4.3 - 4.5 polarity

4.3, 4.5 Polarity Curriculum C2.1 use appropriate terminology related to structure and properties of matter, including, but not limited to: orbital, emission spectrum, energy level, photon, and dipole [C] C2.4 predict the polarity of various chemical compounds, based on their molecular shapes and the difference in the electronegativity values of the atoms [AI] Electronegativity In 1922, an American chemist, Linus Pauling, proposed a way of quantitatively describing the ability of an atom to attract electrons (combining ionization energies, electron affinity, and some other measures of reactivity). This scale is referred to as electronegativity. Con’t electronegativity: a number that describes the relative ability of an atom to attract electrons. Pauling assigned a value of 4.0 to fluorine, the element considered to have the greatest ability to attract electrons. Electronegativity (EN) values for all other elements were assigned relative to the value for fluorine. Con’td According to Pauling, the greater the difference in (EN), the more polar the bond. The smaller the difference, the more nonpolar the bond. A very polar bond is an ionic bond. A nonpolar bond is a covalent bond. A somewhat polar bond is a polar covalent bond. Polar Covalent Bond polar covalent bond - a bond in which electrons are shared somewhat unequally. This means that the electrons spend more of their time closer to one atomic nucleus than 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 ?+. Ex. HCl Degree of Electron Sharing If the difference in EN is less than 0.5, the bond type is non-polar covalent. If the difference in EN 0.5-1.7, the bond type is polar covalent. If the difference in EN is greater than 1.7, the bond type is considered ionic. Samp prob 1 Label the following atoms and bonds with EN and bond polarity, and classify the bond: (a) H-H (b) P-Cl (c) Na-Br 4.5 Molecular Polarity A polar molecule is one in which the charge is not distributed symmetrically among the atoms making up the molecule. Polar Molecules Ex. CO2 Polar or non-polar molecule? The existence of polar bonds in a molecule does not necessarily mean that you have a polar molecule. Carbon dioxide is considered to be a nonpolar molecule, although each of the C=O bonds is a polar bond. Using ENs, we can predict the polarity of each of the bonds. It is customary to show the bond polarity as an arrow, pointing from the positive (?+) to the negative (?-) end of the bond. This arrow represents the bond dipole. These arrows are vectors and when added together produce a zero total. In other words, the bond dipoles cancel to produce no polarity for the complete molecule, or a nonpolar molecule. Another example, H2O The Lewis structure and VSEPR rules predict a V-shaped molecule, shown here with its bond dipoles. The bond dipoles (vectors) do not cancel. Instead, they add together to produce a non-zero molecular dipole (shown in red). The water molecule has an overall polarity and that it is why it is a polar molecule. Note that the water molecule has a partially negative end near the oxygen atom and a partially positive end at the two hydrogen atoms. Although the “ends” of the water molecule are not initially obvious, the V shape produces two oppositely charged regions on the outside of the molecule. This explains why a stream of water is attracted to a positively charged strip or rod. Shape and Polarity Both the shape of the molecule and the polarity of the bonds are necessary to determine if a molecule is polar or nonpolar. In all symmetrical molecules, the sum of the bond dipoles is zero and the molecule is nonpolar. Rules to determine polarity 1 - Draw a Lewis structure for the molecule. 2 - Use the number of electron pairs and VSEPR rules to determine the shape around each central atom. 3 - Use ENs to determine the polarity of each bond. 4 - Add the bond dipole vectors to determine if the final result is zero (nonpolar molecule) or nonzero (polar molecule). Ex. CH4 All of the bond dipoles point into the central carbon atom. There are no positive and negative ends on the outer part of the methane molecule. Samp prob 2 Predict the polarity of the NH3 molecule and including your reasoning. HMWK p. 221 #1-10 (odd letters) P. 229 # 1-7 (odd letters)