### Determining Shape

Posted:

**Wed Oct 21, 2015 11:47 am**When determining the shape of a compound, how can it be labeled as both tetrahedral and trigonal pyramidal?

Created by Dr. Laurence Lavelle

https://lavelle.chem.ucla.edu/forum/

https://lavelle.chem.ucla.edu/forum/viewtopic.php?f=41&t=6548

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Posted: **Wed Oct 21, 2015 11:47 am**

When determining the shape of a compound, how can it be labeled as both tetrahedral and trigonal pyramidal?

Posted: **Wed Oct 21, 2015 11:57 am**

Hi Rachel!

Tetrahedral and trigonal pyramidal structures are the same. A terahedral structure is one in the shape of a tetrahedron, or a 4 sided three-dimensional figure. Tetrahedrons are typically composed of four equal, equilateral triangles: one "base" and three "sides."

Similarly, a trigonal pyramidal structure is a pyramid which has a 3-sided (TRIgonal) base.

Both of these shapes may have been called "triangular pyramid" in a past math course such as geometry.

I hope this helps!

Renee Crippen 3H

Tetrahedral and trigonal pyramidal structures are the same. A terahedral structure is one in the shape of a tetrahedron, or a 4 sided three-dimensional figure. Tetrahedrons are typically composed of four equal, equilateral triangles: one "base" and three "sides."

Similarly, a trigonal pyramidal structure is a pyramid which has a 3-sided (TRIgonal) base.

Both of these shapes may have been called "triangular pyramid" in a past math course such as geometry.

I hope this helps!

Renee Crippen 3H

Posted: **Wed Oct 21, 2015 3:00 pm**

The tetrahedral and trigonal pyramid geometries are not exactly the same (although the general shape is). A tetrahedron contains four equivalent sides, meaning it must have a central atom in the center. A trigonal pyramid is the same shape, but each face is not equivalent (meaning it must have a central atom at the top). I included a picture and hopefully that makes it more clear (the base is shaded in pencil).