What is the geometry of methane?

1 Answer
Feb 12, 2016

Get (or make) a model of a tetrahedron and consider its symmetry.


A tetrahedron has a high-symmetry geometry; in fact, it is one of the Platonic solids. Of course, the methane molecule, #CH_4#, possesses this symmetry, and I will use methane's structure to illustrate the symmetry.

In methane, ALL of the hydrogens can be interchanged by symmetry operations; all of the hydrogen atoms are equivalent because we can use a symmetry operation to interchange each hydrogen with the other. For a start, there is 3-fold symmetry in that each #C-H# bond forms a threefold axis. You will have to consider the other symmetry elements. Once we replace ONE of the hydrogens, with say a halogen, the symmetry descends.

Chloromethane is less symmetric than methane, but a 3-fold axis of symmetry is retained. Likewise, if I take methane, and replace ANY of the FOUR hydrogens with a heteratom, #X#, I get precisely the same molecule. #H_3CX#, whatever hydrogen is replaced,