Lewis structure of phosphate ion is drawn clearly in this tutorial step by step. Total valence electrons concept is used to draw the lewis structure of PO43- ion. In lewis structure, there should be charges on atoms.
Phosphate ion is one of the oxyanion of phosphorous. Phosphorous is at +5 oxidation state in PO43-. Also, phosphate ion has a -3 charge.
In the lewis structure of PO43-, three is a double bond between phosphorous atom and one oxygen atom. Between other oxygen atoms, there are only single bonds with phosphorous atom. Also, each oxygen atom has a -1 charge.
Following steps are required to draw the PO43- lewis structure and they are explained in detail in this tutorial.
Drawing correct lewis structure is important to draw PO43- resonance structures correctly.
Phosphorous is located at 5th group in the periodic table. Therefore phosphorous has five valence electrons in its last shell. Oxygen atom is located at sixth group in the periodic table and has six valence electrons in its last shell.
There are four oxygen atoms in PO43- ion, Therefore
There are -3 charge on PO43- ion. Therefore there are three more electrons which comes from outside to contribute to the total valence electrons.
Total valance electrons pairs = σ bonds + π bonds + lone pairs at valence shells
Total electron pairs are determined by dividing the number total valence electrons by two. For, PO43- ion, Total pairs of electrons are 16.
To be the center atom, ability of having greater valance is important. Therefore sulfur has the more chance to be the center atom (See the figure) because sulfur can show valance of 6. Maximum valence of oxygen is two. So, now we can build a sketch of PO43- ion.
After, marking electron pairs on atoms, we should mark charges of each atom. Each oxygen atom will get a -1 charge and phosphorous atom get a +1 charge. The overall charge of ion is ( -1*4 + (+1) ) = -3.
When charges exist everywhere (on atoms) in a ion or a molecule, that structure is not stable. We should try to reduce charges on atoms as much as possible. Now, we are going to learn how to reduce charges of atoms in sulfate ion.
You see charges of atoms in PO43- are reduced. Now, there is no charge in one oxygen atom and phosphorous atom. So we have an stable ion than previous one.
You should know, phosphorous can keep more than eight electrons in its last shell. Therefore we can convert one more lone pair of another oxygen atom to a bond.
But, when you try to do this, phosphorous will get a -1 charge. Earlier in this tutorial, I told you that, most electronegative element should have the negative charges. Therefore, we cannot reduce charges furthermore.
Three oxygen atoms have charges. One oxygen atom holds - 1 charge and overall there is -3 charge.
There are no any charge on phosphorous atom in phosphate ion lewis structure. Also, there is no lone pair or pairs in phosphate ion lewis structure.
Three hydrogen atoms are linked to oxygen atoms of PO43- in H3PO4 molecule. There were -1 negative charge on each of oxygen atom and they will be lost due to joining with hydrogen atoms.
If a hydrogen atom is joint to the phosphorous atom, phosphours atom will get a minus charge which is not acceptable because phosphorous has a lower electronegativity than oxygen.
Around phosphorus atom, three single bonds and one double bond exist. No lone pairs exist on phosphorus atom. Also, no charge exists on phosphorus atom in Lewis structure of PO43-