A-Level Chemistry help please - Exceptions to the octet rule

Watch
Anonymous -
Badges: 12
Rep:
?
#1
Report Thread starter 1 month ago
#1
I'm in year 12 and have been making revision cards for dative covalent bonding etc. However, I am really unsure about one of the exceptions to the octet rule? I don't understand how elements in groups 5, 6 and 7 in Period 3 and above can expand their octet.
I know that they expand their octet by accessing empty d-orbitals, but was wondering how this is possible, as I thought that the outer electrons go into a p-orbital??

I don't know if my question really makes sense, but basically, I don't understand how/why they can access the empty d-orbitals.

Any help would be really appreciated
0
reply
golgiapparatus31
Badges: 13
Rep:
?
#2
Report 1 month ago
#2
Period 1 elements have s orbitals
Period 2 elements have s, p orbitals
Period 2 elements obey the octet rule without exception, since they do not have access to the d subshell.

Period 3 elements have s, p, d orbitals. Once s and p subshells are filled, d orbitals can be used. d-orbitals are higher in energy than p orbitals. But the energy released from making a new bond using a d orbital compensates for this, making the use of d orbitals energetically feasible. e.g Al2Cl6, PCl5

Useful reading: https://courses.lumenlearning.com/bo...he-octet-rule/
0
reply
Anonymous -
Badges: 12
Rep:
?
#3
Report Thread starter 1 month ago
#3
(Original post by golgiapparatus31)
Period 1 elements have s orbitals
Period 2 elements have s, p orbitals
Period 2 elements obey the octet rule without exception, since they do not have access to the d subshell.

Period 3 elements have s, p, d orbitals. Once s and p subshells are filled, d orbitals can be used. d-orbitals are higher in energy than p orbitals. But the energy released from making a new bond using a d orbital compensates for this, making the use of d orbitals energetically feasible. e.g Al2Cl6, PCl5

Useful reading: https://courses.lumenlearning.com/bo...he-octet-rule/
Thank you for the help! I think I understand now

Just to check that I am understanding correctly, it's because the third shell can hold up to 18 electrons, and because the highest used orbital in Period 3 elements is the 3p orbital, the 3d orbital is unused?

How does this work during the actual bonding? As in, how is this empty orbital used? (I don't think I have worded that too well, so my apologies).
0
reply
golgiapparatus31
Badges: 13
Rep:
?
#4
Report 1 month ago
#4
(Original post by Anonymous -)
Thank you for the help! I think I understand now

Just to check that I am understanding correctly, it's because the third shell can hold up to 18 electrons, and because the highest used orbital in Period 3 elements is the 3p orbital, the 3d orbital is unused?

How does this work during the actual bonding? As in, how is this empty orbital used? (I don't think I have worded that too well, so my apologies).
Yup, the 3d subshell is empty in Period 3 elements.

d orbitals can be involved in sigma bonding, pi bonding or delta bonding (involvement of d-orbitals in bonding is beyond A-level). Example: https://s3-us-west-2.amazonaws.com/c...wrnd6nl49w.png (pi bonding involving p and d orbitals)

Read here for more: https://www.chemguide.co.uk/atoms/bo...3dhybrids.html
The link does not go into much detail as this is beyond the syllabus. An elementary understanding that d orbitals can undergo hybridisation and form bonds is sufficient. It isn't worth going into more detail as you won't actually be asked this at A-level.
Last edited by golgiapparatus31; 1 month ago
1
reply
Anonymous -
Badges: 12
Rep:
?
#5
Report Thread starter 1 month ago
#5
(Original post by golgiapparatus31)
Yup, the 3d subshell is empty in Period 3 elements.

d orbitals can be involved in sigma bonding, pi bonding or delta bonding (involvement of d-orbitals in bonding is beyond A-level). Example: https://s3-us-west-2.amazonaws.com/c...wrnd6nl49w.png (pi bonding involving p and d orbitals)

Read here for more: https://www.chemguide.co.uk/atoms/bo...3dhybrids.html
The link does not go into much detail as this is beyond the syllabus. An elementary understanding that d orbitals can undergo hybridisation and form bonds is sufficient. It isn't worth going into more detail as you won't actually be asked this at A-level.
Ohhh okay, that makes so much more sense - thank you very much!

I had a look at the second link about the hybridisation which has made it very clear 👍. I find it quite interesting how the bonding occurs in compounds such as PCl5!

Thanks again, you've been a massive help and I really appreciate it
1
reply
X

Quick Reply

Attached files
Write a reply...
Reply
new posts
Back
to top
Latest
My Feed

See more of what you like on
The Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

Personalise

Should there be a new university admissions system that ditches predicted grades?

No, I think predicted grades should still be used to make offers (715)
33.87%
Yes, I like the idea of applying to uni after I received my grades (PQA) (904)
42.82%
Yes, I like the idea of receiving offers only after I receive my grades (PQO) (399)
18.9%
I think there is a better option than the ones suggested (let us know in the thread!) (93)
4.41%

Watched Threads

View All
Latest
My Feed

See more of what you like on
The Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

Personalise