Principles of Chemistry I
Principles of Chemistry I CHM 211
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This 0 page Class Notes was uploaded by Houston Kovacek on Sunday November 1, 2015. The Class Notes belongs to CHM 211 at Marshall University taught by Michael Castellani in Fall. Since its upload, it has received 12 views. For similar materials see /class/233283/chm-211-marshall-university in Chemistry at Marshall University.
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Date Created: 11/01/15
3 bound atoms It is the shape you would see if molecules were composed of balls held together by solid rods If all pairs of electrons are bonding electrons the molecular shape is the same as the base shape In general use the following rules to determine the shape of a molecule 1 Draw the correct Lewis structure 2 Determine the total number of electron pairs around the nucleus in question by adding the number of lone pairs and bonding pairs on the central atom Treat multiple bonds as a single bonding pair 3 Use this total to select the base shape from the table above 4 Place the electron pairs around the atom in the least crowded way This requires lone pairs go into the least hindered positions Multiple bonds are larger than single bonds Rule 4 affects only atoms with 5 or 6 pairs of electrons We ll now go over some examples Total electron pairs 2 The molecule is lin with 2 bonding pairs or one each bonding pair lone pair Total electron pairs 3 base shape trigonal planar bonding prs 3 lone pairs O gt trigonal planar BF3 2 1 gt bent O3 1 2 gt linear 2 Total electron pairs 4 base shape tetrahedral bonding prs 4 lone pairs O gt tetrahedral CH4 3 l gt trigonal pyramidal NH3 2 2 gt bent H2O l 3 gt linear HF For the neXt two base geometries we must consider lone pair placement The 5 positions on a trigonal bipyramid are not all equivalent Those in the axial A position are more crowded because they have 3 X 90quot contacts while those in the equatorial E positions have 2 X 90quot and 2 X 120 contacts Since the 120 positions are swept back these are very weak interactions and may be ignored The gures below show the different positions the 90 interactions the 180 interactions and an alternative way of viewing the molecule respectively It is important to remember that the last view does not show correct atom connectivity for all atoms but may make visualizing the molecule easier Total electron pairs 5 base shape trigonal bipyramidal bonding prs 5 lone pairs O gt trigonal bipyramidal PF5 4 l gt seesaw 3 2 gt Tshaped 2 3 3 linear 5 In an octahedron all six sites are initially equivalent However after the rst lone pair is placed the least hindered position lies opposite to the rst lone pair This is because a position 90quot to the rst lone pair forces an interaction between the lone pairs that doesn t occur at 180 Since lone pairs are larger than bonding pairs placing lone pairs 1800 to each other is favored As for the trigonal bipyramid above the rst depiction of the octahedron below shows the correct atom connectivity while the second may help visualize an octahedron better 39 Total electron pairs 6 base shape octahedral p 3 F1 bondmg prs 6 lone pa1rs O 3 octahedral SF6 H39SVF F 1 u E quotAF quot 5 l 3 square pyram1dal IFS 1 1 quot F quot 4 2 3 square planar XeF4 X6 Finally one does not normally discuss the geometry of a molecule with no central atom e g CH3CHZOH rather the geometry around each atom in the molecule is considered Skip the section The Effect of Nonbondmg Electrons and Multiple Bonds on Bond Angles p 348349 93 Molecular Shape and Molecular Polarity In Chapter 8 we discussed polar and nonpolar bonds Remember that in polar bonds the electrons are not shared equally so a charge separation occurs along the bond