1.1 What is Chemistry?

9.2. Molecular Geometry http://www.ck12.org

• Draw the Lewis electron dot structure for the molecule.


• Count the total number of electron pairs around the central atom. This is referred to as theelectron domain
  geometry.


• If there are no lone pairs around the central atom, refer toTable9.1, to determine the molecular geometry,
  which is the same as the electron domain geometry.


• If there are one or more lone pairs on the central atom, the molecular geometry (the actual shape of the
  molecule) will not be the same as the electron domain geometry. Refer toTable9.2.


• In predicting bond angles, remember that a lone pair takes up more space than a bonding pair or pairs of
  electrons.

TABLE9.1: Geometries of Molecules in Which the Central Atom Has No Lone Pairs

Atoms Around Central

Atom

Electron Domain Geome-

try

Molecular Geometry Example

2 linear linear BeCl 2

3 trigonal planar trigonal planar BF 3

4 tetrahedral tetrahedral CH 4

5 trigonal bipyramidal trigonal bipyramidal PCl 5

6 octahedral octahedral SF 6

TABLE9.2: Geometries of Molecules in Which the Central Atom Has One or More Lone Pairs

Atoms Plus

Lone Pairs

Around Central

Atom

Number of Sur-

rounding Atoms

Number of Lone

Pairs

Electron

Domain

Geometry

Molecular

Geometry

Example

3 2 1 trigonal planar bent O 3

4 3 1 tetrahedral trigonal pyrami-

dal

NH 3

4 2 2 tetrahedral bent H 2 O

5 4 1 trigonal bipyra-

midal

seesaw SF 4

5 3 2 trigonal bipyra-

midal

T-shaped ClF 3

5 2 3 trigonal bipyra-

midal

linear XeF 2

6 5 1 octahedral square pyrami-

dal

BrF 5

6 4 2 octahedral square planar XeF 4

Practice with basic molecule shapes at http://phet.colorado.edu/en/simulation/molecule-shapes-basics. Practice
building molecules at http://phet.colorado.edu/en/simulation/build-a-molecule. Build 3D molecules at http://phet.
colorado.edu/en/simulation/molecule-shapes.