TETRAHEDRAL ELECTRONIC GEOMETRY: AB 3 U SPECIES
(ONE LONE PAIR OF ELECTRONS ON A)We are now ready to study the structures of some simple molecules with unshared valence
electron pairs (lone pairs) on the central atom. In this and subsequent sections, we use
generalized chemical formulas in which “A” represents the central atom, “B” represents
an atom bonded to A, and “U” represents an unshared valence shell electron pair (lone
pair) on the central atom A. For instance, AB 3 U would represent any molecule with three
B atoms bonded to a central atom A, with one unshared valence pair on A.
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Let us first consider the left-hand carbon atom. The arrangements of its regions of
high electron density allows us to locate the other C and three H atoms with respect to
that C atom (the atoms outlined in red). Then we carry out a similar analysis for the
right-hand C atom to deduce the arrangements of itsneighbors (outlined in blue).
Each C atom in C 2 H 6 has four regions of high electron density. The VSEPR theory
tells us that each C atom has tetrahedral electronic geometry; the resulting atomic
arrangement around each C atom has one C and three H atoms at the corners of this
tetrahedral arrangement. The VB interpretation is that each C atom is sp^3 hybridized.
The CXC bond is formed by overlap of a half-filled sp^3 hybrid orbital of one C atom
with a half-filled sp^3 hybrid orbital of the other C atom. Each CXH bond is formed
by the overlap of a half-filled sp^3 hybrid orbital on C with the half-filled 1sorbital of an
H atom.