Chem 161 Chapter 9 Molecular Geometry Worksheet - Shape Determines Function Page 7
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40Molecules Where Central Atom Has One Or More Lone Pairs
From Trigonal Planar (SN=3)
AB
E: bent
2
– start with AB
molecule (trigonal planar) and replace a B atom with a lone pair
3
– bond angles are now <120°
From Tetrahedral (SN=4)
AB
E: trigonal pyramidal (central atom + 3 outer atoms make a pyramid)
3
– start with AB
molecule (tetrahedral) and replace a B atom with lone pair
4
– bond angles are now less than 109.5°
AB
E
:
bent
2
2
– start with AB
molecule (tetrahedral) and replace 2 B atoms with 2 lone pairs
4
From Trigonal Bipyramidal (SN=5)
AB
E: seesaw
4
– start with AB
molecule and replace one B atom with one lone pair
5
– an outer atom can be taken from an axial or an equatorial position
– from axial: lone pair is 90° from equatorial atoms and 180° from other axial atom
– from equatorial: lone pair is 90° from two axial and 120° from two other
equatorial B atoms
– taking the outer atom from the equatorial position maximizes the distance between
the lone pair (lp) and bonding pairs (bp)
– bond angles are now <90° and <120°
CHEM
1 61:
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