Chem 161 Chapter 9 Molecular Geometry Worksheet - Shape Determines Function Page 4
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40Molecular Geometries with 2 to 6 Outer Atoms on the Central Atom
(where the Central Atom Has No Lone Pairs)
Consider a molecule composed of only two types of atoms, A and B:
A=central atom
B=outer atoms
For three or more atoms in a molecule, general formula: AB
(where #=2 to 6)
#
AB
: linear
2
– the two outer atoms are 180° from each other
– steric number (SN) = 2
AB
: trigonal planar
3
– three outer atoms at the corners of an equilateral triangle
– each outer atom is 120° from the other two outer atoms
– SN = 3
AB
: tetrahedral (tetra = four) since four-sided, or four faces
4
– maximum distance between electrons requires 3D
structure with 109.5° bond angles
– each outer atom is 109.5° from the other outer atoms
AB
: trigonal bipyramidal
5
– trigonal = three outer atoms form a planar triangle around
central atom
– bipyramidal = two outer atom directly above and below central
atom, connecting outer atom forms two 3-sided pyramids
– 3 outer atoms are at equatorial positions, 120° from one another at the ends of a
planar triangle
– 2 outer atoms are at axial positions, above and below central
atom and 90° from the equatorial atoms
AB
: octahedral (octa=eight) connecting the B atoms → eight faces
6
– all outer atoms are 90° away from each other
– terms "axial" and "equatorial" do not apply because all six
positions are identical
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