CHAPTER 4.4: POLAR MOLECULES

Chemists believe that molecules are made up of charged particles (electron and nuclei)

A molecule is considered to be polar or to have a molecular polarity, when the molecule has an overall imbalance of charge.

That is, the molecule has a region with a partial positive charge, and a region with a partial negative charge.

Not all molecules with polar bonds are polar molecules.

Eg. A carbon dioxide molecule has two polar C=O bonds, but it is not a polar molecule.

A water molecule has two polar O-H bonds, and it is a polar molecule.

How do you predict whether or not a molecule that contains polar bonds has an overall molecular polarity?

To determine molecular polarity, you must consider the shape of the molecule and the bond dipoles within the molecule.

Before we continue discussing the dipole nature of a molecule, let us go back and determine what is meant by the dipole nature of a molecule.

ELECTRONEGATIVITY AND POLARITY OF BONDS:

Linus Pauling realized that while two atoms can share one or more pairs of electrons, there is nothing that requires them to share those electron pairs equally.

He saw the need for a theory to explain and predict the polarity of molecules and so combined properties such as bond energies with valence bond theory to create a new property of atoms called electronegativity.

Electronegativity: is a measure of how strongly an atom attracts electrons in a chemical bond (the "hog").

Because Pauling's calculations produced differences in the attraction of nuclei for a shared pair of electrons in a covalent bond, he arbitrarily assigned values so that fluorine, the most electronegative atom, had a value of about 4.0.

All other atoms have a lower electronegativity and, therefore, a lower attraction for electrons when bonded.

Draw the periodic table and indicate, with arrows, the trend that occurs (page 252, fig.2)

Pauling used the difference in electronegativity to explain polarity of a chemical bond.

The greater the difference in electronegativity, the more polar the bond.

The smaller the difference, the more non-polar the bond.

A very polar bond is an ionic bond.

A non-polar bond is a covalent bond.

A somewhat polar bond is a polar covalent bond.

When two different kinds of atoms (usually two non-
metals) share their electrons to form a bond there may be unequal sharing between the two due to their electronegativities.

This means that the electrons spend more of their time closer to one atomic nucleus that the other.

The end of the bond where the negatively charged electrons spend more time is labelled as being partially negative ( ).

The end of the bond that is partially positive is labelled ( ). Observe fig.3 page 252.

Thus, every polar bond has a bond dipole: a partial negative charge and a partial positive charge, separated by the length of the bond.

Figure: Dipoles are often represented using vectors. Vectors are arrows that have direction and location in space.

Partial positive charge Partial negative charge

C = O

Dipole vector points from positive charge to negative charge

Other examples of polar covalent bonds include: C-O, O-H and N-H.

Carbon and hydrogen attract electrons to almost the same degree.

Therefore, when carbon is bonded to another carbon atom or to a hydrogen atom, the bond is not usually considered to be polar.

To answer the question above:

If equal bond dipoles act in opposite directions in three-
dimensional space, they counteract each other.

A molecule with identical polar bonds that point in opposite directions is not polar.

If the bond dipoles in a molecule do not counteract each other exactly, the molecule is polar.

Question:

(a)	Determine whether the following molecules are polar or non-polar. Justify your answer.

(a) CCl4 (b) CCl3H (c) BeF2 (d) HCl

2.	Which of the following is more polar? Justify your answer.

(a) NF3 or NCl3 (b) Icl4 or TeCl4

Pauling like to think of chemical bonds as being different in degree rather than different in kind.

To him, all chemical bonds involved a sharing of electrons, even ionic bonds.

The degree of sharing depends upon the difference in electronegativities of the bonded atoms.

Percent ionic character

Ionic Polar Covalent Covalent

Electronegativity Difference

What if you know the formula of a compound but do not know how to classify it? In this case, you can use the electronegativity difference between the bonding atoms to predict the type of bond.

Eg. Two atoms with identical electronegativities, such as chlorine ( EN= 3.16- 3.16 = 0) share electrons equally. They are bonded covalently.

When two atoms have different electronegativities, the atom with the higher electronegativity value attracts electrons more strongly than the atom with the lower electronegativity value.

In sodium chloride, for instance, chlorine (EN = 3.16) attracts an electron more strongly than sodium (EN = 0.93). Therefore, sodium's valence electron has a very high probability of being found near chlorine, and a very low probability of being found near sodium. A high electronegativity difference is characteristic of ionic compounds.

EN = 3.16 – 0.93

= 2.23

According to the scale given above, this substance can be classified as ionic.

Let us try to predict whether a substance is polar or non-polar without using the values of electronegativities.

Both the shape of the molecule and the polarity of the bonds are necessary to determine if a molecule is polar or non-polar.

Let us observe the methane molecule (CH4).

Methane is a non-polar substance and its C-H bonds are polar. The shape diagram with bond dipoles shows that the outer part of the molecule is uniformly positive and therefore the molecule has no ends that are charged differently. A nearby molecule would "see" the same charge from all sides of the methane molecule. This is true because the CH4 molecule is symmetrical. In fact, all symmetrical molecules are non-polar for this reason.

In all symmetrical molecules, the sum of the bond dipoles is zero and the molecule is non-polar.

Question:

Predict the polarity of the ammonia, NH3, molecule, including your reasoning.

Step 1:

-	draw the Lewis structure

Step 2:

-	based on the Lewis structure, draw the shape diagram

Step 3:

- add the electronegativities of the atoms, from the periodic table, and assign and to the bonds.

Step 4:

-	draw in the bond dipoles

The ammonia molecule is polar because it has polar bonds that do not cancel to zero. The electron pairs are in a tetrahedral arrangement, but one of these pairs is a lone pair and three are bonding pairs. Therefore, the bond dipoles do not cancel.

Practice Problems: Page 255-256

#6 (a) (b), 7 (a) (c) (d), 8 (b) (d), 10

Questions: Page 256

#1 (a) (b) (e), 2 (a) (b) (e), 3 (a) (b)