Molecular Geometries

[sentrosi]

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The question is:

What is the geometry of chloroamine?
A. Trigonal planar
B. Trigonal pyramid
C. Tetrahedral
D. T-Shaped


Chloroamine is NH2Cl

 

[sentrosi]

The answer key is:
The geometry of chloroamine is completely determined by the number of groups of electrons around the central nitrogen. Since the Lewis structure for chloroamine is the same as ammonia, chloroamine has four groups of electrons. Therefore, the geometry of chloroamine is tetrahedral (choice C), and the shape of chloroamine is trigonal planar.


I disagreed since "shape" and "geometry" basically mean the same thing. Either way, I looked it up in my chem book and sure enough if there are four electon groups and one is a lone pair (as it is in this case), the geometry is trigonal planar. My book says nothing about "shape" but uses "molecular geometry".

I'm wondering what I should do on the real test if I get a similar question. Go purely by the number of electron groups or the way it is in my book.

 

[45408]

I would call that trigonal planar, unless the question specifically said to treat the lone pair like any other ligand.

 

[MDtoBe777]

The book is right. The answer is C because they are asking for a geometry not shape. The MCAT will specify which one they want (according to my TPR teacher). If they ask about shape do not include lone pairs. If they ask for geometry include everything! Hope this helps.

 

[liverotcod]

Yup, the geometry is tetrahedral. But I would argue that the shape is closer to trigonal pyramid than trigonal planar. Ammonia is trigonal pyramid, and although the chlorine atom is larger, it still isn't going to make the molecule flat like, say, BH3.

 

[sentrosi]

Is not geometry the study of shapes? My AP chem book and my MIT general chem book both don't ever mention "shape" when talking about molecules...they always say geometry and both say ammonia is trigonal pyramidal.

However since you guys have cleared up that the MCAT uses "shape" and "geometry" differently, I will attempt to go by that for the test.


PS I said trigonal planar in my second post, but I really meant trigonal pyramidal which was the answer I chose.

 

[ASDIC]

i dont see the difference between shape and geometry

 

[liverotcod]

i dont see the difference between shape and geometry

Water, for example, has tetrahedral geometry, and a bent shape. Geometry refers to the orientation of the charge clouds around the central atom, whereas shape refers to the orientation of the atoms.

 

[willthatsall]

Yeah, that's an interesting question. I don't like the wording very much. On the last Kaplan test I took, one of the questions was, "Which of the following molecules has the same geometric configuration of electron pairs about the central nucleus as CH4?" And the answer was H2O. That seems like a better worded question to me. The question asked by the OP should really say, "What is the geometric configuration of the electron pairs," IMO.

 

[liverotcod]

Maybe I'm way off base, here, but it seems like in general the test prep materials have far more ambiguous and poorly written questions that the actual MCAT. It's a shame, because you wind up wasting effort trying to figure out what the question's trying to ask instead of thinking about the answers.

 

[GBFKicks]

Maybe I'm way off base, here, but it seems like in general the test prep materials have far more ambiguous and poorly written questions that the actual MCAT. It's a shame, because you wind up wasting effort trying to figure out what the question's trying to ask instead of thinking about the answers.

The one good thing is that it forces you to understand the question. In this case, it forced people to undersand shape and geometry and the difference between the two. But, I agree with you that those are poor questions.

 

[DieselPetrolGrl]

The book is right. The answer is C because they are asking for a geometry not shape. The MCAT will specify which one they want (according to my TPR teacher). If they ask about shape do not include lone pairs. If they ask for geometry include everything! Hope this helps.

is it true to distinguish between shape and geometry in this way???????...sorry if i sound stupid but i always took them to mean the same thing -- i always count lone pairs as an electron rich region when determining shape

(ps :thanks for your patience)

 

[DieselPetrolGrl]

Water, for example, has tetrahedral geometry, and a bent shape. Geometry refers to the orientation of the charge clouds around the central atom, whereas shape refers to the orientation of the atoms.

nevermind this clarified me! Thanks anyways guys!

 

[CJMPre-Med]

Yeah, that's an interesting question. I don't like the wording very much. On the last Kaplan test I took, one of the questions was, "Which of the following molecules has the same geometric configuration of electron pairs about the central nucleus as CH4?" And the answer was H2O. That seems like a better worded question to me. The question asked by the OP should really say, "What is the geometric configuration of the electron pairs," IMO.

I agree; the text (and professor) I learned chem from made the distinction between molecular geometry (I suppose this would be the "shape" of the molecule that the OP was looking for) and so-called electron-domain geometry, which follows the same shape patterns (trigonal bipyramidal, tetrahedral etc.), but the nonbonding pairs of electrons placed at the vertices of these electron domain geometries served only to affect the molecular geometry by compressing some of the "ideal" bond angles depending on which ligand it was closest to. God I need to review this stuff. 😛

 

[DieselPetrolGrl]

omg this is crazy i have never heard this distinction before -- this prep is blowing my mind on the new junk i am learning everyday

 

[willthatsall]

omg this is crazy i have never heard this distinction before -- this prep is blowing my mind on the new junk i am learning everyday

It's nothing complicated, it's just the reason why NH3 is trigonal pyrimidal instead of trigonal planar. Because the lone pair takes up space too, so the regions of electron density are basically the same as for CH4 (tetrahedral). This causes the bonds to form a pyrimidal shape. Same thing for the 2 lone pairs in water. That is why water is bent instead of linear.