a Q. on Vsepr

[gomawum]

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According to VSEPR, the H-Te-H bond angle in H2Te should be cloest to
a. 90
b. 120
c. 109.5
d. 149.5
e. 180

since the valence e- is 8, and 4 e- are bonded and 2 pairs are lone,
I would assume that it has bent shape from (tetrahedrea -> trigonal planar -> bent)

so I assumed that angle is b/t 180 ~ 120, but the answer was c,

how come?

 

[hoss19]

i would have said answer A is the best choice. of similar molecules, the bond angles decrease down a group. this makes sense b/c the size of the atom down the group will generally increase so the lone electron pairs of Te can move around a greater area (lower elctron density) which in effect pinches the attached H's closer together. so if H2O H-O-H angle = 104.5, then T2O must be smaller than 104.5.

This leaves A as the only logical choice...which goes against the "given" answer C. Anybody else agree?

 

[bodiddely]

Without thinking about it too much I would just say that with 2 lone pairs and 2 hydrogens, there are 4 electron groups around the central atom, so the electron geometry is going to be tetrahedral, resulting in angles of ~109.5 between the electron groups.

You can also get a "bent" shaped molecule from 3 electron groups (2 atoms and one lone pair), with angles of ~120 but it's the electron group geometry not the molecular geometry that determines angles.

I could be overlooking something really obvious but that's the explanation that first comes to mind.

 

[A8]

You are right. It is bent, but the angle for the bent is 109.5 not 120.

120 is for trigonal planar not the bent structure.

 

[bodiddely]

You are right. It is bent, but the angle for the bent is 109.5 not 120.

120 is for trigonal planar not the bent structure.

You have to keep in mind the distinction between molecular geometry and electron group geometry. Both tetrahedral and trigonal planar electron geometries can result in a bent molecular geometry, but it's the electron group geometry that determines the angles. Simply knowing the molecular geometry doesn't tell you what the bond angles will be.

 

[hoss19]

http://books.google.com/books?id=Lm...ts=FsUaYQvTke&sig=20M83sqlk76K9Dy3OXbuqb-RbbI

http://www.newton.dep.anl.gov/askasci/chem03/chem03447.htm

i will not "post" these b/c some of this is copyrighted, but the links are for you to explore if you want to know the real answer....

 

[2009er]

According to VSEPR, the H-Te-H bond angle in H2Te should be cloest to
a. 90
b. 120
c. 109.5
d. 149.5
e. 180

don't think too much into the question!!! when looking at the molecule u know that it resembles that of H2O since Te is int he same group of the periodic table as O. since H2Te has 4 electron pairs, an approximate tetrahedral arrangement is formed. tetrahedral= 109.5 (same as CH4).
H2O has a bent shape which is at 105, just slightly less than the tetrahedral. if there was a 105 choice i would have chosen that, however since there isn't a 105 then the closest is 109.5.
u know that it can not be larger than 109.5, and 90 is too small!!!

hope this helps.

 

[chessxwizard]

I'd have to agree with the other posts. It can't be greater than 109.5. Since there are lone pairs attached to the center molecule, they repel each other and make the bond angle even smaller, close to 105.