(Polar molecules, Non-polar molecules, etc.)
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For part b in homework problem 13 in chapter 4, it wants you to write the Lewis structure for POCl3. The answer has P as the central atom and Cl, Cl, Cl, and O surrounding it in a tetrahedral shape. However, oxygen has a double bond. How do you know that oxygen has a double bond and it doesn't just have three electron pairs and a bond to P, like the other three Cl atoms do?
P can have an expanded octet. You want to achieve the lowest normal charge possible. That is when O has two bonds and two lone pairs. This gives it a formal charge of 0. If it only had one bond, it would have a formal charge of -1.
This has to do with recognizing the overall charge of the molecule. Because no charge is specified we assume that the molecule is neutral. Next we need to draw the lewis structure that would lead to a neutral molecule. In order to do this you just need to calculate the sum of the formal charges for each atom that is part of the molecule. The chlorines and phosphorus have 0 formal charges so we know that the oxygen must also have a formal charge of 0 and an oxygen double bonded to another atom has a formal charge of 0 whereas an oxygen single bonded has a formal charge of -1.
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