Step 1: Find valence e- for all atoms. Add them together.

P:5
O:6x4=24

Total=29+3=32*

*Add 3 electrons since the molecule has 3- charge on it.

Step 2: Find octet e- for each atom and add them together.

Si:8
O=8x4=32

Total=40

Step 3: Gives you bonding e-. Subtract step 1 total from step 2

40-32=8e-

Step 4: Find number of bonds by diving the number in step 3 by 2(because each bond is made of 2 e-)

8e-/2= 4 bond pairs

Step 5: Find the number of nonbonding (lone pairs) e-. Subtract step 3 number from step 1.

32-8= 24e-=12 lone pair

Use information from step 4 and 5 to draw the lewis structure.

Phosphorous goes in the centre.

Lewis dot structure of PO₄^3-

Alternatively a dot method can be used to draw the lewis structure.
Calculate the total valence electrons in the molecule.
P:5
O:6x4=24

Total=29+3=32*

Put phosphorous in center and arrange oxygen atoms on the sides.Arrange electrons until each atom get 8 electrons.

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