Shape Memorization [ENDORSED]

[MadisonFuentes1G]

With all of the VSEPR names of the shapes that we have to memorize, does anyone have any ways of memorizing the number of the types and the names of the shapes we need to know?

[Hazem Nasef 1I]

I would say a good way to memorize them is to organize the shapes by electron domain geometry. Then, you should know that there are shapes whose molecular geometry matches its electron domain geometry, and shapes with one or more lone pairs replacing an atom.

For example, for tetrahedral electron domain, you know that a molecule can be tetrahedral in shape with 4 atoms around a central atom. Also, a molecule can have only 3 atoms and one lone pair around a central atom, which is trigonal pyramidal. Lastly, a molecule can have only 2 atoms and 2 lone pairs around the central atom, which is a bent molecule.

[Michael Cheng 1C]

What is the difference between trigonal planar and trigonal bipyramidal?

[Vincent Grospe 3C]

@Michael

In general, trigonal planar has a VSEPR formula of AX₃ — three bonded atoms to the central atom, while trigonal bipyramidal has a VESPR formula of AX₅ — five bonded atoms to the central atom.

When I think of the word “planar”, I think one-dimensional or one plane. So the bonded atoms are attached to the central atom with bond angles of 120 degrees.


On the other hand, when I think of the word “bipyramidal”, I think of multidimensional or two pyramids. So in the horizontal plane, three atoms are bonded to central atom with bond angles of 120 degrees. While viewing the molecule more vertically, there is are two atoms vertically attached to both sides of central atom with a bond angles of 90 degrees with those three other atoms. So the shape looks like two pyramids — one pointing up and the other point down — while they have the same base. However, we see the multi-dimensional aspect, in that we must distinguish the dimensions/axis of those three atoms when drawing them in the plane of the paper. So when writing their VSEPR models, one has wedged lines (to show it is facing toward the viewer), one has dashed lines (facing away from the viewer), and one has solid lines (in the same plane of the paper).

[Rakhi Ratanjee 1D]

Re: What is the difference between trigonal planar and trigonal bipyramidal?

Trigonal planar has the VSEPR formula of AX3E0, meaning that the molecule has 3 bonded pairs and 0 lone pairs on the central atom, making the bond angles each 120 degrees. Ex) BH3
Trigonal bipyramidal has a VSEPR formula of AX5E0, meaning that the molecule has 5 bonded pairs and 0 lone pairs on the central atom, making the bond angles 90 degrees and 120 degrees. Ex) PCl5
Therefore, essentially the only difference is number of bonds the the central atom makes. The more bonded and/or lone pairs on the central atom, the smaller the bond angles are to maximize electron repulsion, although lone pairs have a greater region of electron concentration.

[Karan Singh Lecture 3]

I agree the best way to memorize all of these different structures is to arrange the electrons by the total number of bonds in each geometry.

[Alexia Joseph 2B]

It's fairly easy to remember the shapes when you classify molecules by their electron domain geometry. Try to visualize the structures Dr. Lavelle showed during lectures to remember the names. Molecules with the same formula have the same general shape (bond angles may differ slightly).

For example:
AX2 (2 regions of electron density)
-AX2: 2 bonding pairs, linear, bond angle of 180

AX3 (3 regions of electron density)
-AX3: 3 bonding pairs, trigonal planar, bond angle of 120

AX4 (4 regions of electron density)
-AX4: 4 bonding pairs, tetrahedral, bond angle of 109.5
-AX3E: 1 lone pair, 3 bonding sites, trigonal pyramidal, bond angle less than 109.5
-AX2E2: 2 lone pairs, 2 bonding sites, bent, bond angle less than 109.5

AX5 (5 regions of electron density)
-AX5: 5 bonding pairs, trigonal bipyramidal, bond angles of 90, 120, 180
-AX4E: 1 lone pair, 4 bonding sites, see-saw, bond angles less than 90, less than 120, more than 180

AX6 (6 regions of electron density)
-AX6: 6 bonding pairs, octrahedral, bond angles of 90, 180
-AX4E2: 2 lone pairs, 4 bonding sites, square planar, bond angles are 90 degres (axial)

[Reyna Alonso 1E]

I would say that looking at visual examples will be the most helpful way to memorize the shapes.