octet question

[JohnDoeDDS]

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guys why doesnt SeF4 have an octet on the central atom??? Se has 4 bonds so 8 electrons no?

And PBr4+ has an octet on the central atom. can someone exaplin this please.

 

[midwestboy]

SeF4 has 34 electrons (Se has 6 and each F has 7). the equation for no. of electrons shared is S=N-A each atom needs (N) 8 electrons x 5=40. so 40-34=6, meaning Se should only share 3 bonds consisting of 2 electrons each. However, when i drew the structure i found Se to be surrounded by 10 electrons. 4 bonds and a nonbonding electron

 

[desert]

The valance electron of Se is this:
4s2 4p6 3d
// // / /

As you see, we have two full orbitals. But if one electron jump from one p orbital into an empty d orbital, the valence electron configuration will be like this:
4s2 4p6 3d
// / / / /

Now we have four half orbitals that can participate in covalent bond with F.
Considering all electrons that Se can get from covalent bond, Se will have 10 electrons in the valence layer.

Therefore Se will not follow octet rule anymore.

In PBr+, P has the following configuration in the valance layer:
3s2 3p3
// / / /
When P engage covalent bound with Br, it uses three half filled orbitals to Br atom and one full orbital to Br ion (Br+). So the total number of electrons after covalent bonding is 8 that follow the octet rule.

Desert

 

[joonkimdds]

guys why doesnt SeF4 have an octet on the central atom??? Se has 4 bonds so 8 electrons no?

And PBr4+ has an octet on the central atom. can someone exaplin this please.

I don't study chemistry on August due to my religion that says August is BIO month but let me disobey that verse of my bible and answer ur question.

SeF4 will have 34 electrons.
Se in the middle, 4 of F surrounding it. u connect them with single bond.
single bond = 2 electrons , thus we used total of 8 electrons and end up with 26 electrons.
surround F with 6 dots each and that's 26-(4*6)= 2 electrons left.
Now you can put them around Se.

and i believe here is where u got confused.
maybe u think(or i think...)that u can spend those two extra electrons and make double bonds and think it can have resonance form. This is the first option I would have.

then secondly i would be wondering if i can put those two dots around Se and disobey octet rule.

Let me tell u this. There are some exception to this octet rule and u need to either memorize them or be familiar with them. I drew thousands of octet and thus i know that Se is one of those exceptions that can have more than 8.

if u ask me about why!?!?!? explain logically, i can't. The only person who knows what I can't answer is Tinman, lol. he knows everything.

anyway let me write what Kaplan blue book says.

"exceptions to this rule are as follows: hydrogen, which can have only two valence electrons(the configuration of He); lithium and beryllium, which bond to attain two and four valence electrons, respectively; boron, which bonds to attain six; and elements beyond the second row, such as phosphorus and SULFUR, which can expand their octets to include more than eight electrons by incorporating d oritals.