Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 110	General Chemistry	Summer 2006
Lecture Notes::Lec 12_15 June	© R. Paselk 2006
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VSEPR Theory and Molecular Geometry

VSEPR (Valence Shell Electron Pair Repulsion) Theory is based on three assumptions:

• Electron pairs will orient around a central point to minimize repulsion.
• Lone-pairs of electrons will have greater repulsion than bonded pairs of electrons (note that the atoms are ignored in terms of repulsion).
• Repulsion is strong at 90° and weaker at 120° (weakest at 180°).

VSEPR predicts geometry based on these assumptions in a few simple, sequential, steps:

1. Draw a correct Lewis Structure.
2. Determine the Steric Number = the number of bonded atoms + the number of lone pairs (= regions of electron density).
3. Maximize the angles between electron pairs, placing the lone (unbonded) pairs at the extremes.

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:

1. Linear with angles of 180° ( a single pair and a triple bond, or two double bonds).
2. Trigonal planar with angles of 120° (one double bond and two single pairs).
3. 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)
  • CO₂
    • valence electrons = 4 + 2x6 = 16
    • 6y + 2 = 20, thus 4 fewer electrons than required for all single bonds, 4/2 = 2 multi-bonds (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 lone-pairs, therefore
    • steric number = 2, so linear electronic geometry, and
    • linear molecular geometry
• Trigonal planar (e.g. formaldehyde, CH₂O)
  • formaldehyde, CH₂O
    • 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 lone-pairs, therefore
    • steric number = 3, so trigonal planar electronic geometry, and 3 atoms so
    • trigonal planar molecular geometry
• Tetrahedral (e.g. methane, CH₄)
  • 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 lone-pairs, therefore
  • steric number = 4, so tetrahedral electronic geometry, and 4 atoms so
  • tetrahedral molecular geometry
• Trigonal pyramidal (e.g. ammonia, NH₃)
  • 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 lone-pair, therefore
  • steric number = 4, so tetrahedral electronic geometry, but only 3 atoms so
  • trigonal pyramidal molecular geometry
• Bent (e.g. water, H₂O)
  • 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 lone-pairs, therefore
  • steric number = 4, so tetrahedral electronic geometry, but only 2 atoms so
  • bent molecular geometry

Small atoms:

• Linear with angles of 180° ( BeCl₂) Note that this is an example where the "hi-lo" 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° ( BCl₃)
  • 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 d-shells can also have what are sometimes referred to as expanded valence shells. In these cases the d-orbitals also participate in bonding enabling more bonds to be formed. Thus two additional electronic geometries are possible:

• Trigonal bipyramidal with angles of 90° & 120° results when the valance shell is expanded to accommodate 10 electrons in five pairs.
• Octahedral with angles of 90° results when the valence shell is expanded to accommodate 12 electrons in six pairs.

These two electron pair geometries can lead to six new molecular geometries in addition to another way to make a linear molecule:

• Trigonal bipyramidal with angles of 90° & 120° (PCl₅)
  • valence electrons = 5 + 5x7 = 40
  • 40 > 6y + 2 = 38, delta = 2 therefore expanded valence with one extra pair
  • from symmetry P is central atom
  • valence electrons around P = P electrons + one from each Cl = 5 + 5
  • LS:
  • steric number = 5, so trigonal bipyramidal electronic geometry
  • 5 bonded atoms so Trigonal bipyramidal molecular geometry
• Seesaw with angles of 90° & 120° (SF₄)
  • valence electrons = 6 + 4x7 = 34
  • 34 > 6y + 2 = 32, delta = 2 therefore expanded valence with one extra pair
  • from symmetry S is central atom
  • valence electrons around S = S electrons + one from each F = 6 + 4
  • LS:
  • steric number = 5, so trigonal bipyramidal electronic geometry
  • 4 bonded atoms so Seesaw molecular geometry

© R A Paselk

Last modified 15 June 2006