Transcript Lewis Dot Structures

Lewis Dot Structures
Valence Electrons

Electrons in the “outer shell” (Bohr) or the
highest energy s and p orbitals (Wave
Mechanical) of an atom

Sulfur
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1s22s22p63s23p4
Highest Principal Energy Level is 3
2 electrons in 3s, 4 electrons in 3p = 6 valence
electrons
Valence Electron Patterns

The number of valence electrons can be easily
read from the top of the periodic table.
Lewis Dot Structures
Show us the NUMBER and TYPES of bonds
in each molecule.
 We will also be able to tell the SHAPE of
the molecules
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Drawing Lewis Dot Structures
1.
2.
3.
Obtain the TOTAL number of valence
electrons in the entire molecules.
Use one pair of electrons to form a bond
between each pair of bound atoms.
Draw a LINE to represent the bonding
pair. (In general, if there is a single
element, it is the center atom and all
others bond to it).
Arrange the remaining electrons to
satisfy the octet/duet rule in the outer
atoms.
Drawing Lewis Dot Structures…
If there are any remaining electrons, place
them on the center atom.
 Check to see that there is an octet on
EVERY atom (except Hydrogen).
 If there are not enough electrons to make
octets on all atoms, SHARE!
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Make a DOUBLE BOND
If there are still not enough, make a TRIPLE
BOND
Practice Problems

Draw the Lewis Dot Structures for the
following molecules. Make sure:
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there are no more electrons around the atoms
than the number of total valence electrons
all atoms have obey the octet rule
O2
N2
CO2
Answers
O2
2(6)=12 v.e.
N2
2(5)=10 v.e.
CO2
4+2(6)=16
Resonance Structures
When two or more correct Lewis Dot
Structures can be drawn for a single
molecule, they are known as resonance
structures.
 The following molecules have resonance
structures. Can you show how many?
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CO2
NO3-1
SO2
CO3-2
•3
•3
•2
•3
Exceptions to the Octet Rule

Every rule has an exception, eh?
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BORON
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Tends to act more like an ionic compound and
forms only 3 bonds
BF3 makes a molecule with 3 single bonds and
only 6 electrons around the B
BF3 reacts vigorously with NH3 to make H3NBF3
which further proves the structure
Exceptions to the Octet Rule
Any ionic compound
 Ionic compounds GIVE and TAKE electrons
to form their bonds. The ones that GIVE
electrons get an octet or duet from their
inner electrons.
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Examples:
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NaCl
MgCl2
Exceptions to the Octet Rule
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Paramagnetism
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When an object displays paramagnetism, it
reacts to a strong magnet, indicating that it
has UNPAIRED ELECTRONS
OXYGEN GAS (O2)
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The LDS that satisfies the octet rule shows
that it has a double bond, yet its observed
behavior is paramagnetic.
Exceptions to the Octet Rule
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Odd numbers of electrons
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NITROGEN
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NO and NO2 have eleven and fifteen valence
electrons, respectively, yet both are observed
molecules
Exceptions to the Octet Rule
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More than an octet
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Some molecules have more than an octet
Only if they have d orbitals that electrons can
go into
N, O, F CANNOT ever have more than an octet
Examples
IF5
AsF6
IBr3
Bond Order
When referring to bond order, it is usually
the type of bond.
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Single bond = bond order of 1
Double bond = bond order of 2
Higher bond order means:
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Stronger bonds (harder to break)
Shorter bond lengths (atoms closer together)
Bond Order
When a molecule has resonance
structures, the bond order tends to be an
average of the resonant structures.
 Predict the bond order of each bond in the
following molecules that you already wrote
resonance structures for:
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CO2
SO2
NO3-1
CO3-2
•2
•1.3
•1.5
•1.3