How to determine the molecular shape?

[joey.jiayi]

Hi guys,
I'm a Unit 1&2 Chemistry student and I am struggling to determine the molecular shape of covalent compounds. Is there any tips to identify the molecular shape of the covalent compounds? Also, I was at the ATAR notes chem 1&2 lecture and it says "electron domain". How do we determine the number of electron domains?

Thank you

[Sine]

Hi guys,
I'm a Unit 1&2 Chemistry student and I am struggling to determine the molecular shape of covalent compounds. Is there any tips to identify the molecular shape of the covalent compounds? Also, I was at the ATAR notes chem 1&2 lecture and it says "electron domain". How do we determine the number of electron domains?

Thank you

Electron domains is the number of lone pairs + number of bonds of a molecule.

By molecular shape do you mean stuff like "bent", "tetrahedron", "planar", "linear"?

[joey.jiayi]

Electron domains is the number of lone pairs + number of bonds of a molecule.

By molecular shape do you mean stuff like "bent", "tetrahedron", "planar", "linear"?

Is it the number of lone pairs + number of bonds of the central atom (eg. the C in carbon dioxide) or the entire compound?

Yes, those are the molecular shapes.

[coldairballoon]

Hey, I'm doing chem 1/2 as well! I can't say if my explanation is completely correct, but it's gotten me by pretty well.

Firstly, the main shapes you need to know are: linear, bent, trigonal planar, (trigonal) pyramidal, and tetrahedral.
You can use VSEPR to determine the shape. Basically it just means:
1. Electron pairs repel other pairs as much as possible, so they try to be equally spaced from each other.
2. BUT: lone pairs repel other lone pairs AND bonded pairs just a teeny bit more than bonded pairs repel lone pairs or other bonded pairs.
3. Double/triple bonds don't move
4. In VSEPR you only focus on the electron pairs around the 'central' atom of the molecule.

It might also help to image in your head that even though free pairs repel other free pairs more than they repel bonded pairs, they kinda keep to themselves - they won't go between 2 bonded pairs if they can help it. (If that doesn't make sense (sorry) look at the attached h2o diagram). This helps for bent/pyramidal shapes.

Apart from these rules, VSEPR is fairly intuitive (in this I'm assuming that there are no double/triple bonds).
- If there are only 2 atoms it's linear (duh)

For 3-atom molecules:
- Linear if there are no lone pairs. Or if there double/triple bonds. A lot of the time that's the same thing. Anyway, it's because the bonded pairs mutually repel each other with the same strength.
- Bent if there are 2 lone pairs and 2 bonded pairs. Because remember, that the 2 lone pairs repel each other more than the bonded pairs, so they kind of 'hog' the space and force the 2 bonded pairs to be closer together, bending the actual non-central atoms closer together. Hence bent.

For 4-atom molecules:
- Trigonal planar if there are no lone pairs. All the bonded pairs repel each other w/ the same strength so they're equally spaced apart (the diagrams make them look 'flat' but they're not actually). So the 3 outer atoms are also spaced equally apart.
- Trigonal pyramidal if there's 1 lone pair. The lone pair repels the 3 bonded pairs more than the bonded pairs repel the free pair or the other bonded pairs. Hence the free pair hogs more space and the 3 bonded pairs (as well as the 3 outer atoms) are clumped closer together.

One thing that tripped me up about VSEPR at first was that I kept forgetting that the diagrams are actually 3D. (But then when I actually understood it, the fact that it was 3D sometimes confused me more, haha)

I've also attached a pic here that my teacher used (it might be a confusing at the start though). Hope this helps!

[joey.jiayi]

Hey, I'm doing chem 1/2 as well! I can't say if my explanation is completely correct, but it's gotten me by pretty well.

Firstly, the main shapes you need to know are: linear, bent, trigonal planar, (trigonal) pyramidal, and tetrahedral.
You can use VSEPR to determine the shape. Basically it just means:
1. Electron pairs repel other pairs as much as possible, so they try to be equally spaced from each other.
2. BUT: lone pairs repel other lone pairs AND bonded pairs just a teeny bit more than bonded pairs repel lone pairs or other bonded pairs.
3. Double/triple bonds don't move
4. In VSEPR you only focus on the electron pairs around the 'central' atom of the molecule.

It might also help to image in your head that even though free pairs repel other free pairs more than they repel bonded pairs, they kinda keep to themselves - they won't go between 2 bonded pairs if they can help it. (If that doesn't make sense (sorry) look at the attached h2o diagram). This helps for bent/pyramidal shapes.

Apart from these rules, VSEPR is fairly intuitive (in this I'm assuming that there are no double/triple bonds).
- If there are only 2 atoms it's linear (duh)

For 3-atom molecules:
- Linear if there are no lone pairs. Or if there double/triple bonds. A lot of the time that's the same thing. Anyway, it's because the bonded pairs mutually repel each other with the same strength.
- Bent if there are 2 lone pairs and 2 bonded pairs. Because remember, that the 2 lone pairs repel each other more than the bonded pairs, so they kind of 'hog' the space and force the 2 bonded pairs to be closer together, bending the actual non-central atoms closer together. Hence bent.

For 4-atom molecules:
- Trigonal planar if there are no lone pairs. All the bonded pairs repel each other w/ the same strength so they're equally spaced apart (the diagrams make them look 'flat' but they're not actually). So the 3 outer atoms are also spaced equally apart.
- Trigonal pyramidal if there's 1 lone pair. The lone pair repels the 3 bonded pairs more than the bonded pairs repel the free pair or the other bonded pairs. Hence the free pair hogs more space and the 3 bonded pairs (as well as the 3 outer atoms) are clumped closer together.

One thing that tripped me up about VSEPR at first was that I kept forgetting that the diagrams are actually 3D. (But then when I actually understood it, the fact that it was 3D sometimes confused me more, haha)

I've also attached a pic here that my teacher used (it might be a confusing at the start though). Hope this helps!

Thank you so much! I think I'm starting to get the hang of it