How do we know when an element will have more than 8 electrons?

[OwenSumter_2F]

As the title says, how do we know when an element will have more than 8 electrons? Is it just when making a structure and we realize we need more bonds, or is it something we can predict beforehand? Thanks!

[Ava Nickman]

One way to tell is by looking at which orbital it lands on. For example, certain molecules on n=3 will be able to accept more electrons. Typically you will know based on the total number of valence electrons you calculate at the beginning of the problem and then you can look at the sub shells to double check your work.

[OwenSumter_2F]

Got it, so it relates back to orbitals and where on the periodic table it is?

[Samantha Lee 1A]

Today's lecture discussed the exceptions to the octet rule in detail. An element can have more than the octet due to d-orbitals. He also discussed elements that can have less than the octet rule. Looking at the placement of elements in the periodic table is the best approach.

[Emmeline Phu 1G]

Hi! The octet guideline states that an element is usually most stable when it has 8 electrons; however, there are exceptions to this rule. We know when an element is allowed to have more than 8 electrons when it is in row 3 or greater. This is because starting from row 3 the d-orbital is included which allows the element to have an expanded octet or more electrons. For row 2, only 8 electrons maximum are allowed because there are 2 electrons in the s orbital and 6 electrons in the p orbital. Hope this helps! :)

[AnnaNovoselov1G]

One way to know is by examining the formal charge, which is FC= V (L+ s/2). L represents lone pair electrons and s represents shared electrons. The closer the formal charge is to 0, the more stable the element is.

Usually, elements with an atomic number less than 20 form an octet (based on their orbitals, which can be figured out by their arrangement on the periodic table). Boron and Aluminum are exceptions because they have too few valence electrons (only 3 each) and need to gain 5 e- to form an octet. These elements don't always get an octet. Other elements like S, P, Cl, and F form an expanded octet because they are in the n=3 shell and can have the d-orbitals. The d-orbitals can thus participate in bonding.

Basically, you need to consider the orbital number and type as well as how many electrons an atom wants and how much it wants those electrons.

[Libby Dillon - 1A]

I believe you can know if an atom is able to have more than 8 electrons depending on its principle quantum number. Atoms with n=3 or higher have d-orbitals in valence shells that can accommodate additional electrons, as written in Dr. Lavelle's notes. This would include P, S, and Cl. Based on how many electrons you need to account for in the Lewis structure, you can add extra bonds based on this.