Science Concepts and Questions (K to 12): November 2017

Molecular geometry is the three-dimensional arrangement of atoms in a molecule. This is also called molecular shape. To determine the molecular geometry of molecule you have to know how to write lewis structure of different molecules. Molecular geometry of molecule affects the physical and chemical properties such as melting point, boiling point, density and the types of reactions.

In predicting molecular geometry of molecules, VSEPR (Valence-Shell Electron-Pair Repulsion) Model is used. VSEPR Model accounts for the geometric arrangements of electrons pairs around the central atom in terms of electrostatic attraction between electron pairs. Valence shell means the outermost shell occupied by the valence electrons, responsible for the bonding.

There are rules to be followed in predicting molecular geometry using VSEPR Model.

1. Double bonds and triple bonds can be treated as single bond. However, multiple bonds are larger than single bond, and the electron densities occupies more space.

2. If a molecule has two or more resonance structures, we can apply VSEPR Model to any one of them. Formal charges are usually not shown.

In my discussions below, we will be using letters which will represent the central atom, bonded atoms and lone pairs. For the central atom, we will use C for easy recall, B for bonded atoms and L for lone pairs.

Molecules without Lone Pairs on the Central Atom

The molecules that will be mention here are those molecules that do not contain lone pairs or non-bonding electron pairs on the central atom. And so we will just be using C and B letters, where C stands for the central atom while B the bonded atom. The pattern to predict the molecular geometry of molecules without lone pairs are CB₂, CB₃, CB₄, CB₅ and CB₆

1. CB₂ means that there is only one central atom and two bonded atoms, example of this is Berrylium Chloride (BeCl₂). The lewis structure of this molecule is shown below.

Since there is no lone pair on the central atom, the bonding pairs repel each other, forming a straight line having an angle of 180^o. The molecule is considered to be linear as shown in the model below:

2. CB₃ is another pattern which means 1 central atom and 3 bonded atoms without lone pair. Example molecule having this pattern is Boron triflouride (BF₃). The lewis structure of this molecule is shown below:

The geometry of the molecule having CB₃ pattern is triangular planar, where the angle between bonds is 120^o. The angle is equal because there is no non-bonding electron pair between the atom. See the ball and stick model below:

3. CB₄, is a pattern which shows molecule having 1 central atom and 4 bonded atoms without the lone pair. Example of a molecule with this pattern is CH₄, in this molecule the carbon atom is bonded to four hydrogen atoms. To show the bonding and non-bonding electrons, lewis structure must be written. Below is the lewis structure:

Lewis structure shows that there are four bonded atoms and no non-bonding electron pair on the central atom and therefore the angle between bonds will be 109.5^o with a tetrahedral molecular geometry as shown below:

4. Another pattern with no non-bonding electrons on the central atom is CB₅. Example molecule is PCl₅ with the lewis structure below.

Lewis structure shows that there is only 1 central atom the P (Phosphorus) bonded to 5 Chlorine atoms. Central atom has no non-bonding electrons and therefore the possible geometry of this kind of molecule is trigonal bipyramidal having 120^o and 90^o between bonds as shown below:

5. CB₆ is another pattern with no non-bonding electron pair or lone pair on the central atom. One example of this is SF₆ in which the central atom sulfur is bonded to 6 flourine atoms. The lewis structure of this molecule is

From the Lewis structure, there is one central atom the sulfur and 6 surrounding atoms. Since there is no non-bonding electrons on the central atom the probable molecular geometry of SF₆ is octahedral as shown below:

SUMMARY OF MOLECULAR GEOMETRY OF MOLECULES WITHOUT LONE PAIR ON THE CENTRAL ATOM

Molecular Geometry of Molecules with the lone pair on the central atom.

The geometry of molecules with lone pair on the central atom is more complicated than those without the lone pair. These kinds of molecules have three types of repulsive forces that we have to consider:
1. between bonding pairs
2. between lone pairs
3. between bonding pair and lone pair

VSEPR Model predicted the arrangement of repulsive forces in decreasing order:

lone pair vs lone pair > lone pair vs. bonding pair > bonding pair vs. bonding pair

So here we will use the same symbol for the pattern, C for central atom, B for bonded atoms and L for lone pair.

1. CB₂L, is the simplest pattern, which means 1 central atom, two bonded atoms with 1 lone pair. Example of this is SO₂. The lewis structure is shown below:

The lewis structure shows double bond between oxygen and sulfur, and VSEPR consider this as if single bond. It has also lone pair on the central atom in which this lone pair exhibits repulsive force with the bonding pair. Since the lone pair to bonding pair repulsive force is greater than that of bonding to bonding pair, the angle between O-S-O will be less than 120^o. And so the molecular shape of this kind of molecule is bent as shown below:

The molecular model of SO₂ is shown below:

2. CB₃L is another pattern having 1 central atom, three bonded atoms and 1 lone pair. Example of this kind is NH₃. Nitrogen is bonded to 3 hydrogen atoms and 1 lone pair.

The Lewis structure above shows the bonding atoms and the lone pair. The molecular shape of this kind of geometry is pyramidal, because it looks like a pyramid. The angle between H-N-H bond will be less than 109.5^o. Examine the illustration below:

In the tetrahedral molecular shape the angle is 109.5^o, but once the bonded atom is removed and is changed with a lone pair there is a decrease in the H-N-H bond angle due to the repulsion between the bonding pair and the lone pair. The angle decreases to 107.3^o.

3. Another pattern is CB₂L₂, which means molecule having 1 central atom, 2 bonded atoms and 2 lone pairs. Example of this molecule is the water, H₂O molecule. Look at the lewis structure below:

Oxygen is the central atom being the atom which has smaller number of molecule, surrounded by 2 hydrogen atoms. There are also 2 lone pairs on the central atom. The overall arrangement of electron pairs in water is tetrahedral just like NH₃, only that there are two lone pairs. These two lone pairs tend to be far apart from each other as possible the tendency the O-H bonding pair is push towards each other decreasing the angle to 104.5^o, as shown below:

Therefore the shape of water molecule is bent.

4. CB₄L is a pattern which means 1 central atom is bonded to 4 atoms with 1 lone pair, like the SF₄ molecule. Below is the lewis structure of SF₄:

This lewis structure shows that S being the central atom is surrounded with 4 F atoms and 1 lone pair. The observe shape experimentally is the seesaw, as shown below:

Another example is the XeF₄,

5. CB₃L₂ is another pattern for a molecule having lone pair on the central atom. Example of this is ClF₃. The lewis structure is shown below:

As shown above the Cl is the central atom surrounded by 3 bonded atoms the F, and there are 2 lone pairs on the central atom. The possible molecular geometry of this molecule is T-shaped, as shown below:

6. CB₂L₃ is another pattern without lone pair on the central atom. This pattern means that there is 1 central atom, 2 bonded atoms and 3 lone pairs on the central atom, Example molecule is XeF₂. Let us look at the lewis structure of this molecule.

The possible shape of this molecule is linear, as shown in the model below:

7. CB₅L, a pattern for a molecule having 1 central atom, 5 bonded atoms with 1 lone pair in the central atom. Example of this is BrF₅,

Lewis structure above shows that Br is surrounded by 5 F atoms with 1 lone pair. The molecular shape of this kind of molecule is square pyramidal as shown below:

8. CB₄L₂, is another pattern showing molecule with 1 central atom, 4 bonded atoms with 2 lone pairs, Example molecule is XeF₄ with the lewis structure of