Molecular Geometry

Molecular Geometry

The valence shell electron pair repulsion theory or VSEPR theory helps predict the spatial arrangement of atoms in a polyatomic molecule. The shapes are designed to minimize the

repulsion within a molecule.

The five different geometric shapes under the VSEPR theory, namely, linear, trigonal planar, tetrahedral, trigonal bipyramidal, and octahedral.

Symmetry plays an important role in determining the polarity of a molecule.

Guidelines to determine the VSEPR shape of a molecule:

1. Determine the central atom of a molecule. The central atom is the least electronegative element.

2. Count how many valence electrons the central atom has.

3. Count how many valence electrons the side atoms have.

4. Create the appropriate Lewis structure of the molecule.

5. Using the Lewis structure as a guide, determine the appropriate VSEPR shape for the molecule.

6. Note how many electrons are shared and unshared. This will help determine the appropriate VSEPR shape.

How to determine and draw different molecular shapes using the examples below.

Lone pairs has a big factor in making a molecule polar.

Polar molecule:

H₂O      Bent - polar due to two lone pairs

NH₃      Trigonal pyramidal - polar due to one lone pair

NO       Linear - polar due to unequal sharing of electrons