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1) hypervalent molecules or ions
molecules or polyatomic ions in which the central atom has more than the octet of valence electrons
2) hypovalent molecules or ions
molecules or polyatomic ions in which the central atom has few than the octet of valence electrons
3) odd-electron molecules or ions
molecules or polyatomic ions with an odd number of total electrons
(hypervalent) molecules or ions with central atoms in the second period never exceed 8 valence electrons
but in the third period & beyond, they do
(hypovalent) relatively rare & found only in compounds of
boron, berryllium, or alluminum
EXCEPTION
berryllium combines with nonmetals and makes covalent compounds
hypovalent molecules on their own are
very stable & exist in nature
hypovalent molecules are very reactive with many other molecules
achieve energetically more favored configuration with an octet of valence electrons (does not always achieve this, but it is the goal)
hypovalent molcules forms
coordinate covalent bonds
(odd-electron) total # of valence electrons is an odd number
not all of the electrons can be paired
resonance
both are equivalent lewis structures
more bond order (such as triple bonds)
shorter bond length
resonance structure
use a double-headed arrow between the 2 structures to indicate that the real molecule is described by an average of the resonance structures
resonance hybrid
average structure; represents true description of bonding in the molecule that explains its properties (NO LONE PAIRS)
electron pair in the resonance hybrid is
spread out/ delocalized over entire molecule
ex of NO2-
bond order= 3 bonding pairs of e-/ 2 resonance structures= 1.5
most desired resonance structure will
contribute/ show up more in the hybrid
if an element is more electronegative and there is a negative charge on it
it is most preferred
Note 
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