Planar vs Nonplanar

[woox]

I need help identifying between planar and non planar for example why is NH3 planar but not BF3?

Thanks

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[mh0000]

You have them backwards. NH3 is not planar, BF3 is planar.

Count the number of valence electrons; draw the lewis dot structure with the more electropositive element as teh center atom; distribute the electrons around the outer elements, placing any remaining electrons on the center atom; Finally look at the number of electron domains to determine the base geometry. The question might ask for the molecular geometry, which doesn't include any lone pairs of electrons. In such instances, you will take your electronic geometry and determine which group the lone pair of electrons will reside in, and name the geometry based on that.

 

[yappy]

BF3 has 24 electrons. Boron is unique because it is an octet violator (6 e-). When I draw the structure I see that it is trigonal planar with no lone pairs. Therefore, BF3 is planar.

NH3 has 8 electrons. When I draw the structure I have three bonds to the H's and one lone pair. Therefore the electron geometry is tetrahedral (four areas of electron density) and the molecular geometry is trigonal pyramidal (three single bonds to three atoms and one lone pair). This structure is not planar.

Do you guys agree?

 

[Barking Lamb]

Drawing Lewis Dot Structures will certainly get you the right answer, however, I think it's easier to consider the number of bonds and lone pairs on a molecule.

Nitrogen typically forms three bonds. NH3 has three bonds, but, it also has a lone pair (in order to be electrically neutral). The compound is really :NH3. Four regions of electron density are trying to spread out over as much space as possible.

Again, they each want the MAXIMUM amount of space possible. This is a tetrahedral shape. This repulsion gives each area of electron density the most space possible. The -H takes up a slot, the other -H another, the third -H yet another and the electron pair another slot! Four slots, spread out make a tetrahedron.

Imagine if all four bonds were laying flat. They would be at 90 degree angles to each other. Why would this happen? They have the choice to spread out more, so they do.

These same rules apply to BH3; you just need to remember that Boron only forms three bonds (it is an exception to the rule) and has no extra electrons. Three regions of electrons spread out to give each other as much room as possible. That means they lay flat in 120 degrees sections.

The universe wants to spread out as much as possible. Lower energy is better.

The number of bonds atoms form should be intuitive after taking Organic Chemistry.

 

[woox]

So is there a set in stone rule I can memorize to determine if something is planar or non planar. Like BF3 has a electron domain of Trigonal Planar, so all Trigonal Planar compounds are planar.

Which other electron domoin geometry/moleculuar geometery are planar/nonplanar?

 

[jwnichols21]

So is there a set in stone rule I can memorize to determine if something is planar or non planar. Like BF3 has a electron domain of Trigonal Planar, so all Trigonal Planar compounds are planar.

Which other electron domoin geometry/moleculuar geometery are planar/nonplanar?

bump

 

[Demps]

I will introduce something called steric number. It exists from 1 to 6 for the interest of GC questions that we deal with. I'm sure many of you know, but I guess I'm doing the reinforcement.

1 = s
2 = sp
3 = sp2
4 = sp3
5 = sp3d
6 = sp3d2

First draw the Lewis Structure for the molecule. Count all the bonds and lone pairs around the atom of interest. Count and sum up the bonds and lone pairs you have to determine the steric number. Double bonds and triple bonds get the same designations as single bonds. For example, X=C=X, that carbon has two double bonds, but it only counts once, so it has total steric number of 2, not 4.

We all know planar is sp2, this also means that the steric number has to come out to 3 to be planar as well.

so NH3 for this case is 4, so it is sp3 not sp2, so it is not planar.

On the other hand, BF3 has a steric number of 3, so it is sp2, and it is planar.

Hope this clarified. If anything is vague, I will be glad to explain.

 

[jwnichols21]

I will introduce something called steric number. It exists from 1 to 6 for the interest of GC questions that we deal with. I'm sure many of you know, but I guess I'm doing the reinforcement.

1 = s
2 = sp
3 = sp2
4 = sp3
5 = sp3d
6 = sp3d2

First draw the Lewis Structure for the molecule. Count all the bonds and lone pairs around the atom of interest. Count and sum up the bonds and lone pairs you have to determine the steric number. Double bonds and triple bonds get the same designations as single bonds. For example, X=C=X, that carbon has two double bonds, but it only counts once, so it has total steric number of 2, not 4.

We all know planar is sp2, this also means that the steric number has to come out to 3 to be planar as well.

so NH3 for this case is 4, so it is sp3 not sp2, so it is not planar.

On the other hand, BF3 has a steric number of 3, so it is sp2, and it is planar.

Hope this clarified. If anything is vague, I will be glad to explain.

This was really good. But what about a GENERAL rule? Is a steric number of 3 the ONLY possibility of a molecule being planar?

 

[Demps]

This was really good. But what about a GENERAL rule? Is a steric number of 3 the ONLY possibility of a molecule being planar?

yes. We generally see three bonds by separated 120 degrees (no lone pair) as primary examples and I feel like that is the only thing we will ever encounter in depth of DAT.

Something like X=C=X, that is sp

but say that carbon exists as an anion, X=C-X with lone pair with it in the middle, then it should be sp2, and it is planar (Steric number 3 as well).

 

[jwnichols21]

yes. We generally see three bonds by separated 120 degrees (no lone pair) as primary examples and I feel like that is the only thing we will ever encounter in depth of DAT.

Something like X=C=X, that is sp

but say that carbon exists as an anion, X=C-X with lone pair with it in the middle, then it should be sp2, and it is planar (Steric number 3 as well).

Okay, wow this is more clean-cut than I expected. Thank you. Steric # 3 or DIE

 

[Demps]

Okay, wow this is more clean-cut than I expected. Thank you. Steric # 3 or DIE

np. my pleasure

 

[2PacClone23]

Did you guys all memorize the rules for shape/geometry?

Like AX2 = Linear, but AX3E = Trigonal Pyramidal....etc etc?

 

[Demps]

Did you guys all memorize the rules for shape/geometry?

Like AX2 = Linear, but AX3E = Trigonal Pyramidal....etc etc?

I did. You should know your VSEPR.

 

[AlbinoPolarBear]

I did. You should know your VSEPR.

YOU SHOULD DEFINITELY KNOW YOUR VSEPR.

Shouldn't use any gimmicks for this part of studying.