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Shapes of Molecules and Ions with 5 and 6 negative
centers
•Section 14.1
Sulfur Hexafluoride Octahedral with bond angles of 90º and 180º Notice there are 12 electrons around the
sulfur atom
VSEPR Theory Can be used for 5 and 6 negative centers Elements in the 3rd and lower periods of the
p-block can promote one or more electrons from a filled s or p orbital to an unfilled, low energy d orbital
More unpaired electrons are available for bonding
Continued Have an expanded valence shell with more
than 8 electrons Usually occurs when the atoms are
relatively small and very electronegative so they can fit around the central atom
Lewis structures are a little more challenging as you have to consider the total number of electrons available
Examples: SF6, PCl5, XeF4, PF6-
Phosphorus Pentachloride Trigonal bipyramidal with angles of 90º and
120º
Xenon Tetrafluoride Unshared (lone) pairs of electrons distort the
shape to make it a square planar with bond angles of 90º
Sulfur Hexafluoride Shape is an octahedron See pages 107 or 375 for additional pictures
and examples
Resonance Structures Consider the nitrate ion and its Lewis
structure It has three possible structures Note the use of lines and dots whereas you
must use one or the other
Continued Experiments have shown that the bonds do
not really switch back and forth Instead, all the bonds have the same length
and energy A better structure is called a resonance
hybrid:
Continued The shape is trigonal planar The electrons are said to be delocalized as in
they are shared evenly and not with just one nucleus
Consider the nitrite ion. The resonance structures are:
More on the Nitrite Ion The shape is bent The resonance hybrid is:
Consider the Carbonate Ion Has three resonance structures The resonance hybrid is trigonal planar
Ozone (O3) Has two resonance structures The resonance hybrid is bent
RCOO-
R represents a combination of CH3 with a number of CH2 attached (or not)
The important part is that the two oxygens experience resonance
The shape is trigonal planar
Benzene Ring The resonance structures are called Kelule
structures
Resonance structures can be drawn as any of the examples below