Expanding on Lewis
Structures
Lewis Structures
• You can use lewis structures to
predict some patterns in bonding.
• Example : Nitrogen
• Lewis Structure:
• How many unpaired electrons?
• 3
• How many bonds will it form?
• 3
Practice Lewis Structures
• Draw the Lewis Structures for:
• SeF2
• SnCl4
• BF3
• Octet rule exception
• B is fine with 3 bonds – it has 3 valence,
so 3 bonds are expected
• F NEVER FORMS A DOUBLE BOND
Practice Lewis Structures
• Draw NH3 and BF3
• When they bond, the nitrogen will
donate it’s lone electron pair to the
boron
• Coordinate Covalent Bond
• When a bonding pair of electrons originates on
one of the bonded atoms
• Both electrons in a bond come from a single
atom
Practice Lewis Structures
• Draw the Lewis Structures for:
• NO
• Odd electron molecules can’t obey the
octet rule… why?
• ClO2
• Free Radicals – chemical species with an
unpaired electron
Practice Lewis Structures
• Draw the Lewis Structures for:
• SF4
• Expanded octet… empty 3d orbitals in S
• I3 • F3• Can’t happen… no “d” orbitals to hold
electrons
• Only elements of the 3rd or higher periods in the
periodic table may form compounds or ions in
which an octet is exceeded
Practice Lewis Structures
•
•
•
•
SiF5PF5
ClF3
XeF2
Resonance
• Draw CO3-2
• Each structure is equally likely,
“TRUE” CO3-2 is an average of the 3
structures
Resonance
• Draw Ozone (O3)
• One lewis structure cannot accurately
describe ozone
Resonance
• Resonance
• when a single Lewis structure fails to
describe accurately the actual electronic
structure
• Resonance Structures alternative
structures
• Actual structure is a composite : resonance
hybrid
Resonance Practice
• C6H6
• H2O4-2
• OCN-
Writing Formal Charges
• Every atom has an assigned formal
charge:
Formal Charge = [# of valence e-] –
[(# of nonbonding electrons)+1/2(#
of bonding electrons)]
Writing Formal Charges
• Formal charges must add up to the
overall charge
• Large formal charges are unlikely,
and negative formal charges “want”
to reside on very electronegative
elements
Writing Formal Charges
• Calculate the formal charge for:
• CN• SO3-2