What would cause the shape of a molecule to be tetrahedral?

1 Answer
Jun 21, 2016

The NUMBER, and preferred disposition of electron pairs, bonding and non-bonding, around the central atom.


If there are 4 electron pairs around a central atom, the shape, the geometry that results in LEAST electrostatic interaction is the tetrahedron. For methane, #CH_4#, the 8 electrons that comprise the 4 #C-H# bonds demonstrably tend to adopt a shape that minimizes electrostatic repulsion. Methane thus assumes the shape of a tetrahedron, with #/_H-C-H=109.5""^@#.

The same geometry is adopted in the water molecule, but here there are ONLY TWO #O-H# bonds, and 2 lone pairs. The most stable electronic geometry is still the tetrahedron, but because we describe geometry on the basis of actual atoms, not lone pairs, we describe the geometry of the water molecule as bent, and the #/_H-O-H# is compressed from #109.5""^@# to approx #105""^@#, because the lone pairs exert a disproportionate influence.