C and C have the same electronegativity. QUESTION: Which pair of atoms would form a bond where the polarity is directed towards the first atom? A. C and C, B. H and F, C. O and C, D. I and Cl Polar covalent bonding refers to unequal sharing of electrons. Polarity is directed towards the more electronegative element. The question rephrased: in which pair is the first atom more electronegative than the second atom? C and C have the same electronegativity. H is less electronegative than F O is more electronegative than C I is less electronegative than Cl SCRIPT: Which pair of atoms would form a bond where the polarity is directed towards the first atom? A. C and C, B. H and F, C. O and C, D. I and Cl PAUSE CLICK Polar covalent bonding refers to unequal sharing of electrons. Polarity refers to the extent that the sharing is unequal. This atom is said to be the more electronegative atom and polarity is said to be directed to the more electronegative atom. CLICK This question is asking for a pair of atoms where the polarity is directed towards the first one. Therefore, we need for the pair where the first atom is more electronegative than the second atom. Obviously, choice A is wrong. There’s no reason why electrons would prefer one carbon atom over another. Two carbon atoms have the same electronegativity. Electrons would be shared equally between these two atoms and the bond is called a pure covalent bond. CALLOUT “Pure covalent bond” Choice B is also wrong. You should know that fluorine is the most electronegative atom. So between H and F, the polarity would be directed towards F. CLICK Choice C is correct. Oxygen is more electronegative than carbon. Both elements are in the second period of the periodic table. Oxygen is farther to the right and the trend in electronegativity, from left to right, is increasing Choice D is wrong. Chlorine and iodine both belong to group 7A. Electronegativity increases going up a column. Iodine is below Chlorine. Iodine is in period 5, while chlorine is in period 3. We can represent the polarity of the HF , OC and I Cl bonds using the following diagrams. Note that the arrows are drawn pointing towards the more electronegative atom. CONTINUED ON NEXT SLIDE

F > O > Cl > N, Br > I > S, Se > C > P, As, H Increasing 2.5 3.0 3.4 4.0 2.2 2.6 3.2 Increasing 2.2 2.6 3.0 It’s good to remember that the trend in electronegativity is increasing towards fluorine. It’s increasing from left to right across a row CLICK So we can see that carbon is less electronegative than oxygen It’s increasing going up a group So iodine CLICK is less electronegative than chlorine In some cases, just knowing the general trend in electronegativities may not be enough to determine which element is more electronegative. In such cases, you can look up electronegativity values. You should be able to find these in your textbook from the Internet. The most electronegative element is fluorine, with a value of 4.0 on the Pauling scale Flourine is followed by oxygen, with 3.4 CLICK Then, chlorine, with 3.2 CLICK Then nitrogen and bromine, with 3.0 CLICK CLICK Then iodine, with 2.7 CLICK Sulfur and selenium, with 2.6 CLICK CLICK Phosphorus and Arsenic and hydrogen, with 2.2 Although you’re not expected to memorize these numbers, you should at least memorize the sequence, so that you can predict bond polarities. PAUSE END RECORDING 2.7

Video ID: 3-2-1 © 2008, Project VALUE (Video Assessment Library for Undergraduate Education), Department of Physical Sciences Nicholls State University Author: Glenn V. Lo Narrator: Funded by Louisiana Board of Regents Contract No. LA-DL-SELECT-13-07/08