How to tell

[Annabella_Amato_1I]

How can you tell if an element is an exception to the octet rule? Or is this just something that needs to be memorized?

[Alexandra Ahlschlager 1L]

An element can have an expanded octet if its energy level has access to a d orbital. For example, the elements in the 3rd period have access to the 3d subshell, so elements like Phosphorus and Sulfur can have more than the traditional octet. However, elements in the second period like Nitrogen or Oxygen can only have 8 valence electrons, since they only have access to the 2s and 2p subshells. For elements that have less than an octet I think these have to be memorized, but a common one is Boron. Hope this helps!

[Neel Bonthala 2G]

Hey! So just to add on, the first 4 elements on the periodic table are all exceptions to the octet rule. Hydrogen, Helium, Lithium, and Beryllium don't have a p subshell, so they only have 2 orbitals rather than a full 8.

[Melody Wu 2L]

One of the UAs said a good way to remember the 3 elements that can hold more than 8 electrons (P, S, Cl) is to think of them as the "pascal" elements (PaSCal. No tied meaning there but just a helpful mnemonic so to speak. For the ones that hold less than eight, Boron is the most common one and I think any of them that do not have a 2p orbital (only 1s and 2s)

[Gicelle Rubin 1E]

An element can have an expanded octet if its energy level has access to a d orbital. For example, the elements in the 3rd period have access to the 3d subshell, so elements like Phosphorus and Sulfur can have more than the traditional octet. However, elements in the second period like Nitrogen or Oxygen can only have 8 valence electrons, since they only have access to the 2s and 2p subshells. For elements that have less than an octet I think these have to be memorized, but a common one is Boron. Hope this helps!

This was a great way to explain it. Thank you very much!

[Nan_Guan_1L]

Just to add on to the discussion above: I found this on a chem learning website that says there are three general exceptions to the Octet rule:
1. Molecules, such as NO, with an odd number of electrons;
2. Molecules in which one or more atoms possess more than eight electrons, such as SF6; and
3. Molecules such as BCl3, in which one or more atoms possess less than eight electrons.

here is the link to the website:
https://chem.libretexts.org/Bookshelves ... d_Eldredge)/06%3A_The_Structure_of_Atoms/9.6%3A_Exceptions_to_the_Octet_Rule

But according to what Dr. Lavelle's said, I don't really think this is sth we should memorize. From my own personal experience, I find that most of the times it wouldn't be too hard to identify if this is an exception or not because you would notice sth not quite right ie. an atom not reaching octet etc. hope this helps!

[MMorcus2E]

There are multiple exceptions to the octet rule such as B, H, He, Li, and B. I would just memorize the exceptions that I come across just to be safe.