Lewis structure Chemistry

[Baller97]

Hey guys, so I have my mock Chemistry exam tomorrow morning.

 

For the Lewis structure, what are some exceptions to the octet rule??

Also, are there any specific topics/information that I should know or focus on?

 

Thank you!

[Emmi]

There are two main exceptions you'll see:

 

1. Third row elements and below can have expanded octets. This is because with the n=3 quantum number, you're able to start filling up d atomic orbitals in addition to the s and p orbitals, and these d orbitals have room to accommodate these extra electrons needed to form bonds. A common exam is PCl5.

 

2. Some atoms tend to form incomplete octets. Boron compounds often do this, as do compounds with beryllium. When drawing possible structures for compounds containing these (such as BF3 as an example), the most stable configuration that gives the lowest charges on the individual atoms is that involving an incomplete octet. The highest possible stability is what bonding atoms want to achieve, and this gives the greatest stability for the molecule. Any other arrangement is less stable.

 

Sometimes you'll have an incomplete octet if there are an odd number of valence electrons to share.

 

As for what to study, focusing on what you've covered so far is your best bet most likely. Know what's on the syllabus and you should be fine.