Chem 110 

Summer 2006 
Lecture Notes::Lec 12_15 June 


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VSEPR (Valence Shell Electron Pair Repulsion) Theory is based on three assumptions:
VSEPR predicts geometry based on these assumptions in a few simple, sequential, steps:
Examples of the various molecular geometries discussed below may be found in the Molecular Geometry Supplement.
For central atoms with eight outer electrons (octets) there are three possible electron pair geometries:
 Linear with angles of 180° ( a single pair and a triple bond, or two double bonds).
 Trigonal planar with angles of 120° (one double bond and two single pairs).
 Tetrahedral with angles of 109.5° (four single pairs). [model]
These three electron pair geometries can lead to five molecular geometries:
 Linear (e.g. carbon monoxide)
 CO_{2}
 valence electrons = 4 + 2x6 = 16
 6y + 2 = 20, thus 4 fewer electrons than required for all single bonds, 4/2 = 2 multibonds (2 double or 1 triple)
 LS: from symmetry C will be central atom, therefore= :O::C::O:
 Considering C as the central atom, have 2 bonded atoms and no lonepairs, therefore
 steric number = 2, so linear electronic geometry, and
 linear molecular geometry
 Trigonal planar (e.g. formaldehyde, CH_{2}O)
 formaldehyde, CH_{2}O
 valence electrons = 4 + 2x1 + 6 = 12
 6y + 2 = 6 x 2 + 2 = 14; so molecule has 2 fewer electrons than required for all single bonds, 1 double bond
 LS: from symmetry C will be central atom, therefore=
 Considering C as the central atom, have 3 bonded atoms and no lonepairs, therefore
 steric number = 3, so trigonal planar electronic geometry, and 3 atoms so
 trigonal planar molecular geometry
 Tetrahedral (e.g. methane, CH_{4})
 valence electrons = 4 + 4x1= 8
 four bonds possible, since only 4 pairs, single bonds because only have H's bound to C.
 LS: from symmetry C will be central atom, therefore=
 Considering C as the central atom, have 4 bonded atoms and no lonepairs, therefore
 steric number = 4, so tetrahedral electronic geometry, and 4 atoms so
 tetrahedral molecular geometry
 Trigonal pyramidal (e.g. ammonia, NH_{3})
 valence electrons = 5 + 3x1= 8
 only 4 pairs, single bonds because only have H's bound to N, 3 bonds, since only 3 H's
 LS: from symmetry N will be central atom, therefore=
 Considering N as the central atom, have 3 bonded atoms and one lonepair, therefore
 steric number = 4, so tetrahedral electronic geometry, but only 3 atoms so
 trigonal pyramidal molecular geometry
 Bent (e.g. water, H_{2}O)
 valence electrons = 6 + 2x1= 8
 only 4 pairs, single bonds because only have H's bound to O, 2 bonds, since only 2 H's
 LS: from symmetry O will be central atom, therefore=
 Considering O as the central atom, have 2 bonded atoms and 2 lonepairs, therefore
 steric number = 4, so tetrahedral electronic geometry, but only 2 atoms so
 bent molecular geometry
Small atoms:
 Linear with angles of 180° ( BeCl_{2}) Note that this is an example where the "hilo" rules will lead us astray. The "1.7 rule" for electronegativities on the other hand predict covalent bonding. Also, the extreme small size of Be also leads to a prediction of greater covalent bond character than expected for other alkaline earth elements.
 valence electrons = 2 + 2x7 = 16
 from symmetry Be will be central atom, therefore only two pairs on Be with single bonds to Cl
 LS:
 steric number = 2, so linear electronic geometry, so
 linear molecular geometry
 Trigonal planar with angles of 120° ( BCl_{3})
 valence electrons = 3 + 3x7 = 24
 from symmetry B will be central atom, therefore only three pairs on B with single bonds to Cl
 LS:
 steric number = 3, so trigonal planar electronic geometry, & three bonded atoms so
 trigonal planar molecular geometry
Representative atoms with empty dshells can also have what are sometimes referred to as expanded valence shells. In these cases the dorbitals also participate in bonding enabling more bonds to be formed. Thus two additional electronic geometries are possible:
These two electron pair geometries can lead to six new molecular geometries in addition to another way to make a linear molecule:
Syllabus/Schedule 
© R A Paselk
Last modified 15 June 2006