M OLECULAR P OLARITY Dipole Theory Section 3.3 – Part B Pg )Determine the polarity of a molecule based on simple structural shapes and unequal charge distribution 2)Describe bonding as a continuum ranging from complete electron transfer to equal sharing of electrons.

P OLARITY Chemists believe that molecules are made up of charged particles (electrons and nuclei). A polar molecule is one in which the negative (electron) charge is not distributed symmetrically among the atoms making up the molecule. Thus, it will have partial positive and negative charges on opposite sides of the molecule. A molecule with symmetrical electron distribution is a nonpolar molecule. The existence of polar molecules can be demonstrated by running a stream of water past a charged object. Demo: See Figure 9

TESTING A LIQUID WITH A CHARGED OBJECT: In a liquid, molecules are able to rotate freely. Polar molecules in a liquid will rotate so that their positive sides are closer to a negatively charged material. Near a positively charged material they become oriented in the opposite direction.

EMPIRICAL RULES FOR POLAR AND NONPOLAR MOLECULES TypeDescription of molecule Examples PolarABdiatomic with different atoms HCl(g), CO(g) NxAyNxAy containing nitrogen and other atoms NH 3 (g), NF 3 (g) OxAyOxAy containing oxygen and other atoms H 2 O(l), OCl 2 (g) CxAyBzCxAyBz containing carbon and two other kinds of atoms CHCl 3 (l), C 2 H 5 OH(l) NonpolarAxAx all elementsCl 2 (g), N 2 (g) CxAyCxAy containing carbon and only one other kind of atom (except CO(g)) CO 2 (g), CH 4 (g) When the water test was repeated with a large number of pure liquids, this provided the set of empirical rules above. Pg. 99

PREDICTING AND EXPLAINING POLARITY Linus Pauling explained polarity by creating the concept of electronegativity. Introduced in Section 3.1 Electronegativity increases as you go up or to the right on the periodic table.

Pauling explained the polarity of a covalent bond as the difference in electronegativity of the bonded atoms. If the bonded atoms have the same electronegativity, they will attract any shared electrons equally and form a nonpolar covalent bond. If the atoms have different electronegativities, they will form a polar covalent bond. The greater the electronegativity difference, the more polar the bond will be. For a very large electronegativity difference, the difference in attraction may transfer one or more electrons resulting in ionic bonding. PREDICTING AND EXPLAINING POLARITY Cl 2(g) We use the Greek symbol delta to show partial charges

PREDICTING AND EXPLAINING POLARITY Pauling liked to think of chemical bonds as being different in degree rather than different in kind. According to him, all chemical bonds involve a sharing of electrons, with ionic bonds and nonpolar covalent bonds being just the two extreme cases The bonding in substances therefore ranges anywhere along a continuum from nonpolar covalent to polar covalent to ionic. For polar covalent bonds, the greater the electronegativity difference of the atoms, the more polar the bond. ~EN difference: nonpolar (< 0.4) polar (0.4+) ionic (m + nm)

PRACTICE See pg. 100 Sample Problem 3.4 Try on your own pg. 100 #9 (a-c), 10, 11(a only)

DOES BOND POLARITY = MOLECULAR POLARITY?? NO! Chemists have found that the existence of polar bonds in a molecule does not necessarily mean that you have a polar molecule. Example: Carbon dioxide is found to be a nonpolar molecule, although each of the CO bonds is a polar bond. WHY?? According to VSEPR (two bonds, no lone pairs) = linear arrangement We will start showing bond polarity as arrows pointing in the negative direction (where e - ’s want to go) = bond dipole Points from lower to higher electronegativity The arrows are vectors and when added together, the equal but opposite bond dipoles equal zero. Non-polar molecules are ones where the bond dipoles balance each other; producing a molecular dipole (vector sum) of zero δ– δ+ δ– O = C = O

P REDICTION M OLECULAR P OLARITY Step 1: Draw a Lewis formula for the molecule. Step 2: Use the number of electron pairs and VSEPR rules to determine the shape around each central atom. Step 3: Use electronegativities to determine the polarity of each bond. Step 4: Add the bond dipole vectors to determine whether the final result is zero (nonpolar molecule) or nonzero (polar molecule).

G UIDED P RACTICE #1 Predict the polarity of the water molecule. 1) Draw the Lewis formula 2) VSEPR: Draw the stereochemical formula 3) Assign the EN of the atoms, assign δ– and δ+ to the bonds 4) Draw in the bond dipoles O H Angular (bent) The bond dipoles (vectors) do not balance. Instead, they add together to produce a nonzero molecular dipole (shown in red). This results in a polar molecule (explains bending water) Go to Learning Tip pg. 102

G UIDED P RACTICE #2 Predict the polarity of the methane molecule. 1) Draw the Lewis formula 2) VSEPR: Draw the stereochemical formula 3) Assign the EN of the atoms, assign δ– and δ+ to the bonds 4) Draw in the bond dipoles Notice how all the bond dipoles point into the central atom. There are no positive or negative areas on the outer part of the molecule. A tetrahedral molecule is symmetrical in 3-D and four equal tetrahedral bond diploes always sum to zero Tetrahedral

P RACTICE Predict the bond polarity of the ammonia, NH 3(g) molecule. Include your reasoning. Answer: See pg. 102

FYI: DO YOU REMEMBER “LIKE DISSOLVES LIKE”? Means: “Polar substances are soluble in polar substances; Non- polar substances are soluble in non-polar substances” Mixing non-polar and polar substances results in them forming layers, with the least dense one on top. This occurs because polar molecules attract each other more strongly; thus they stay close together excluding nonpolar molecules Two clear liquids formed layers in this tube: nonpolar hexane C 6 H 14(l) on top, and polar water, H 2 O (l) below. Nonpolar dark orange liquid bromine, Br 2(l) was then added. The bromine dissolves much more readily in the nonpolar hexane.

WHY DO WE CARE ABOUT POLAR MOLECULES? Cleaning!! Water (polar) is useless at removing oil (nonpolar) so detergents are artificially created molecules that overcome this problem Detergents have long, nonpolar sections which are attracted to (dissolve in) a tiny oil droplet. The polar end of each of these detergent molecules helps form a polar “layer” around the droplet, which attracts polar water molecules. This allows them to pull the oil droplet away from a stained area of fabric and hold it suspended in the wash water.

H OMEWORK : Pg #13-16 Pg. 104 # 4, 5, 10