Ask A Scientist© Chemistry Archive

name         Sandesh S.
status       student
age          16

Question -   What exactly is a tetrahedral?
Our teacher says that it's supposed to be a six sided figure; but the
structure of methane looks somewhat triangular. And, to add to the
confusion, she says that a square is tetrahedron !! Well, what exactly is it ?

(Can it be formed by sticking two egyptian pyramids along their bases?)

A tetrahedron is a solid with four faces.  It has six edges.  You can think
of is as an Egyptian pyramid in which the base is a triangle instead of a
square.  Methane is tetrahedral in that its four hydrogen atoms define the
vertices (points) of a tetrahedron.  A square is not a tetrahedron.

Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois
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Sandesh,

A tetrahedron is a triangular pyramid . It has four faces, all of which are
equilateral triangles. It cannot be formed by placing the bases of "two 
Egyptian
pyramids" together because those pyramids have square bases. As mentioned, all
faces of a tetrahedron are triangular.

Regards,
ProfHoff
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The "official" definition of a "regular tetrahedron" is: A convex solid
figure having 4 vertices, 6 edges, and 4 equivalent equilateral triangular
faces. In methane the carbon atom lies at the "center" of the tetrahedron,
which is probably causing part of the confusion. In methane the H--C--H
angles are all equal and about 109 degrees, but any H--H--H angle is 120
degrees. A "solid" tetrahedron looks like a pyramid with a triangular base
and faces, all of which are equivalent, which means it doesn't matter which
face is the "bottom" one, they all look the same.

Vince Calder
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A tetrahedron is a 4 (tetra) sided object.  A tetrahedron has six edges, four
(triangular) sides, and four vertices (corners).  The methane structure is
tetrahedral.  Lines connecting any three hydrogens outline a triangular side.

Combining two Egyptian pyramids at their bases forms an octahedron (8-sided
object).

sp{3} bonding, such as for the carbon atom in methane is a tetrahedral 
bond.  The
bonds between carbons in diamond are also tetrahedral and these tetrahedra 
form an
extended 3-D network that is diamond.  On a larger scale than a single 
tetrahedron
the diamond crystal structure is face-centered cubic.  To see this you 
might need
to locate a chemistry text that talks about crystal structures.

Greg Bradburn
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Hi Sandesh!
Ok...let us get things  straight....
probably you misunderstood what your teacher said...
A tetrahedron, in mathematics, is a polyhedron
of four faces, and  tetrahedral is the quality of beeing a tetrahedron.
To see it imagine a triangular basis pyramid.
In chemistry, back in 1864 the structural formulas for organic compounds
originated by Kekule and others, were expanded into 3 dimensions by
the independent work of van't Hoff and  Le Be that demonstrated that
the 4 bonds of the Carbon atom in methane, for example, are arranged
in such a way that they would point toward the corners of a regular
tetrahedron, if the Carbon atom were  placed at its center with the 4
Hydrogen atoms at the corners, with bond angles of 109.5 degrees.
  That is a tetrahedral structure.
If we assume that the bonding electron pairs of methane are located
somewhere between the central Carbon atom and each of the
Hydrogen atoms, we see that the tetahedral arrangement allows
the electron pairs to be as far apart as possible.
Thanks for asking...and tell your friends about NEWTON!
Mabel
(Dr. Mabel Rodrigues)
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