Only certain central atoms have a full octet?

(Polar molecules, Non-polar molecules, etc.)

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dstemp
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Joined: Fri Sep 29, 2017 7:07 am

Only certain central atoms have a full octet?

Postby dstemp » Fri Nov 03, 2017 2:51 pm

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?

Varsha Sivaganesh 1A
Posts: 51
Joined: Thu Jul 13, 2017 3:00 am

Re: Only certain central atoms have a full octet?

Postby Varsha Sivaganesh 1A » Fri Nov 03, 2017 4:53 pm

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
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Joined: Fri Sep 29, 2017 7:04 am

Re: Only certain central atoms have a full octet?

Postby leilawilliams16 » Fri Nov 03, 2017 5:12 pm

So should we always calculate formal charge before drawing the diagram or is this semi-unique?

Hena Sihota 1L
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Re: Only certain central atoms have a full octet?

Postby Hena Sihota 1L » Fri Nov 03, 2017 8:05 pm

@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.

Beza Ayalew 1I
Posts: 58
Joined: Fri Sep 29, 2017 7:07 am

Re: Only certain central atoms have a full octet?

Postby Beza Ayalew 1I » Fri Nov 03, 2017 9:19 pm

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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