CH%20301%20Chapter%203%20notes%20part1

CH%20301%20Chapter%203%20notes%20part1 - Chapter 3 part 1...

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Chapter 3 part 1 VSEPR and VB theories VSEPR = V alence S hell E lectron R epulsion - determines the likely shape of a molecule - bonds and lone pairs repel- keep them apart! - does not describe how the bonding occurs VB = V alence B ond - tries to explain how bonds form - predicts which orbitals / hybrid orbitals are used - Hybrid orbitals created from s,p,d orbitals as required . USING VSEPR THEORY: 1. Draw correct Lewis dot structure for molecule or ion 2. Count RHED around central atom. 3. Predict ELECTRONIC geometry: shape includes all RHED - lone pairs far apart as possible 4. Predict MOLECULAR geometry: (‘shape’ of molecule) - lone pairs are “invisible” WHAT’S A RHED? REGION of HIGH ELECTRON DENSITY Any area where valence electrons are found. Look at the central atom of the molecule: SINGLE, DOUBLE, TRIPLE bonds AND lone pairs are EACH considered ONE RHED. USING VSEPR THEORY: Electrons repel each other. What 3D shapes permit the regions of electrons to be as separated as possible? VSEPR THEORY : POSSIBLE ELECTRONIC GEOMETRIES (Fig 3.2) RHED ELECTRONIC GEOM. ANGLES 2 linear 180° 3 trigonal planar 120° 4 tetrahedral 109.5° 5 trigonal bipyramidal 90°, 180°, 120° 6 octahedral 90°, 180° LONE PAIRS AFFECT GEOMETRY ! *ZERO LONE PAIRS: MOLECULAR GEOMETRY = ELECTRONIC GEOMETRY ANY LONE PAIRS: POSITION TO KEEP THEM FAR APART: DEPENDS ON ELECTRONIC GEOMETRY. (most tricky for trigonal bipyramidal) VSEPR THEORY : POSSIBLE MOLECULAR GEOMETRIES WITH LONE PAIRS (Fig 3.1, 3.7) RHED ELEC. GEOM. # lone pairs MOLEC. GEOM ANGLES 2 linear 0 linear 180 ° 3 trigonal planar 0 trigonal planar 120° 1 bent 120° 4 tetrahedral 0 tetrahedral 109.5° 1 trigonal pyramidal 109.5° 2 bent <109.5° 5 trigonal bipyramidal 0 trigonal bipyramidal 90°, 180°, 120° 1 seesaw 90°, 180°, 120° 2 T shape 90°, 180° 3 linear 180° 6 octahedral 0 octahedral 90°, 180° 1 square pyramidal 90°, 180° 2 square planar 90°, 180° 3 T shape 90°, 180° 4 linear 180°
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CH%20301%20Chapter%203%20notes%20part1 - Chapter 3 part 1...

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