Many 5-atom, and 4-atom, 3-atom, and 2-atom MOLECULES have a geometry based on the tetrahedron. And here we will attempt to argue the reason.
The molecule methane is #CH_4#, there are #4xxC-H# bonds, i.e. FOUR regions of electrons density arranged around a central carbon atom. And these regions of electron density, THESE BONDS, assume a shape and distribution that minimizes interaction between the negatively charged bond. And here this shape corresponds to a tetrahedron, which is one of the so-called Platonic solids, regular convex polyhedra, whose faces are triangles (tetrahedra and octahedra), squares (cubes), etc.
And so we can immediately assign a value to the bond angle in methane, #/_H-C-H=109.5^@#.
And to determine molecular geometries, it is usually a simple matter to identify the central atom, and to count how many electron pairs, bonding, and non-bonding, that surround the central atom.
We do this for methane we get a tetrahedral molecule. We do this for ammonia, #NH_3#, or #H_2O# we also get a tetrahedron for the ELECTRONIC GEOMETRY. But because we describe molecular geometry in terms of bonds, not electron pairs,we describe the geometry of ammonia, and water as trigonal pyramidal, and bent respectively, and the lone pairs of electrons bend down #/_H-N-H# and #/_H-O-H# to give values of approx. #106^@# and #104^@#..
