Download How to Determine the Molecular Geometry for a Compound

How to Determine the Molecular Geometry for a Compound
1. Draw the Lewis Dot Structure for the compound.
2. Establish the AXE designation for the Dot Structure as follows:
A
# of X’s
# of E’s
= central atom
= # of BONDED atoms to the central atom
= # of LONE PAIRS of electrons on the central atom
The numbers of X’s and E’s are then written as subscripts.
DO NOT count Lone Pair electrons on the atoms which are connected to the central
atom. ONLY the lone pairs of electrons on the central atom count as E’s.
3. The total number of X’s and E’s determine the ELECTRON PAIR geometry. (There are only 5
basic electron pair geometries)
NOTES:
1. If you only have 2 atoms bonded together, there is NO central atom and NO AXE
designation. Since two points make a straight line, this geometry is always LINEAR (bond angles
of 180 degrees, sp hybridization)
2. If you have more than one central atom, determine the geometry on each atom separately.
If the molecule is symmetrical, the geometry on each atom will be the same. If not, assign the
geometry individually to each central atom.
3. If an atom has multiple bonds to a central atom, it is only counted ONCE. You count # of
bonded atoms in this model, NOT # of bonds.
How to Determine the Hybridization for a Compound
1. Draw the Lewis diagram for the compound.
2. The hybridization must follow the format spxdy (where x cannot be higher than 3 and y
cannot be higher than 2 in this class)

The total number of bonds to the central atom (X + E) must add up to the same
number of letters in the hybridization.
Ex.) AX3 = 3 bonds to the central atom so hybridization = sp2 (3 total letters)
AX4E = 5 total bonds to central atom so hybridization = sp 3d (5 total letters)
Molecular Geometry and Hybridization to Memorize!!
Total
Bonds
(X + E)
2
(Linear)
3
(Trigonal
Planar)
4
(Tetrahedral)
5
(Trigonal
bipyramid)
6
(Octahedral)
# bonded
atoms (X)
2
# lone
pairs (E)
0
AXE
Notation
Molecular
Geometry
Bond
Angles
AX2
Linear
120°
118°
sp2
3
0
AX3
Trigonal
Planar
2
1
AX2E
Bent
180°
HybridExample
ization
sp
sp2
Polarity
BeCl2
POLAR, for AX2
ONLY if X groups
not identical.
BCl3
POLAR, ONLY if X
groups not
identical.
NO2-
YES, always polar.
CCl4
POLAR, ONLY if X
groups not
identical.
4
0
AX4
Tetrahedral
109.5°
sp3
3
1
AX3E
Trigonal
Pyramid
< 109.5°
sp3
NH3
YES, always polar.
2
2
AX2E2
Bent
< 109.5°
sp3
H2O
YES, always polar.
5
0
AX5
Trigonal
Bipyramid
90°,
180°,120°
sp3d
AsCl5
POLAR, ONLY if X
groups not
identical.
See-Saw
(Distorted
Tetrahedral)
< 90°,
180°,120°
sp3d
SeCl4
YES, always polar.
T-Shaped
<90°,180°
sp3d
BrCl3
YES, always polar.
Linear
<180°
sp3d
XeF2
POLAR, ONLY if X
groups not
identical.
AX6
Octahedral
90°, 180°
sp3d2
TeBr6
POLAR, ONLY if X
groups not
identical.
AX5E
Square
Pyramid
< 90°,180°
sp3d2
BrF5
YES, always polar.
AX4E2
Square
Planar
< 90°
sp3d2
XeF4
POLAR, ONLY if X
groups not
identical.
4
1
3
2
2
3
6
0
5
4
1
2
AX4E
AX3E2
AX2E3