When to expand an octet

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Isabelle Bautista 3H
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When to expand an octet

Postby Isabelle Bautista 3H » Mon Nov 06, 2017 2:33 pm

So for question 3.93 part (b) is asks us to draw the Lewis structure and all resonance structures for BrO+. Since Bromine can have an expanded octet, why wouldn't there be a resonance structure that has a triple bond between O and Br, then 2 lone pairs on Br, and one lone pair on O. Do expanded octets only occur when they are needed to fit more atoms around the central atom?

Johann Park 2B
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Re: When to expand an octet

Postby Johann Park 2B » Mon Nov 06, 2017 2:56 pm

If I am understanding your question correctly, in looking at BrO+, we're split between choosing a double bond, Br and O with two lone pairs vs. a triple bond where Br has two lone pairs and O has one?

Because we can draw the structure of :Br: = :O:, with a double bond and two lone pairs on each atom, we have a formal charge of +1 and also satisfy the octet rule. I think that we only break the octet rule on these "exception atoms" when we have to. Otherwise, satisfying the octet rule when we can requires less energy/is more stable.

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Re: When to expand an octet

Postby JonathanLam1G » Mon Nov 06, 2017 4:46 pm

I agree. I believe the order of priority when drawing Lewis Structures should be:
1) Can the atom have an expanded octet? or can it only hold 8 valence electrons?
2) Are the formal charges of the atoms the lowest possible?
3) If there must be a negative charge, is it placed on the most electronegative atom?

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