LECTURE 20- Drawing Lewis Structures for Exceptions to the Octet Rule

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