1. Module 2 Intro

1.15. Page 3

Lesson 3

Module 2—Chemical Compounds

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In Lesson 3 of Module 1 you were introduced to electronegativity, the attraction that an atom has for a shared pair of electrons. In that lesson you calculated differences in electronegativity between atoms; and when the difference was large, you classified these bonds as being polar.

 

You may wish to reread the section titled “Electronegativity and Bond Polarity” on pages 99 and 100 in your textbook.

 

Your observation of the bending of the stream of water when exposed to a charged object, demonstrates the polarity of the water molecule. Polarity means having two different regions of charge.

 

bond dipole: the charge separation that occurs when the electronegativity difference of two bonded atoms shifts the shared electrons, making one end of the bond partially positive and the other partially negative

As you have seen, it is possible to have a bond dipole. A bond dipole is the charge separation that occurs when the electronegativity difference of two bonded atoms shifts the shared electrons, making one end of the bond partially positive and the other partially negative.

 

What happens when a molecule is composed of many atoms?

 

Read the section, “Bond Polarity and Molecular Polarity” on pages 101 and 102 in your textbook. Work through “SAMPLE problem 3.5” on page 102 in your textbook.

 

Try This

 

TR 1. Explain why the stereochemistry (shape) of a molecule needs to be considered when determining if a substance is polar. To support your answer, use one example other than the ones discussed in your textbook. Save a copy of your answer to this question to your course folder. Send a second copy to your teacher.

 

polar molecule: a molecule in which the negative charge is not distributed symmetrically among the atoms, resulting in partial positive and negative charges on opposite ends of the molecule

 

Water is a polar molecule. This means that although water molecules are neutral, the electron charge within the molecule is not symmetrically distributed. The oxygen portion of the water molecule is slightly negative, while the hydrogen portions are slightly positive. Because there is a difference of charge within the molecule, there is a negative pole and a positive pole.

 

A diagram of a water molecule is formed from a large red circle with two smaller blue circles located at the 10 and 2 o’clock positions. Each of the blue circles has a positive sign inside it, and the blue circles are labeled the positive pole of the water molecule. The bottom section of the red circle contains two negative signs and is labeled the negative pole of the water molecule.

 

Notice that the overall charge of the molecule is zero, although the side with the hydrogen atoms is slightly positive and the side with the oxygen atom is slightly negative.

 

The molecules in liquid form are loosely packed together and are able to rotate. If a negatively charged object is brought close to the water stream, the molecules within the water will orient themselves so that the slightly positive hydrogen atoms will be attracted to the negatively charged object.

 

Conversely, if a positively charged object is brought close to the stream of water molecules, the molecules will orient themselves so that the slightly negative oxygen atoms are attracted to the positively charged object.

 

Two diagrams show a stream of water molecules bending towards a charge rod. The diagram on the left shows a rod with a negative charge. The water molecules in the stream are oriented so that the blue coloured hydrogen atoms are located closest to the charged rod. The diagram on the right shows a positively charged rod. The water molecules in the stream are oriented so that the red coloured oxygen atoms are located closest to the charged rod.

 

This action leads to the water being attracted to both types of charges; therefore, the water always bends toward the object. A bond dipole is the charge separation that occurs when the electronegativity difference of two bonded atoms shifts the shared electrons, making one end of the bond partially positive and the other partially negative.

 

nonpolar molecule: a molecule in which the negative (electron) charge is distributed symmetrically among the atoms making up the molecule

 

A nonpolar molecule has a symmetrical electron distribution—there is no positive pole or negative pole. Diatomic elements, such as fluorine, are nonpolar.

 

Self-Check

 

SC 1. Complete question 14 at the top of page 103 in your textbook.

 

Check your work.

 

Module 2: Lesson 3 Assignment

 

You will use the following lab to complete part of your Module 2: Lesson 3 Assignment.