Video Transcript
What is the proposed molecular
geometry for CH4?
Let’s begin by drawing a Lewis
structure for methane. To determine the molecular geometry
or shape of this molecule, we need to determine how many electron pairs are
organized around the central atom, which is carbon. To do this, we need to make use of
the valence shell electron pair repulsion model. This model explains how the
three-dimensional shape of a molecule results from minimization of repulsive
electrostatic forces between electron pairs.
The methane molecule contains four
bonded pairs of electrons. Each bonded pair is shared in a
covalent bond between the central carbon atom and one hydrogen atom.
Now, to determine the shape of the
methane molecule, let’s first determine its AXE notation. In this notation, A represents the
central atom, X 𝑚 represents the number of bonded electron pairs, and E 𝑛
represents the number of nonbonded electron pairs. Carbon is the central atom, and
there are four bonded pairs and zero nonbonded pairs. Therefore, methane belongs to the
AX4 group and has the tetrahedral shape.
In methane, there are equal
repulsive forces between the four bonded electron pairs. Therefore, each H-C-H bond angle is
identical and is predicted to be 109.5 degrees. The following dash-and-wedge
structure better represents the tetrahedral shape of the methane molecule. In conclusion, the proposed
molecular geometry for CH4, or methane, is tetrahedral.
