Chemistry 545 Inorganic Chemistry Lecture 1.

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Chemistry 545Inorganic ChemistryLecture 1.

• Lewis Dot Structures
• VSEPR

G. N. Lewis was probably the best chemist who
never won the Nobel Prize
Gilbert Newton Lewis (1875-1946)
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Lewis Dot Structures (revision)

• Lewis dot structures present a simple approach
  to bonding that allows us to rationalize much
  molecular structure. The idea is that atoms share
  electrons in the valence shell to form the
  chemical bond, with one pair of electrons per
  bond. Note that each H-atom has two electrons,
  which is the structure of He, the next inert gas.

Electron pair single bond
Valence electrons
H-atom H-atom H2 molecule
(Each H-atom has one valence electron)
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• Lewis Dot Structures (contd.)

Two shared pairs of electrons double bond
O-atom O-atom
O2 molecule
Periodic table
1 2
3 4 5 6 7 8
Oxygen has six valence electrons
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The octet rule

• Electrons are shared in forming bonds such that
  atoms have the same number of electrons in their
  valence shells as the nearest noble gas,
  including the electrons shared with the atom to
  which they are bonded.

O-atom O-atom
O2 molecule
Each oxygen atom in the O2 molecule now has
eight valence electrons, including those it
shares with the other oxygen atom number of
electrons (8 octet) in the nearest inert gas
neon.
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8.1 Chemical Bonds, Lewis Symbols, and the Octet
rule.

• Chemical bonding involves mainly the attempt to
  achieve the rare gas number of valence electrons,
  i.e. an octet. This can be achieved in several
  ways.
• Ionic bond Electrons are mainly the property of
  one of the two atoms forming the bond.
• Covalent bond Electrons are shared so that each
  atom has a noble gas electronic configuration.
• Metallic bonds. Electrons are lost into the
  conduction band.

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8.2 Ionic Bonding.

• This occurs between metallic elements from the
  left-hand side of the periodic table and
  non-metallic elements from the right hand side of
  the periodic table.
• Note that Na gives up its lone valence electron
  to Cl, so that they both end up with an octet of
  electrons.

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8.3 Covalent bonding.

• Here the two atoms share the electrons to
  achieve a covalent bond.

two pairs of electrons equally shared between the
two oxygen atoms
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Multiple Bonds and bond order

• The sharing of a single pair of electrons
  consititutes a single bond. Sharing of two pairs
  of electrons constitutes a double bond, and
  sharing three pairs of electrons constitutes a
  triple bond.
• HH OO NN
• Single bond double bond triple bond
• Bond order a single bond has bond order 1, a
  double bond has bond order 2, and a triple bond
  has a bond order 3. Fractional bond orders such
  as 1½ or 1? are also possible, as discussed below.

.. ..
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Some more examples of Lewis dot structures

• The N2 molecule

triple bond
N-atom N-atom
N2 molecule
Periodic table
1 2
3 4 5 6
7 8
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Examples of Lewis dot diagrams

• Methane, CH4

One shared pair of electrons single bond
Carbon has four valence electrons (red)
Hydrogens achieve two electrons like He
Carbon achieves octet of electrons
single line single bond
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Examples of Lewis dot diagrams

• Carbon dioxide (CO2)

Carbon has four valence electrons (red)
oxygens have six valence electrons (black)
OCO
double line double bond
two shared pairs of electrons double bond
Carbon and both oxygens achieve an octet of
electrons
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Examples of Lewis dot diagrams

• Sulfur dioxide (SO2)

single bond?
double bond?
OS-O (or O-SO ?)
actual structure is average of the two (bond
order 1½)
SO2 is an example where a molecule can be
written in two ways and actual structure is the
average of the two. This is called RESONANCE (see
later)
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Slightly different Lewis dot representations

• One can also represent molecules/ions with a
  combination of dots and lines for bonds,
  remembering that each line represents a shared
  pair of electrons, e.g. the phosphate anion

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8.6 Resonance structures Ozone (O3)
bond order 1½
double arrow resonance
O-O bonds 2.78 Å
The ozone molecule can be written with two
equivalent Lewis dot structures. In such a
situation the actual structure is the average of
these two structures, with the two O-O bond
lengths equal.
O
O
O
The ozone molecule
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Resonance structures the nitrite anion (NO2-)
In drawing up a Lewis dot diagram, if we are
dealing with an anion, we must put in an extra
electron for each negative charge on the anion
negative charge on anion
One extra electron in Lewis dot diagram
because of single negative charge on anion
Bond order 1½
Two resonance structures average
structure
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The nitrate anion
B.O. 2
B.O. 1
B.O. 1
average bond order (B.O.) 2 1 1 1?
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to work out bond order, pick the same bond
in each structure and average the bond order for
that bond
Number of canonical structures
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Resonance in benzene.
There are two canonical structures for benzene,
which means that the C to C bonds have a bond
order of (21)/2 1.5. The benzene ring has a
very high stability due to this resonance, which
is called aromaticity.
or
Short-hand versions for the benzene ring
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8.7. Exceptions to the octet rule.

• BF3. This can be written as F2BF with three
  resonance structures. To complete its octet, BF3
  readily reacts with e.g. H2O to form BF3.H2O. The
  actual structure of BF3 appears not to involve a
  double bond and does not obey the octet rule

Best repre- sentation of BF3 with B having
only 6 electrons in its valence shell
Possible resonance structure for BF3, but is not
important as this would involve the very
electronegative F donating es to B
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Exceptions to the octet rule free radicals

• There are some molecules that do not obey the
  octet rule because they have an odd number of
  electrons. Such molecules are very reactive,
  because they do not achieve an inert gas
  structure, and are known as free radicals.
  Examples of free radicals are chlorine dioxide,
  nitric oxide, nitrogen dioxide, and the
  superoxide radical

odd electrons
nitric oxide chlorine dioxide
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Exceptions to the Octet rule Heavier atoms (P,
As, S, Se, Cl, Br, I) may attain more than an
octet of electrons

• Example PF5.
• In PF5, the P atom has ten electrons in its
  valence shell, which occurs commonly for heavier
  non-metal atoms

leave off F electrons not shared with P
F
F
P
F
F
P has 10 valence electrons
F
PF5
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Many phosphorus compounds do obey the octet rule

• PF3 and PO43-

three blue electrons are from charge on anion
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Some compounds greatly exceed an octet of
electrons

• IF7 XeF6
• (both I and Xe have 14 valence es)
• (Think about XeF82-)