Molecular Modeling Web Site Reedley College

This page is made to assist students who wish to draw correct Lewis structures of molecules and complex ions. Correct molecular shapes and bond angles are determined with the VSEPR theory. This theory is explained to derive the geometrical shape of before mentioned species. 

The method is explained in five easy steps.

1. Electron pairs to draw: add together all the outer shell electrons of the atoms in the molecule. Divide the total by two to make electron pairs (= e^- pairs).

2. Each atom strives for a noble gas configuration in its direct environment. For hydrogen this is two electrons, which is the helium configuration and for the other elements this is eight electrons, which is called the octet rule*). Add together those numbers and subsequently, divide the total by two to make e^-pairs.

3. Subtract 2-1 to find the number of covalent bonds. Each covalent bond will be represented by a line.

4. Subtract 1-3 to find the number of lone pairs. Each lone pair will be represented by two dots.

5. Use the values for electronegativity, valency and your common sense to determine the central atom(s) and draw the Lewis structure.

Examples in which the octet rule is applied:

a. Draw the Lewis structure of N₂, nitrogen gas, and determine its correct geometrical shape.

1. 5+5 = 10 e^- = 5 e^-pairs

2. 8+8 = 16 e^- = 8 e-^pairs

3. Number of covalent bonds in N₂: 8-5 = 3

4. Number of lone pairs in N₂: 5-3 = 2

5. There is no central atom in a diatomic molecule, so the answer is

6. The shape is LINEAR. This is the logical case for all diatomic molecules.

It means that nitrogen gas contains linear molecules, that are non polar, because of the even electron distribution around the two N atoms.

b. Draw the Lewis structure of SO₂, sulfur dioxide gas, and determine its correct geometrical shape.

1. 6+6+6 = 18 e- = 9 e-pairs

2. 8+8+8 = 24 e- = 12 e-pairs

3. Number of covalent bonds in SO₂: 12-9 = 3

4. Number of lone pairs in SO₂: 9-3 = 6

5. The central atom is S because it has the highest possible valency (6+) of the two atoms. Therefore the Lewis structure is

6. The correct geometrical shape is ANGULAR BENT, with an O-S-O bond angle of approximately 120⁰, which means that SO₂ molecules are polar, because the electron distribution is uneven and the O atoms are positioned under an angle with respect to each other.

c. Draw the Lewis structure of CH₄, methane = natural gas.

1. 4+1+1+1+1 = 8 e- = 4 e-pairs

2. 8+2+2+2+2 = 16 e-= 8 e-pairs

3. Number of covalent bonds in CH₄: 8-4=4

4. Number of lone pairs in CH₄: 4-4=0

5. The central atom is C because it has a valency of 4+. The Lewis structure is

6. The correct geometrical shape is tetrahedral with an H-C-H bond angle of 109.5⁰.

d. Now, draw the Lewis structure of the carbonate ion, CO₃^2-.

The rule for ions is: for each negative charge add one electron in line 1; for each positive charge subtract one electron in line 1.

1. For the 2- charge we add 2e-, so to draw: 4+6+6+6+2 = 24 e-= 12 e- pairs

2. 8+8+8+8 = 32 e-= 16 e- pairs

3. Number of covalent bonds in CO₃^2- 16-12=4

4. Number of lone pairs in CO₃^2- 12- 4=8

5. The central atom is C because it has the highest valency 4+. The Lewis structure is

Note that the overall charge of this ion is -2.

6. The geometrical shape is TRIANGULAR PLANAR with O-C-O bond angles of 120⁰. The following figure shows the convention we use to include the charge.

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*) Exception: for atoms such as Be and B that have stable configurations with respectively 4e^- and 6e^- in their outer shell, this octet-rule-method does not work. In addition, you need to be careful with atoms that are capable of expanding their octet, such as S, P, Cl etc. In those situations you should exclusively use line 1 and (again) common sense to put the Lewis structure together. Try BeCl₂, BH₃, PBr₅ and SF₆ to check these exceptions.

More information on shapes of molecules can be found here.

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Using the awesome Molecular Modeling Software PC Spartan Plus, available in room PHY 77, you may want to try and build these molecules and ions, and rotate them on the screen. It is amazing how that will increase your insight in the true shape of molecules and it will ultimately give you an understanding how medicinal drugs do their work in your body.

Let 's take a look at four molecules built with a simple molecular model kit, four substances that recently made headlines.