无机化学67948.ppt

Molecules having steric number = 5 Molecules having steric number = 5 Electronegativity and atomic size effect Electronegativity is a measure of the tendency of an atom to attract a bonding pair of electrons. The Pauling scale is the most commonly used. Fluorine (the most electronegative element) is assigned a value of 4.0, and values range down to caesium and francium which are the least electronegative at 0.7. Chapter 3 p64 Electronegativity and bond angle The compounds containing F have smaller angles than those containing Cl, which in turn have small angles than those containing Br or I. As a result, the lone pair effect is relatively larger and forces smaller bond angles. The same result is obtained if size is considered: as the size of the outer atom increases in the FClBrI series, the angle increase * Octet rule: ns2np6 (n1); e.g., C4-, N3-, O2-, F-, Na+, Mg2+, etc. Lewis symbols e.g., the 2nd period elements Lewis structures of CH4 and H2O Lewis structure Resonance When a molecule or ion can be represented by two or more plausible Lewis structures that differ only in the distribution of electrons, the true structure is a composite, or hybrid, of them. The different plausible structures are called resonance structures. The actual molecule or ion that is a hybrid of the resonance structures is called a resonance hybrid. the resonance structures of ozone Exceptions of the octet rule molecules with an odd number of valence electrons, e.g., NO, 11 valence electrons; NO2, 17 valence electrons; ClO2, 19 valence electrons  molecules with incomplete octets, e.g., BF3, the central boron atom is electron deficient due to one empty p orbital, which makes it a Lewis acid and an important catalyst in organic synthesis intramolecular stabilization intermolecular stabilization  bond distance: BF3, 130 pm; BF4-, 145 pm structures with expanded valence shells, e.g., the 3rd-period elements are allowed to have more than 8 electrons due to the av