Molecular Geometry

This simplified view of molecular geometry has limitations. Whenever there is more than one electron in an atom (i.e., all atoms heavier than hydrogen), the electron orbitals interact or hybridize. For example, the presence of a s orbital electron makes a p orbital electron somewhat s-like in shape (electron cloud density) and, in turn, presence of the p orbital electron makes an s orbital electron more p-like. Hybrid orbitals are named by combining the names of the participating orbitals. An s orbital and three p orbitals will hybridize to form an sp³ orbital that has the characteristics of both s and p orbitals.

The carbon atom at the center of a methane (CH₄) molecule spreads out its four valence electrons into four sp³ orbitals pointed at the corners of a tetrahedron with bond angles of 109.5°.

In the case of ammonia, the five valence electrons surrounding nitrogen—two electrons occupying the outermost 2s orbital and three electrons in three 2p orbitals—hybridize to form four sp³ orbitals that, if equal, would separate themselves in three dimensional space by pointing at the corners of a tetrahedron. One of these orbitals, however, contains the lone electron pair, and the three remaining sp³ orbitals make additional space available by pointing at the corners of a pyramid with a triangular base.