University:
Purdue University
Course:
CHM 12500 | Introduction To Chemistry I
Academic year:

2021
Views:

336

Pages:

6
Author:

Elliott James

CHM 123 - Lecture (Monday, 10:30am) Quiz this week 11/15 and 11/17 (during recitation) Exam 3 next week 11/21/05 (Monday, 7:15 pm in EE 129) CHAPTER 9 Molecular Structures Predicting Molecular Shapes Lewis structures show us connectivity in atoms, but don’t tell us the actual shapes of the molecules. Molecules are three-dimensional. Hence an extension to Lewis theory is needed to help us predict the arrangement of atoms in space. VSEPR (Valence Shell Electron Pair Repulsion): regions of high electron concentration repel each other (bonds, lone pairs, etc.) In this theory, we will focus only on the central atom of the molecule and what is connected to it. VSEPR (a) Central atom w/ Bonding pairs only * Central atom w/ lone pairs * 2 atoms attached to central: O=C=O To be as far apart as possible, bonding pairs (and oxygens) must be 180° apart. Electron-pair geometry: linear Molecular geometry: linear Multiple bonds count as only one attachment. In CO₂, 2 oxygen atoms are connected to carbon. 4 bonds are not required in this case. It's only the atoms that are connected to the central atom that concern us. 3 atoms attached to central atom H / \ B H \ / H To be as far apart as possible, bonding pairs (and hydrogens) must be 120° apart. Electron-pair geometry: Trigonal (triangular) planar. Molecular geometry: Trigonal (triangular) planar. 4 atoms attached to central atom H /|\ H-C-H \|/ H To be as far apart as possible, bonding pairs must be 109.5° apart. Electron-pair geometry: Tetrahedral Molecular geometry: Tetrahedral 5 atoms attached to central atom PCl5 Cl | Cl-P-Cl | Cl Axial position Equatorial position To be as far apart: bonding pairs must be a combination of 90° and 120° apart. Electron-pair geometry: Trigonal (triangular) bipyramidal Molecular geometry: Trigonal (triangular) bipyramidal 6 atoms attached to central atom F | F-S-F | | F F To be as far apart: bonding pairs must be 90° apart. Electron-pair geometry: Octahedral Molecular geometry: Octahedral

Molecular structures Lecture

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