Molecular Structures

Chemistry Unit 7 Lesson 1: Molecular Lewis Structures

By Ryan McCluskey

In this unit we will learn to predict the Lewis structure of covalent molecules and learn to draw their molecular structure as well as determine the geometry(Shape) of these structures.

Molecular Lewis Structures

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Multiple Choice

Which of the following is a covalent molecule?

• CO₂

• NaCl

• H₂O

• N₂O

Covalent molecules are formed between nonmetals only. Ionic compounds only form as salts in what is known as a crystal lattice.

Molecular Lewis Structures

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A crystal lattice must form so that no like charges may line up or touch each other due to their repulsion of one another.

Molecular Lewis Structures

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This is also why ionic structures like glass can be hard but very brittle. If you hit the lattice hard enough to align these like charges they will repel and somewhat explode.

Molecular Lewis Structures

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Open Ended

What is a substance other than glass that must be made from ionic bonds?

The process of drawing molecular structures can be confusing but it becomes a lot easier if we break it down into steps.

For this class, I like to break the process into 6 Steps​

Molecular Lewis Structures

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To demonstrate these steps we will walk through them using CO₂ as our example molecule. Remember Lewis structures only apply to covalent molecules not ionic bonds. ​

Molecular Lewis Structures

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Step 1: COUNT THE TOTAL NUMBER OF VALENCE ELECTRONS

During covalent bonds valence electrons are being shared between the atoms to help all elements fulfill the octet rule.​

Molecular Lewis Structures

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Match

Match each element with how many valence electrons it has available to "share" when forming molecular structures.

• Carbon

• Nitrogen

• Oxygen

• Fluorine

• Hydrogen

4

5

6

7

1

• 4

• 5

• 6

• 7

• 1

Valence electrons are the outermost electrons. They can be shared with other elements in order for both to reach stability or fulfill the octet rule.

Molecular Lewis Structures

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Step 1: COUNT THE TOTAL NUMBER OF VALENCE ELECTRONS

C= 1 atom

O = 2 atoms

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Molecular Lewis Structures

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​Back to it.....

Step 1: COUNT THE TOTAL NUMBER OF VALENCE ELECTRONS

C= 1 atom x 4 valence e's = 4

O = 2 atoms x 6 valence e's = 12

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Molecular Lewis Structures

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Step 1: COUNT THE TOTAL NUMBER OF VALENCE ELECTRONS

C= 1 atom x 4 valence e's = 4

O = 2 atoms x 6 valence e's = 12

So CO₂ has 16 total valence e-’s​

Molecular Lewis Structures

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Step 1: COUNT THE TOTAL NUMBER OF VALENCE ELECTRONS

C= 1 atom x 4 valence e's = 4

O = 2 atoms x 6 valence e's = 12

So CO₂ has 16 total valence e-’s​

Molecular Lewis Structures

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​Some exceptions:

If a molecule shows an overall positive charge you subtract these electrons from the total Ex. NH₄⁺

Step 1: COUNT THE TOTAL NUMBER OF VALENCE ELECTRONS

C= 1 atom x 4 valence e's = 4

O = 2 atoms x 6 valence e's = 12

So CO₂ has 16 total valence e-’s​

Molecular Lewis Structures

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​Some exceptions:

If a molecule shows an overall positive charge you subtract these electrons from the total Ex. NH₄⁺

If a molecule shows an overall negative charge you add these electrons to the the total Ex. SO₄ ^2-​

Step 1: COUNT THE TOTAL NUMBER OF VALENCE ELECTRONS

​

Molecular Lewis Structures

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​Some exceptions:

If a molecule shows an overall positive charge you subtract these electrons from the total Ex. NH₄⁺

If a molecule shows an overall negative charge you add these electrons to the the total Ex. SO₄ ^2-​

​This is NOT common and only occurs when drawing structures for polyatomic ions.

Step 1: COUNT THE TOTAL NUMBER OF VALENCE ELECTRONS

H= 2 atom x 1 valence e's = 2

O = 1 atoms x 6 valence e's = 6

So H₂O has 8 total valence e-’s​

Molecular Lewis Structures

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Step 1: COUNT THE TOTAL NUMBER OF VALENCE ELECTRONS

N= 1 atom x 5 valence e's = 5

O = 3 atoms x 6 valence e's = 18

with the - charge add one more e = 1

So NO₃^- has 24 total valence e-’s​

Molecular Lewis Structures

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The count of electrons should always be EVEN. If not then I know I've made a mistake somewhere.

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Molecular Lewis Structures

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Step 2: DRAW THE SKELETON

The smallest bond that must form is a single bond. So to start we place the central atom and single bond the peripheral atoms to it. ​

Molecular Lewis Structures

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Step 2: DRAW THE SKELETON

• The "Single" element is usually the central atom 

• Carbon is a very common central atom

• Hydrogen/Halogens are very common peripheral atoms.

• When in doubt the element that is found furthest to the left is the central atom.

Molecular Lewis Structures

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Step 2: DRAW THE SKELETON

​So the in CO₂ Carbon is the central atom and the oxygens are peripheral.

Molecular Lewis Structures

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Step 3: ADD PERIPHERAL ELECTRONS

After subtracting the single bonded electrons (2 per bond) from the total, start adding valence electrons beginning with peripheral(outside) atoms first then moving to the central atom. Each atom should have 8 valence electrons (H=2) including the shared bonds. (Octet Rule) Do this until you run out of valence electrons.​

Molecular Lewis Structures

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Step 3: ADD PERIPHERAL ELECTRONS

Remember with CO₂ we had 16 total electrons. The 2 single bonds accounted for 4 of those electrons. This means we have 12 left to add. We must start by adding them to the peripheral or oxygen atoms. ​

Molecular Lewis Structures

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Step 3: ADD PERIPHERAL ELECTRONS

Once we run out electrons we must stop. Even if we haven't fulfilled the octet. We can only use the valence electrons provided by the atoms to create a predicted lewis molecular structure. ​

Molecular Lewis Structures

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Draw

Determine how many valence electrons remain after drawing your skeleton and add them to your molecule now.

Draw

Determine how many valence electrons remain after drawing your skeleton and add them to your molecule now.

Step 4: CHECK FOR OCTET

If everybody fulfills the octet rule at this point stop. You have all single bonds and no lone pairs. If you ran out early move to step 5. If you actually had extra electrons skip to step 6. ​

Molecular Lewis Structures

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Step 4: CHECK FOR OCTET

For our example both oxygens are satisfied with 8 total electrons but our carbon is not with only 4 total electrons. Remember each bond represents 2 total electrons. So we go on to STEP 5. ​

Molecular Lewis Structures

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Drag and Drop

Check for octet. ​​

Placeholder

is not satisfied. It needs ​

Placeholder

more electrons to fulfill its octet.

Drag these tiles and drop them in the correct blank above

Nitrogen

2

Oxygen

4

6

8

Neither

0

Drag and Drop

Check for octet. ​​

Placeholder

is not satisfied. It needs​ ​

Placeholder

more electrons to fulfill its octet.

Drag these tiles and drop them in the correct blank above

Neither

0

Oxygen

4

6

8

Hydrogen

Step 5: SLIDE IN DOUBLE BONDS

If you ran out before making the central atom happy you will have to slide in more valence electrons pairs to share; creating double bonds. This will make atoms happy without adding more valence e-’s then you have; which you can’t do.​

Molecular Lewis Structures

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Step 5: SLIDE IN DOUBLE BONDS

Notice you have to remove these electrons from the peripheral atom to form this new bond. 2 electrons per new bond. Also realize that we generally create symmetry in our molecule because it exists most often in nature. ​

Molecular Lewis Structures

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Draw

Since Nitrogen does not fulfill its octet draw on the molecule to show how you would change it's structure.

Step 6: EXPANDED OCTET

If you have extra valence electrons you will place them around the corners of the central atom. This results from something called an expanded octet.​ This happens commonly with elements like sulfur and phosphorus.

Molecular Lewis Structures

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Draw

Draw your best CCl₄ molecule here.

Draw

Draw your best NO₂^- molecule here.

Open Ended

How would you rate yourself on a scale of 1-5 so far on confidence with this topic?

Open Ended

Which of the 5 steps do you find the most difficult?

(We really haven't practiced STEP 6 yet.)