Hybridization

[Shinobiz11]

Hey I am learning about hybridization in my chem class and I don't really understand the reading. My teacher hasn't explained it yet so I was wondering if someone could clear it up and make it easier to understand.

So far I understand that a molecule which is suppose to be a certain VSEPR shape but it changes due to some factor. This is the part I do not understand very well. Also I get how the atoms change so their is a large one and a small one.

My main questions are, how do you know when the shape is suppose to change and why does the central atom have four different large parts and each one has a smaller part ( in CH4-Methane).

CH4 is suppose to have 90* angles between all of the H atoms, but instead it changes to a tetrahedral and has 109.5*. Can someone explain why this happens?

Thanks in advance

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[silverhorse84]

Get a molecular model set (either the cheap way of marshmellows and toothpicks, or an actual model set) and try to get four things (the Hydrogens) around a circle (the Carbon) so that they are as far apart from each other as possible. From what I can tell, you're thinking of this in the 2D and you need to think of these in the 3D (that's where the models come it - it can be hard to think in 3D space in your head).
It's difficult for me to explain in text like this - I used to teach this, but in person is a lot easier. You might want to take the model set to the teacher and ask him/her to explain it.

Also, you're really not supposed to post homework help on this site. Just FYI

 

[Procyon]

Hey I am learning about hybridization in my chem class and I don't really understand the reading. My teacher hasn't explained it yet so I was wondering if someone could clear it up and make it easier to understand.

So far I understand that a molecule which is suppose to be a certain VSEPR shape but it changes due to some factor. This is the part I do not understand very well. Also I get how the atoms change so their is a large one and a small one.

My main questions are, how do you know when the shape is suppose to change and why does the central atom have four different large parts and each one has a smaller part ( in CH4-Methane).

CH4 is suppose to have 90* angles between all of the H atoms, but instead it changes to a tetrahedral and has 109.5*. Can someone explain why this happens?

Thanks in advance

I'm not quite sure what you mean by "large parts" and "smaller parts". Can you elaborate on that a little?

I can tell you why some atoms have less than ideal bond angles though. CH4 has four bonds and no lone pairs, so it's tetrahedral; the bond angles for a tretrahedral molecular geometry, are in fact, 109.5 degrees, so that isn't really abnormal. (90 degree angles are normally seen in octahedral geometries, and in the axial bonds on a triagonal bipyramidal molecule.)

But let's take a look at water instead of CH4. If you drew out the Lewis dot structure for H2O, you would notice that there's two single bonds, and two lone pairs of electrons on the central atom. Electrons, by their nature, repel one another. So, in a nutshell, the repulsive force between the two lone pairs of electrons and the two bonds pushes the bonds away from the lone pairs, giving the molecule a bent shape and 104.5 degree bond angles.

Now let's look at phosgene, COCl2. The idea behind why the bond angles in phosgene are less than ideal is similar. There is one double bond in phosgene, and two single bonds. This means that there should be a triagonal planar geometry, and ideally the bond angles would be 120 degrees, right? Not quite. Double and triple bonds, like the lone pairs, have greater repulsive force, so the bond angle in phosgene is 111.8 degrees rather than 120.

So basically, if the central atom has any lone electron pairs, or if double/triple bonding is involved, the bond angles will be less than assumed by VSEPR theory, because the repulsive forces of the electrons push the bonds in other directions, distoring the molecule.

Hope I explained that ok. I'm sure if I screwed up, someone else can help.

 

[Shinobiz11]

I'm not quite sure what you mean by "large parts" and "smaller parts". Can you elaborate on that a little?

I can tell you why some atoms have less than ideal bond angles though. CH4 has four bonds and no lone pairs, so it's tetrahedral; the bond angles for a tretrahedral molecular geometry, are in fact, 109.5 degrees, so that isn't really abnormal. (90 degree angles are normally seen in octahedral geometries, and in the axial bonds on a triagonal bipyramidal molecule.)

But let's take a look at water instead of CH4. If you drew out the Lewis dot structure for H2O, you would notice that there's two single bonds, and two lone pairs of electrons on the central atom. Electrons, by their nature, repel one another. So, in a nutshell, the repulsive force between the two lone pairs of electrons and the two bonds pushes the bonds away from the lone pairs, giving the molecule a bent shape and 104.5 degree bond angles.

Now let's look at phosgene, COCl2. The idea behind why the bond angles in phosgene are less than ideal is similar. There is one double bond in phosgene, and two single bonds. This means that there should be a triagonal planar geometry, and ideally the bond angles would be 120 degrees, right? Not quite. Double and triple bonds, like the lone pairs, have greater repulsive force, so the bond angle in phosgene is 111.8 degrees rather than 120.

So basically, if the central atom has any lone electron pairs, or if double/triple bonding is involved, the bond angles will be less than assumed by VSEPR theory, because the repulsive forces of the electrons push the bonds in other directions, distoring the molecule.

Hope I explained that ok. I'm sure if I screwed up, someone else can help.

Ahhh, thank you, this makes a lot more sense.

My textbook says that CH4 should have all 90* angles because of the structure and the hydrogen's are perfectly spaced, but instead it changes to a tetrahedral. I guess it doesn't really matter.

Also with the big shape and little shape, here is a picture.

the "an sp2 hybrid orbital"

Also I guess I don't understand this to the fullest because I am not too good with spacial reasoning. BTW, this is not for homework, you are allowed to post topics if you need help with a subject.

Thanks for the explanation though, it really helped. So for example if I was doing BrF3, which has two lone pairs, instead of being triagonal planer it would be T-shaped?

 

[circulus vitios]

My textbook says that CH4 should have all 90* angles because of the structure and the hydrogen's are perfectly spaced, but instead it changes to a tetrahedral. I guess it doesn't really matter.

Carbon's electron configuration after electron promotion is 2S1 2P3. The S orbital is a sphere, and the 3 P orbitals are Px, Py, and Pz...all 90 degrees apart. This is too close together, so the orbitals hybridize into SP3 orbitals which are then spaced in a tetrahedral arrangement 109.5 degrees apart.

Thanks for the explanation though, it really helped. So for example if I was doing BrF3, which has two lone pairs, instead of being triagonal planer it would be T-shaped?

The electron pair geometry is trigonal bipyramidal and the molecular geometry is T shaped.

 

[Shinobiz11]

Yeah, my teacher explained it today in class, I got this.

 

[rctriplefresh5]

Carbon's electron configuration after electron promotion is 2S1 2P3. The S orbital is a sphere, and the 3 P orbitals are Px, Py, and Pz...all 90 degrees apart. This is too close together, so the orbitals hybridize into SP3 orbitals which are then spaced in a tetrahedral arrangement 109.5 degrees apart.



The electron pair geometry is trigonal bipyramidal and the molecular geometry is T shaped.

no the electron arrangement would be trigonal bipyramidal. you are correct about the geometrys though;(tshaped)