(Polar molecules, Non-polar molecules, etc.)
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In the lecture today, in a couple different molecules the central atom did not have a full octet. For example, in Beryllium Chloride BeCl2 and in BF3, the Beryllium and Boron only had four shared electrons and six shared electrons respectively. Why is this? Especially when there are enough lone pair electrons to give the central atom a full octet. Furthermore, why can only certain atoms take on more than an octet e.g. Phosphorus in Phosphorus Pentachloride PCl5?
Usually it has to do with the formal charge. Some atoms work better when they don't have an octet because it makes their formal charge closer to 0. For example, if boron is bonded to three fluorine atoms via single bonds, the formal charge of all of those atoms is 0. However, if you were to give boron an octet, its formal charge would be -2 I believe.
@leilawilliams16 You don't always have to calculate the formal charge after drawing a lewis structure, because formal charge is only used to determine the most stable structure when molecules or ions have resonance, and therefore resonance structures. For example, CH4 does not have resonance, so you would not have to calculate the formal charge.
Yea, I think the priority when making a Lewis Structure is to make sure you have the right number of electrons and that the more electronegative atoms (which are the outer atoms since the central atom is usually the less electronegative atom) have an octet, which may mean the central atom doesn't get an octet
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