Chapter 4: Unit 15. Molecular Polarity

Molecular Polarity

Polarity is very important to determine the outcome of most chemical reactions.

In this section polarity of entire molecule will be discussed. A polar molecule is always attracted to the opposite sides of an electric field. (shown in the figure below). Molecular polarity depends on the individual bond polarity as well as symmetry of the molecule.

A polar molecule has either on polar bond or two or more bond dipoles that do not cancel.

A nonpolar molecules has either all nonpolar bonds or two or more polar bonds that do cancel each other.

Watch the following video:

Complete this Phet simulation Activity below:

https://phet.colorado.edu/en/simulation/molecule-polarity

( This simulation is explained in detail in chapter 7) More information about molecular polairy is given in chapter 7.

Modeling Molecular Polarity

Complete the chart for your given substance.

Questions

Determine the following molecules as polar or nonpolar

1. CH₂Cl
2. NH₃
3. HF
4. CO₂

Ans: 1. polar

2. Polar

3. Polar

4. Nonpolar

Following activity has been taken from AACT

Making Connections between Electronegativity, Molecular Shape, and Polarity

Objective

At the end of this activity you should be able to determine the polarity of a molecule based on the electronegativity of its constituents and its molecular shape.

Directions

Complete each of the following tasks using Ptable.com. Answer the questions as you go along.

1. Go to http://ptable.com/ in your web browser.
2. Select the “Properties” tab at the top.
3. Scroll over an element to see its electronegativity (see image below). Fill in the table below with the electronegativity values for the atoms provided. An example is provided for you.

1. Which atom is the most electronegative?  How can you tell?

1. If an atom with a high electronegativity value bonds with an atom with a low electronegativity value, what impact does the difference in electronegativity value have on the bond?

1. Draw the Lewis structures of the molecules below and determine their molecular shape:

1. Go to the “Compounds” tab on Ptable.com and find the “Name or Formula” search bar.  It is circled in the image below.

Search for each of the molecules in question 6. Click the link to the molecule that matches the formula exactly.  An example of CCl₄ is shown below.

After you have clicked on the link, a window will pop up on the screen with information regarding the structure and function of the molecule.  An example of what you should see for of CCl₄ is shown below.  Please note that some structures do not include lone electron pairs.

1. Look at the structure for each molecule and determine if your Lewis structure and shapes were correct. Describe any structures or shapes you had incorrect and explain why they were incorrect:

1. For each molecule, use the correct shape in combination with the electronegativity values you found in question 3 to determine if the molecule is polar or nonpolar. Describe how you know if each molecule is polar or nonpolar.

1. Read about each molecule and write down one interesting fact about each molecule.

1. Considering differences in electronegativity values between bonded atoms and the overall molecular shape, what do nonpolar molecules have in common?

1. Considering differences in electronegativity values between bonded atoms and the overall molecular shape, what do polar molecules have in common?

1. Write a short paragraph describing how electronegativity and molecular shape determine the polarity of a molecule. In your paragraph, describe why chemists consider relative electronegativity values and not absolute electronegativity values when determining the polarity of a bond. Also explain how a molecule with polar bonds can be non-polar overall.

Answer Key: Making Connections between Electronegativity, Molecular Shape, and Polarity

Question 3:

Question 4:

Fluorine is the most electronegative.  You can tell because it has the highest electronegativity number.

Question 5:

Question 7b:

Question 7c:

Answers will vary.

Question 8:

Nonpolar molecules can have polar or nonpolar bonds as long as the molecule is symmetrical.

Question 9:

Polar molecules have polar bonds and are non-symmetrical.