Chapter 4: Unit 14. Molecular Shape and VSEPR

Molecular Shape and VSEPR

This theory helps to determine the shape of polyatomic covalent molecules. In a polyatomic molecule, central atom solely determines the shape. The total number of bond pair and lone pair determines the geometry of the molecule. Shape is determined based on  the number of lone pair on central atom.

The complete form of VSEPR is Valence Shell Electron Pair Repulsion Theory. There are three postulates of VSEPR theory

1. Electrons are arranged in polyatomic molecule I such a way that the repulsion between atoms is minimized.
2. Any Bond pair or lone pair is considered as one density area surrounding area of electron density. VSEPR theory doesn’t distinguish between single and multiple bond.
3. Lone pair on central atom is considered as electron cloud and repels the bond pair of electrons causing the bond angle to decrease. The degree of repulsion

Lone pair-lone pair> lone pair-bond pair> bond pair-bond pair

The geometry based on the total number of electron groups surrounding central atom is called electronic geometry.  3D molecular shape changes from electronic geometry when there is(are) lone pair of electrons on the central atom. Bond angle  is determined by connecting the two adjacent atoms with a central atom.

To determine the shape, count groups, atoms and lone pairs, and keep the groups as far away from each other as possible.

Two groups- linear, 180 degree, three groups- trigonal planar, 120 degree, four groups- Tetrahedral 109.5 degree.

Here is the complete VSEPR chart.

VSEPR Theory – Molecular Geometries

Here is the complete VSEPR table with 2, 3, 4, 5 and 6 regions of electron density area surrounding the central atom.

We will try to understand VSEPR thorugh the simulation below:

Description: Open the Phet simulation link to VSEPR activity

https://phet.colorado.edu/sims/html/molecule-shapes/latest/molecule-shapes_en.html

• Two electron groups:

Click on the “model “ and observe the two electron groups electron geometry surrounding the central atom. Click on the electron geometry, molecular geometry and bond angle.

Then go to the real molecule section and choose CO2. Can you predict the molecular, electronic geometry and bond angle?

• Three electron groups:

Choose BF3 molecule under real molecule section and click on the molecular geometry, electronic geometry and bond angle.

Now choose SO₂, in SO₂, total three electron density areas but two of them are bonding and one is lone pair. What do you observe for electronic, molecular geometry and bond angle? Why?

• Four electron groups:

In methane, there are total bond groups an no lone pair. For four electron density areas, the ideal geometry is tetrahedral and molecular shape is same as electronic geometry. The ideal tetrahedral bond angle is 109.5⁰

Take the water molecule. H2O has two bond pairs and two lone pairs, total four electron density groups. The shape of the molecule is bent although the geometry is tetrahedral. The bond angle is 104.5⁰ which is less than ideal for tetrahedral geometry (109.5⁰) due to presence of two lone lone pairs.

Choose the molecule NH₃. Can you predict the electronic geometry, molecular geometry and bond angle looking at the structure?

The molecular shape of H₂O is bent.

Here are some more examples:

To practice more problems, watch the video.

https://www.youtube.com/watch?v=nxebQZUVvTg&t=947s

Questions:

1. For each of the following draw the Lewis dot structure and determine electronic geometry, molecular shape and bond angle.

a) NF₃

b) BI₃

c) CH₄

d) SO₂

e) C₂H₄

f) CO₃^2-

Ans: a) tetrahedral, trigonal pyramidal, less than 109.5

b) Trigonal planar, trigonal planar, 120

c) Tetrahedral, Tetrahedral, 109.5

d) Trigonal planar, angular, less than 120

e) Trigonal planar, trigonal planar, 120

f) Trigonal planar, trigonal planar, 120