Drawing the Lewis Structure for PO33-

Viewing Notes:

• PO33- has a charge of -3 (and is therefore a negative ion or anion). That means that it has an three extra electrons that needs to be taken into account.
• Since Phosphorous is in Period Three on the periodic table it can have more than eight valence electrons.
• The Lewis structure for PO33- 26 valence electrons available to work with (remember that negative charge?).
• You should check the formal charges to make sure that the total charge of the atom is -3.
• Since it's an ion you need to put brackets around the Lewis structure and then put a -3 outside.

Transcript: Hi, this is Dr. B. Let's do the PO3 3- Lewis structure. Phosphorus, on the periodic table, is in group 5; 5 valence electrons. Oxygen's group 6, but we have three Oxygens. Then we have this minus three up here, 3-, so that means we have an extra three valence electrons so we need to add those on, as well. Five plus 18 plus 3 is 26. So we have 26 valence electrons. We'll put the Phosphorus in the center and then we'll put the Oxygens around the outside.

Two between each of the atoms to form chemical bonds, and then we've used 6. Around the outside, so 8, 10, 24, and then we'll put these last two in the center, 26. So we've used all of our valence electrons. If you calculate the formal charges, you'll see the formal charge on the Phosphorus atom is zero, and negative 1 on each of the Oxygens. This makes sense because we have this negative 3 over here to start out with. Negative 1 plus negative 1 plus negative 1, that's negative 3. So it does make sense to have nonzero formal charges.

What we do need to do, though, is to make sure people realize it is an ion, put brackets around the molecule here to show that. We can then put the 3- up here and everyone will know that this is, indeed, an ion: the phosphite ion.

That's the Lewis structure for PO3 3-. This is Dr. B., and thanks for watching.

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