Exceptions to octet rule?

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Exceptions to octet rule?

Postby JennyCKim1J » Mon Nov 06, 2017 8:11 am

Can someone list the exceptions to octet rule? I know that
1. Any element from 3rd row or below can have more than 8 e-,
2. Be & B usually don't get all 8 e-, but I don't know if there are any other group of elements like them/the reason why.

Priyanka Bhakta 1L
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Re: Exceptions to octet rule?

Postby Priyanka Bhakta 1L » Mon Nov 06, 2017 12:09 pm


So my understanding is that most p-block elements from Period 3 and on are capable of having expanded octets and thus have more than 8 electrons in their valence shell for Lewis structures and this is because they have unfilled d-orbitals that can hold electrons above the octet.

The book mentions that for most Group 13 elements like B and Al, the atoms don't need a complete octet in the Lewis structure, only about 6 electrons. The textbook also introduces the concept that coordinate covalent bonds can be created with those Group 13 elements in which one element (typically halogens) provides both electrons to complete the incomplete octet from the Group 13 element.

Hope this helps!

Tess McDaniel 1F
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Re: Exceptions to octet rule?

Postby Tess McDaniel 1F » Tue Nov 07, 2017 1:30 pm

Hi! I just wanted to add that the reason why atoms in the 3rd row and below can have expanded octets is that they can utilize the d orbitals in the 3rd energy level or higher. Hope this helps clarify!

Suhail Zaveri
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Re: Exceptions to octet rule?

Postby Suhail Zaveri » Tue Nov 07, 2017 3:10 pm

The main exception to the octet rule is that the d-orbitals can be used to prove that electrons can hold more than 8 electrons, due to the d orbital having 10 electrons and the f orbital having even more. Also, atoms like lithium, boron, and beryllium have too few valence electrons to fulfill the octet rule, so they are considered exceptions such as boron trifluoride where boron does not require 8 electrons. And as explained with the d orbitals, sulfur, chlorine, and phosphorus can hold more than 8 electrons...

Nancy Dinh 2J
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Re: Exceptions to octet rule?

Postby Nancy Dinh 2J » Tue Nov 07, 2017 4:37 pm

In addition to Be and B like you mentioned, I wanted to note that H and He also never follow the octet rule because they are more than happy only filling the 1s orbitals, which is the only type of orbitals the n=1 energy level.

McKenna disc 1C
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Re: Exceptions to octet rule?

Postby McKenna disc 1C » Wed Nov 08, 2017 1:06 am

I just did a problem where Cl was given 12 valence electrons-- is this possible because it is in the 3rd period? And is there a way to determine the limit of how many electrons can be added to those atoms with expanded valence shells?

Cassandra Mullen 1E
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Re: Exceptions to octet rule?

Postby Cassandra Mullen 1E » Wed Nov 08, 2017 8:41 am

With access to the 3d orbital (10 extra electrons), the third energy level can technically hold 18 electrons max.

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