonds are canceled, the molecule will be non-polar, such as PO43-.
Hence a non-polar molecule has an equal distribution of the electronic charge. Contrarily, if the electronic charge is not evenly distributed over the molecule, in that case, it will be a polar molecule.
The following three factors influence the polarity of any covalent molecule:
Electronegativity.
Dipole moment.
Molecular geometry or shape.
The next section will uncover how these three factors contribute to the polarity of the PO43- ion.
Factors affecting the polarity of PO43-
Electronegativity
It is defined as the ability of an atom to attract a shared pair of electrons from a covalent chemical bond.
Electronegativity decreases down the group in the Periodic Table of elements while it increases across a period.
Greater the electronegativity difference between the bonded atoms in a molecule, the higher the bond polarity.
Phosphorus belongs to group V-A (or 15) of the Periodic Table. The electronic configuration of phosphorus is 1s2 2s2 2p63s23p3, so it has 5 valence electrons for bonding.
On the other hand, oxygen belongs to group VI-A (or 16) of the Periodic Table. The electronic configuration of oxygen is 1s22s22p4, which hints at the availability of 6 valence electrons for bonding.
The 3 O-atoms share one electron, and 1 O-atom shares two electrons with the central P-atom to form three single and one double covalent bonds, respectively.
Atom
Electronic configuration
Valence electrons
Oxygen (8O)
1s22s22p4
6
Phosphorus (15P)
1s2 2s2 2p63s23p3
5
Oxygen (E.N = 3.44) is more electronegative than phosphorus (E.N = 2.19). There is an electronegativity difference of 1.25 units between these two atoms.
Due to this electronegativity difference, the oxygen atom strongly attracts the shared electron cloud from the P=O and P-O bonds. The bonded electrons are held close to the oxygen atom in the PO43- ion.
The O-atoms thus gain a partial negative (Oδ-) charge, while the central P-atom, less electronegative, obtains a partial positive (Pδ+) charge. In this way, oppositely charged poles develop in the PO43- ion.
Consequently, each bond in the PO43- ion is polar.
Dipole Moment
The dipole moment is the product of electrical charge (Q) and bond length (r) between two bonded atoms. It is a vector quantity expressed in Debye (D) units.
It is represented by a Greek symbol µ and measures the polarity of a bond.
The dipole moment of any molecule depends on the difference in electronegativity between the bonded atoms. The greater the electronegativity difference, the higher the bond polarity, resulting in a high dipole moment value.
It points from the partial positive (δ+) center to the partial negative (δ–) center of a bond or molecule.
The dipole moment of each P=O and P-O bond points from Pδ+ to Oδ- due to the electronegativity difference between the P and O atoms.
Thus, the P=O and P-O bonds in PO43- ion are polar, with a dipole moment value.
Molecular geometry
As discussed earlier, a phosphate (PO43-) ion consists of three single P-O and one double P=O covalent bond. There are a total of 32 valence electrons in the overall molecule. There are two lone pairs of electrons on each O atom, while the central P has no lone pair of electrons.
According to the Valence Shell Electron Pair Repulsion Theory (VSEPR) theory of chemical bonding, the PO43- ion is an AX4-type molecule. Around the central phosphorus atom (A) are four bond pairs of electrons (X).
To minimize the electronic repulsions between the atoms, the phosphate (PO43-) ion adopts a symmetric tetrahedral geometry with an O-P-O bond angle of 109.5°.
As a result of the symmetric arrangement of atoms around the central phosphorus, the individual dipole moments of the bonds get canceled in the phosphate (PO43-) ion. There is an equal distribution of electronic charge over the molecule.
In conclusion, the phosphate (PO43-) ion is non-polar with a net dipole moment value equal to zero.
Difference between polar and nonpolar?
Polar molecule
Non-polar molecule
Atoms must have a difference in
electronegativity
Atoms may have the same or different electronegativity values
Unequal charge distribution overall
Equal charge distribution overall
Net dipole moment greater than zero
Net dipole moment equals to zero
Examples include water (H2O), ethanol (CH3CH2OH), ammonia (NH3), sulfur dioxide (SO2), bromine trifluoride (BrF3), nitric oxide (NO), bromine pentafluoride (BrF5), etc.
Examples include oxygen (O2), nitrogen (N2), methane (CH4), ethane (C2H6), propane (C3H8), ethyne (C2H2), sulfur trioxide (SO3), phosphate ion (PO43-), etc.
The individual P=O and P-O bonds in the PO43- ion is polar due to an electronegativity difference between P and O atoms.
But the ion (PO43-) overall is non-polar because it has a symmetric tetrahedral geometry.
The individual dipole moments get canceled in opposite directions. So, net µ becomes zero in the ion.
Thus, PO43- is a non-polar ion with a zero net dipole moment.
Compare the polarity of PO43- and SO42- ions.
Both PO43- and SO42- ions are non-polar.
Although the individual bonds in the PO43- and SO42- ions are polar, the overall ions are non-polar.
Both ions have symmetrical tetrahedral geometry, due to which the individual dipole moments of all the bonds get canceled in opposite directions. So, net µ becomes zero.
Thus, PO43- and SO42- are non-polar with a zero net dipole moment.
Is there a formal charge on the bonded atoms in the phosphate (PO43-) ion?
Formal charge of an atom = [ valence electrons – non-bonding electrons- ½ (bonding electrons)]
For phosphorus atom
Valence electrons = 5
Bonding electrons = 8
Non-bonding electrons = 0
∴ The formal charge on the phosphorus atom = 5-0-8/2 = 5-4 = +1
For oxygen atoms
Valence electrons = 6
Bonding electrons = 2
Non-bonding electrons = 6
∴ The formal charge on the oxygen atom = 6-6-2/2 = 0-1 = -1
-1 + (-1) + (-1) = -3, which accounts for an overall negative 3 charge.
Thus, The formal charge on the phosphate ion (PO43-) is -3.
Summary
Phosphate ion (PO43-) is non-polar.
The individual P=O and P-O bonds are polar due to an electronegativity difference of 1.25 units between the bonded atoms.
The difference in electronegativity between the bonded atoms leads to the formation of dipoles from Pδ+ to Oδ-.
Due to the symmetric tetrahedral molecular geometry of the PO43- ion, the individual dipole moments of all the bonds get canceled in opposite directions.
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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