5. Place remaining electrons, as nonbonding pairs, on central atom. 6. If the central atom does not have an octet, form multiple bonds by moving lone (nonbonding) pairs into bonds until the central atom has an octet. 7. Check for possible resonance structures. Determine average structure. 8. If non equivalent Lewis structures exist, check formal charge on each atom: minimize charge, minimize charge separation
78 Use of Formal Charge Use formal charge to determine if a particular Lewis structure is a reasonable possibility or to evaluate the importance of various Lewis structures for molecules that show resonance. Use formal charge to predict which atom is the central atom.
A Word of Caution • Formal charges are in general a better estimate of atomic charges in molecules than are oxidation states. • BUT formal charge is only an estimate of charge and should not be considered to give the actual atomic charge. • A valid Lewis structure is one that “obeys the rules”, but it may or may not accurately describe the molecule and its properties. • Experiments and observations of molecular properties must always be considered. 79
Predicting Molecular Geometry • Lewis structures give us a 2D picture of a molecule, but no information about the 3D shape (geometry). • Recall (Chapter 13 notes part 1) that whether a molecule is polar or non polar depends on the net dipole moment – the vector sum of all the individual bond dipole moments. • To determine molecular polarity one must know both the individual bond polarities AND the geometry of the molecule. • The Valence Shell Electron Pair Repulsion model provides us with a relatively simple way to predict the 3D geometry of a molecule. Ronald Gillespie McMaster University
81 13.13 V alence S hell E lectron P air R epulsion Theory VSEPR Basic Concept: Electron pairs (bonding and nonbonding) will arrange themselves as far away from each other as possible. Look at the arrangement of valence electrons around the central atom only Electron pairs in the valence shell of an atom will orient themselves so as to minimize the repulsion between like charges (negative electrons).
82 VSEPR • Draw the Lewis Structure for the molecule of interest • Determine the number of VSEPR pairs (non bonding pairs plus bonding pairs) of electrons around the central atom • NOTE: • The VSEPR pairs are arranged around the central atom so they are as far away from each other as possible to minimize electron-electron repulsion Whether a bond is a single, double or triple bond does not affect the geometry of the molecule. Double and triple bonds should be counted as 1 VSEPR pair
83 Predicting Molecular Geometry VSEPR Basic Shapes (electron pairs around the central atom are as far away from each other as possible) See Table 13.8 1. Two VSEPR electron pairs linear 2. Three VSEPR electron pairs trigonal planar 3. Four VSEPR electron pairs tetrahedral
84 VSEPR basic shapes 4. Five VSEPR pairs of electrons trigonal bipyramidal 5. Six VSEPR pairs of electrons octahedral equatorial and axial positions
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