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Chapter 9 Sections 3-5 Lewis Structures Molecular Geometry Polarity Lewis Structures • Used to show the position of atoms in a molecule • Can help predict the shape of a molecule • Hydrogen will always be on the end (only bonds once) • The most electronegative atom should be placed in the center (farthest up and to the left on table) • 1 line or 2 dots represent a pair of electrons Steps for drawing Lewis Structures 1. 2. 3. 4. 5. 6. 7. Draw a stick figure Give atoms what they want: 1. Group 1= 2 electrons 2. Group 2 = 4 electrons 3. Group 3A = 6 electrons 4. Group 4A-8A = 8 electrons Count up the total number of electrons you USED Determine the total number of electrons AVAILABLE – get this from the periodic table (valence electrons = group numbers). Add any charge electrons also. Compare the USED with the AVAILABLE – if they match you are done If you USED too many – remove a lone pair from 2 bonded atoms and place another bond between them – continue until USED matches AVAILABLE If you have more AVAILABLE than used – add them as lone pairs to the central atom Practice Lewis Structures 1. CH4 2. NH3 3. CO2 4. PO435. PCl5 Lone pairs of electrons take up more space than a bonded pair of electrons. So the more lone pairs you have the smaller the angles become. Normally an atom with 4 attached groups has an angle of 109.5o Determining the type of Bond • Look at the differences in electronegativies Generally: – 0-0.5 difference = nonpolar covalent (equal sharing) – 0.5-1.7 = polar covalent (unequal sharing) – 1.7 and higher = ionic (transfer of electrons) • What type of bond exist between H and Cl if the electronegativity of H is 2.20 and Cl is 3.16? • 3.16 – 2.20 = 0.96 so it is a polar bond A molecule can have polar bonds and still be overall nonpolar if the polar bonds cancel out by going in opposite directions. Polarity • Polarity determines solubility • Remember “like dissolves like” • This means that polar substances dissolve in polar solvents • Nonpolar dissolves in nonpolar • Water is a POLAR molecule! • Why will oil not dissolve in water? Molecular Structures Lab Activity Draw the Lewis Structures for the following, then build the molecule and determine the shape and if it is polar or not. Molecule BeCl2 BH3 CH4 PH3 H2O PCl5 SF6 Lewis Structure Molecular Polar? Shape