9. Molecular Orbitals and      Molecular Structure   Previous PageNext Page
     sp3 Hybrid Orbitals

Most compounds of second- and third-row nonmetals involve tetrahedral or near- tetrahedral geometry around a central atom. We already have discussed methane, CH, which has ideal tetrahedral H-C-H angles of 109.5, ammonia, NH, with H-N-H angles of 107; and water with an H-O-H angle of 105. Tetrahedral bonding geometry can be obtained by combining an s and three p atomic orbitals of the central atom before bringing in other atoms, to produce a set of four new orbitals called hybrid atomic orbitals, as at the right. These hybrid orbitals can be represented by t, t, t, and t, and can be written formally as

t = s + p + p + p
t = s + p -p -p
t = s -p + p -p
t = s -p -p + p

All four hybrid orbitals have an equal contribution from the spherical s orbital, but they point in different directions because they have different contributions from p p and p The four hybrid orbitals extend out in the four directions of the vertices of a tetrahedron, or to four nonneighboring comers of a cube. The signs of the p terms in the set of four equations above are, in effect, the coordinates of each orbital The t orbital, for example, has its maximum electron probability in the -x, +y, -z direction, as can be seen to the right.

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