I. “Molecular Polarity” I. “Molecular Polarity” – A term that is used to distinguish two types of molecules…. Based on the presence or absence of a “molecular dipole”. – A term that is used to distinguish two types of molecules…. Based on the presence or absence of a “molecular dipole”.

A. Some molecules show characteristics indicating that they have oppositely- charged ends (a positive end and a negative end). These molecules are said to be polar. A. Some molecules show characteristics indicating that they have oppositely- charged ends (a positive end and a negative end). These molecules are said to be polar. B. Other molecules show characteristics indicating that their structure doesn’t have oppositely–charged ends. These molecules are said to be nonpolar. B. Other molecules show characteristics indicating that their structure doesn’t have oppositely–charged ends. These molecules are said to be nonpolar.

II. How to determine whether a molecule is polar. A. IMPT: You MUST first make a “sphere and stick” sketch of the molecule; then, follow these steps: Calculate each individual bond’s polarity. Calculate each individual bond’s polarity. First identify the “electronegativity value” (referred to as “EN”) of each atom. First identify the “electronegativity value” (referred to as “EN”) of each atom.

(1) The EN is a number from 0 to 4, given on your periodic table, side 2, within each element’s square…..upper right corner of the square, in black print. (1) The EN is a number from 0 to 4, given on your periodic table, side 2, within each element’s square…..upper right corner of the square, in black print. (2) Atoms whose EN is close to “4” attract electrons strongly; atoms whose EN is close to “0”, attract electrons weakly. (2) Atoms whose EN is close to “4” attract electrons strongly; atoms whose EN is close to “0”, attract electrons weakly.

Subtract the two EN values. Take the absolute value of the answer (make the answer positive). Subtract the two EN values. Take the absolute value of the answer (make the answer positive). Look at the chart below to see where the│EN – EN│answer falls. Look at the chart below to see where the│EN – EN│answer falls.

( │EN – EN│ )Category of Bond (including 0.4) Non-Polar Covalent Bond (over 0.4; including 2) Polar Covalent Bond Indicated by writing δ + and δ – symbols  2.0  (over 2) Ionic Bond

a.When │EN – EN│ falls in the polar category, insert a δ+ symbol next to the atom with the lower EN and a δ– symbol next to the atom with the higher EN. b.[This might be the opposite from what you would have expected…..Remember, it is the atom with the highest the EN, which is the most able to attract electrons (which are negatively-charged).]

(1)A polar bond occurs when one atom attracts the electrons much more strongly than the atom it is connected to, giving the bond a “partial positive end” and a “partial negative end”. (1)A polar bond occurs when one atom attracts the electrons much more strongly than the atom it is connected to, giving the bond a “partial positive end” and a “partial negative end”.

b. Also, draw an arrow parallel to the polar bond and the same length as the polar bond; additionally, the arrow “point” must be pointed toward the δ– symbol. Last, place a “+” over the non-pointy end of the arrow. Ex: + c. When │EN – EN│ falls in the nonpolar category, don’t insert those two symbols or the arrow. In nonpolar bonds, atoms share the bonding electrons more or less equally.

B. Practice bonds: For each bond below, find the EN values, then calculate the │EN – EN│ answer. Then form your conclusion: polar covalent bond or nonpolar covalent bond.

( │EN – EN│ )Category of Bond (including 0.4) Non-Polar Covalent Bond (over 0.4; including 2) Polar Covalent Bond Indicated by writing δ + and δ – symbols  2.0  (over 2) Ionic Bond

X III. The second part of determining “molecular polarity” (the whole molecule’s polarity) involves shape. X III. The second part of determining “molecular polarity” (the whole molecule’s polarity) involves shape. A. A molecule that has all nonpolar bonds must be nonpolar. A. A molecule that has all nonpolar bonds must be nonpolar. B. But, a molecule that has all polar bonds may or may not be polar! Sometimes bond polarities “cancel”, giving a nonpolar molecule. Sometimes bond polarities “add”, giving a polar molecule. B. But, a molecule that has all polar bonds may or may not be polar! Sometimes bond polarities “cancel”, giving a nonpolar molecule. Sometimes bond polarities “add”, giving a polar molecule. C. As long as every bond in a given molecule is identical and polar, then the 3 shapes drawn below to the left represent shapes where polar bonds are canceled. C. As long as every bond in a given molecule is identical and polar, then the 3 shapes drawn below to the left represent shapes where polar bonds are canceled. D. The structures drawn below to the right represent instances where the polar bonds do not cancel. D. The structures drawn below to the right represent instances where the polar bonds do not cancel.

Polar bonds  Nonpolar Molecules: These structures all have polar bonds which cancel due to the molecule’s shape:

Polar bonds  Polar Molecules: These structures all have polar bonds which do not get cancelled. When there is only 1 polar bond in a molecule, there is nothing to cancel with! In the other examples, the polarity isn’t cancelled, it is combined (it adds). Bent Pyramidal Linear diatomic Linear triatomic - Tetrahedral +

OMG, what if I forget in which shapes the polarity gets canceled? OMG, what if I forget in which shapes the polarity gets canceled? Then you use the arrows that I showed you earlier. Then you use the arrows that I showed you earlier. Combine them together, head to tail, and see if they cancel, or if they add. Combine them together, head to tail, and see if they cancel, or if they add. If you end anywhere different from where the first arrow started, then the arrows “add”, which means that the polarity combines (and the molecule stays polar). If you end anywhere different from where the first arrow started, then the arrows “add”, which means that the polarity combines (and the molecule stays polar). If your arrows form a “closed loop” (the last arrow ends at the same spot as where the first arrow started), then the arrows “cancel” (and the bond polarity cancels). If your arrows form a “closed loop” (the last arrow ends at the same spot as where the first arrow started), then the arrows “cancel” (and the bond polarity cancels). Observe the vector addition for linear triatomic, trigonal planar, and tetrahedral molecules: All these molecules are non-polar, even though they all had individually polar bonds. Observe the vector addition for linear triatomic, trigonal planar, and tetrahedral molecules: All these molecules are non-polar, even though they all had individually polar bonds.

XIV. Polar molecules respond to an electric field: A. When placed between a positively charged metal plate and a negatively charged metal plate, the water molecules line up, with their positively-charged region attracted toward the negative plate and their negatively- charged region attracted toward the positive plate.

XIV. Inter- molecular “Bonds” (forces of attraction). A. Inter-molecular bonds (forces) are the attractions between 2 molecules. They hold molecules together. They are much weaker than polar and non- polar covalent bonds.

B. Breaking an inter-molecular bond is a physical change; whereas, breaking an intra-molecular bond is a chemical change. When you boil water, all you really do is break the intermolecular bonds between water molecules

C. Important category of intermolecular bond: The hydrogen bond. 1. Hydrogen bonding occurs when there is a hydrogen atom covalently bonded to one of these three atoms: F, O, N. Ex: H 2 O, HF, NH 3 Life would not occur as we know it were it not for hydrogen bonding: Hydrogen bonds hold the DNA double helix together, cause water to expand as it freezes, causes water to have a “greater than expected” boiling temperature, and play a part in maintaining the structure of proteins.