Octet exceptions

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Marisa Gaitan 2D
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Octet exceptions

Postby Marisa Gaitan 2D » Fri Nov 06, 2020 11:50 am

I know that P, S, and Cl can have more than 8 valence electrons, but I'm still confused as to why this is possible. Also, how will we know when to make these exceptions when drawing Lewis structures? I tried looking back at the lecture, but maybe hearing it in different wording could help me out more. Thanks!

Ethan Laureano 3H
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Re: Octet exceptions

Postby Ethan Laureano 3H » Fri Nov 06, 2020 11:53 am

From my understanding of the lectures, elements greater than or equal to n=3 can have more than 8 electrons. This is because, when thinking about quantum numbers, you can only have l=2 if n is greater than or equal to 3 (l= 0,1... n-1). And when you have l=2, electrons can fill the d-orbital allowing it to exceed 8 electrons.

Jazlyn Romero 1I
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Re: Octet exceptions

Postby Jazlyn Romero 1I » Fri Nov 06, 2020 11:58 am

Hi! From what I understand, the elements P, S, and Cl all have the quantum number of n=3, allowing it to have the possible l values of l= 0,1,2. Because of this, it can have a s (when l=0) , p (when l=1) , and d (when l=2) orbital. This means that these elements can have an expanded octet since they still have the d-orbital in which more electrons can be added to. Hope this helps!

Sedge Greenlee
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Re: Octet exceptions

Postby Sedge Greenlee » Sat Nov 07, 2020 8:17 pm

Remember that the octet guidline is just that, a guidline on how to reach the most stable electron configuration within a molecule. There are however exceptions to this rule, as more stable configurations may be possible. As others have said, expanded octets occur when the atom in question has access to a d-orbital where it can hold more than 8 valence electrons. Additionally, some molecules may not have an even number of valence electrons, leaving one electron unpaired (known as a radical). These molecules are highly reactive as they want to pair that electron with another electron. An easy way to spot when an exception might be used is when it is possible to lower the number of atoms with formal charges that are not equal to zero, as the most stable arrangments of electrons will have all formal charges of 0. As you play with the lewis structure to find the arrangment with the lowest number of atoms with non-zero formal charges, the exceptions should naturally occur. If you need an example, I would recommend his example in lecture on SO42-.

Andre Fabian 1F
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Re: Octet exceptions

Postby Andre Fabian 1F » Sat Nov 07, 2020 8:47 pm

I'm not sure if this answers your question, but I remember hearing from the lecture that Group 13 elements need 5 valence e- to complete their octet. Since it's more difficult for them to gain that many valence electrons, they participate in coordinate covalent bonds, where the Group 13 element forms a molecule with other atoms, is left with 6 valence e-, and another atom/molecule, would "donate" both of its valence e- in a single bond towards the Group 13 element. In this sense, the element with the lowest ionization energy goes in the middle, the lone pair donor is the Lewis base, and the lone pair acceptor is the Lewis acid.

Hope this helps!

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