Resonance structures of PO43- ion can be drawn by using lewis structure of phosphate ion. four stable resonance structures can be drawn for PO43- ion. These resonance structures are used to build resonance hybrid.
Lewis structure of PO43- ion is important because it is required to draw resonance structures of phosphate ion.
Let's draw four stable four resonance structures for the phosphate anion (NO3-).
When we draw resonance structures, we convert lone pairs to bonds and bonds to lone pairs when it is possible. When we do this, you should be careful to protect stability of ion, octal rule.
In lewis structure of PO43- ion, there are three lone pairs (in the last shell) in three oxygen atoms and that oxygen atoms. Also, those oxygen atoms have a -1 charge on each atoms.
There is another oxygen atom. That oxygen atom is connected to the phosphorous atom by a double bond has two lone pairs in its last shell. Also, there is no charge in that oxygen atom.
On phosphorous atom, there are no lone pair or a charge.
You can convert a lone pair of one oxygen atom which already has three lone pairs to make a bond with phosphorous atom. With that, total electrons around phosphorous atom is going to be twelve. It is not a problem because phosphorous can keep more than eight electrons in its valence shell (phosphorous has 3d orbitals which help to keep more than eight electrons).
With that electron transferring, number of bonds around the phosphorous atom become 6. Phosphorous atom gains -1 charge. Therefore, this is not a best resonance structure because oxygen should gain the negative charges (electronegativity of oxygen is higher than phosphorous).
To obtain a stable resonance structure, convert a bond of former double bond as in the figure.
Now, phosphorous do not have a charge and three oxygen atom has charges. As like this, we can convert lone pairs to bonds and vice versa to draw four resonance structures of phosphate ion.
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