Chapter 6: Chemical Bonding Part II – Polarity

How to use this PowerPoint Hi students, You are to look at each slide. If there is a picture or figure, you are to examine it. If there is a question, you are to answer it in as much detail as possible OUT LOUD. This helps you form solid answers when you don’t just think the answer, you say it out loud. Don’t click to move to the next slide until you have done these things. Use a separate sheet of paper to draw out your molecules when asked. Mrs. Meer

Electronegativity - the attraction of an atom for electrons in a bond F is the most electronegative element. In terms of bonding, which child would represent the more electronegative element? (Picture the ice cream as a pair of bonded electrons and the two children as atoms.) WHY?

Electronegativity Values Do you know how to draw in the arrows to represent the trends in electronegativity within a group and a period? What would they look like? Electronegativity Values Within a period, it increases as you move right. Within a group, it increases as you move upwards. Did you notice the noble gases are not shown? Why? The first three do not bond, so they do not have electronegativity values. The other three would align with the normal trends.

What are the three types of bonds? What do the differences in electronegativity indicate between two atoms? the bond type that they will form What are the three types of bonds? Ionic (difference of 1.7+) Polar covalent (difference of 0.5-1.7) Nonpolar covalent (difference of 0.0-0.4)

(Click to remove each box and reveal the answer.) What types of bonds are represented by (a), (b), and (c)? Nonpolar covalent Polar covalent (Click to remove each box and reveal the answer.) Ionic

Review the Following: Don’t go to the next slide until you and your partners review the differences between an ionic bond and covalent bond. Be sure to use the following words: electronegativity taking/taken/take sharing/shared/share metals and nonmetals

Did you get it right? Ionic bond – the electronegativity differences between two elements are so great that the electrons are completely taken by one element (the nonmetal) from another element (the metal). Covalent bond – the electronegativity differences are not as great, allowing the electrons to be shared between two atoms (two nonmetals)

Review the Following: Don’t go to the next slide until you and your partners review the differences between a polar covalent bond and nonpolar covalent bond. Be sure to use the following words: electronegativity unequally and equally sharing/shared/share

Did you get it right? Both polar and nonpolar bonds are covalent, meaning the electrons are shared between them. The electronegativity difference is not great enough to remove an electron from one atom (that would be an ionic bond). A nonpolar bond is when the difference in electronegativity is close to zero or zero and the electrons are shared equally. A polar bond is when the difference in electronegativity is great enough that one atom has a greater attraction to the electrons than the other. This causes the electrons to be shared unequally.

An analogy to a polar covalent bond An analogy to a polar covalent bond. The geographical center of the contiguous 48 states of the United States remains fixed ( D), but the population center (O) is moving to the south and west. The separation between these two centers is analogous to the separation of the centers of positive and negative charge in a polar covalent bond. As the distance between the centers becomes smaller, the bond becomes less polar.

Nonpolar Covalent Bond Nonpolar covalent bond – a covalent bond in which the electrons are shared equally Example: Cl2 Both chlorines have the same electronegativity.

Polar Covalent Bond H Cl Polar covalent bond – a covalent bond in which the electrons are shared unequally Example: HCl H Cl Which end represents the hydrogen end & which end represents the chlorine end? How do you know this answer? Cl has a higher electronegativity

Dipole dipole – partial negative or partial positive charge formed during unequal sharing of electrons (in polar bonds only) d+ d- The direction of a dipole is from the dipole’s positive pole to its negative pole.

Nonpolar vs. Polar Covalent Bond There is no dipole drawn over the H2 molecule… WHY? The dipole is drawn over the HCl molecule. The arrow points towards the more electronegative element, Cl. There is not an uneven distribution of electrons.

Nonpolar vs. Polar Covalent Bond The electrons are not shared equally within the HCl molecule. Compare the electron clouds for the H2 and HCl molecules. Why are they different? Do we usually draw the electron clouds with the Lewis structures? NO

Each dipole represents a polar covalent bond. Example: PCl3 has three dipoles because there are three polar bonds. These three dipoles are drawn beside the bonds, pointing towards the more electronegative element, Cl. What is the more electronegative element, P or Cl? (Use your periodic table and the general trend to figure this out.) P Cl Cl Cl So, how would the dipoles be drawn? Each dipole represents a polar covalent bond.

You Try It! Draw the Lewis Structures on your white board for the following molecules (Aha!…they must all be covalent, then!) and LABEL ALL THE DIPOLES along each bond. CO2 HBr NH3 Draw them on your white board and only when you are ready to check the Lewis structures AND dipoles, go on to the next slide.

Did you draw in the dipoles along each bond????? If so, click to go to the next slide. If not, go back.

Answers Did you make CO2 linear? Did you make NH3 trigonal pyramidal? Did you show the dipoles pointing to the more electronegative element?

Dipoles within Molecules There are dipoles created along bonds, however, a molecule can exhibit overall polarity. Here is the dipole along the first H-O bond… An overall dipole is created. Here is the dipole along the second H-O bond…

An overall dipole is created. Two individual dipoles along each bond cause the molecule to have a greater electron cloud towards the oxygen end leaving the other end of the molecule more positive. This causes a partial negative end towards the oxygen and the a partial positive end towards the hydrogens (you can only have two “ends” to a small molecule. An overall dipole is created.

The bond is polar and so is the overall molecule. When you have a two atom molecule, it is easy to tell if it is a polar molecule. The bond is polar and so is the overall molecule. partially negative end partially positive end

Polarity of a Molecule Polarity of a molecule depends upon two things the polarity of the bonds the shape of the molecule All molecules with only nonpolar bonds are nonpolar molecules. Molecules with polar bonds, may or may not be polar molecules, depending on the shape.

carbon monoxide, CO Is the bond polar? YES Is the molecule polar? YES…a two atom molecule is polar if the bond is polar.

ammonia, NH3 Are the bonds polar? YES YES…the molecule is not symmetrical and does have a partial positive end and a partial negative end. You can draw on overall dipole. Is the molecule polar?

carbon tetrachloride, CCl4 Are the bonds polar? YES NO…the molecule is symmetrical and does NOT have a partial positive end and a partial negative end. (The center and the outside does not count as an “end”.) Is the molecule polar?

Polar or Nonpolar Molecule? Rule #1: All molecules with nonpolar bonds are always nonpolar molecules. Rule #2: Molecules with polar bonds, will be polar only if they are nonsymmetrical (like NH3).

You Try It! #2 Draw the following molecules, including the dipoles along each bond. Determine whether they are polar or nonpolar molecules. CH3Cl SF6 O2 H2S KCl Draw them on a separate sheet of paper. Click to see the answers.

Did you draw in the dipoles along each bond? Did you determine whether the entire molecule was polar or nonpolar? If so, click to go to the next slide. If not, go back.

Dissociation Remember this from earlier in the chapter? Do you notice that the water molecules are oriented in different ways, depending on what ion they are surrounding, Na+ or Cl-? After today, do you know why?

Why is water attracted to this rod Why is water attracted to this rod? The rod has been charged with static electricity. (Discuss this…we will talk about the right answer later.)

Why is water attracted to this comb? (Try it at home )

Are you ready for the challenge? Try the Polar or Nonpolar Molecules? worksheet.