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Unformatted text preview: Chem 210/ WANG / Chapter 8B
Lewis Structures Atom
placement Sum of
valence eLewis 5e- F 7e- X 3 = 21e- Total Hint: for main group elements
group # = valence e- # 26e- • Ions consist of two or more atoms
• Within these ions, groups of atoms
stay together by covalent bonding.
ammonium NO3- nitrate OH- hydroxide Fall 2009 / UM-SJTU JI : F: :Cl C For CH4O Atom
placement Both O and C are
“center” atoms .
H Sum of
valence e- : Cl :
: F: H Follow the same rules as molecules.
Must add or subtract electrons based on
the ionic charge.
Cation Subtract electrons from total.
number subtracted = charge
Anion Add electrons to the total.
number added = charge H Lewis structure with
Single bonds only 6e4 e- 4x 1e- = 4eTotal 32e- Lewis structures of
polyatomic ions O O
4H F & Cl 7e- X 4 = 28e- C
valence e14 - 10 = 4 e- C = 4e- Total Polyatomic ions NH4+ Lewis
only. 14e- Molecular
formula SO42- Atom
valence e32 - 8 = 24 e- O O Sum of
valence e- -2 : N Remaining
valence e32 - 8 = 24 e- Not enough to
give 8e- for all
/ triple bond. Add e- pairs to
8e Lewis structure of
a polyatomic ion
with single bonds
only. :O : : Remaining
valence e- F Sum of
valence e- Step 4 :O S O: : Lewis structure
with single bonds
only. Add e- pairs to each
atom to give 8e-. F : : : F: Cl
Cl C : Sum of
valence e- Atom
placement : Connect atoms with single
bonds (use 2e- per bond)
26 e – 6e in bonding = 20e left.
20e CCl2F2 : : : F: N Molecular
formula Connect atoms with single
bonds (use 2e- per bond). Remaining
valence e- Lewis
with = or ≡
bonds. : : F: Place atom with
lowest EN in center : Atom
placement : For NF3 Step 3 : •Explain bonding in polyatomic ions.
formula Too many eOne
more than 8e- structure
octet •Help determine molecular shape
http://www.up.ac.za/academic/chem/mol_geom/mol_geometry.htm Hint: for A group elements
group # = valence e- # Step 2 : 8.7 Exceptions to the Octet Rule Lewis structures show how electrons
are distributed among all the atoms in
a molecule. Place atom with
lowest EN in center Step 1 : O: : 8.6 Resonance Structures Molecular
formula : 8.5 Drawing Lewis Structures The steps in converting a molecular formula into a
Lewis structure. Lewis structures of
Covalent Compounds : Chapter 8.5- -8.7
8.5Basic Concepts of Chemical Bonding S & O 6e- x 5 = 30eAnion extra 2eTotal 32e- 1 Chem 210/ WANG / Chapter 8B
Lewis Structures Molecular
formula For CO2 Atom
placement Make C the
valence e16 - 4 = 12 e- 2 X 6e- C 4 e16e- count e- 6 e- from S
12 e- from 2O
18 e- total OSO SO2
count e- 6 e- from S
12 e- from 2O
18 e- total Positon atoms OSO : Total SO2 Position atoms &
connect them. C O: : :O : Sum of
valence e- : 2O OC Distribute e- Distribute remaining e- O Only have 18 e! OSO NOT enough e- for C
missing 4emake 4÷2 = 2 double
bonds!! OC O FAILED! NEXT
Make a double bond! OSO Missing 2e! All have octet now. Lewis structure with
two double bonds. SO2 Which Lewis structure is right?
We can write two or more Lewis structures for
They both - satisfy the octet rule
- have the same number of bonds
- have the same types of bonds count e- 6 e- from S
12 e- from 2O
18 e- total Positon atoms OSO Distribute eFAILED!
NEXT Consider again….. The double bond can
just as well be on the other side. Resonance
Resonance structures O- S = O
O - S =O OSO They
They both are! • The actual SO2 molecule is described by a resonance
hybrid of two equivalent Lewis structures, each
contributing 50% to the hybrid.
• Experimental proof : Two S-to-O bonds are found to
be equal (≈ 1½ bond), NOT one single and the other
a double bond. O = S- O O =S - O OSO O Another
Another example of
Benzene C6H6 also has two
equivalent Lewis structures,
each contributing 50% to
the resonance hybrid.
resonance SAMPLE PROBLEM 1 Write resonance structures for the nitrate ion, NO3-.
Nitrate has 1(5) + 3(6) + 1 = 24 valence eO O N O N O O O N O O O O Experimentally, the C-C bonds in benzene are
Call the same length (~ 1½ bond). Fall 2009 / UM-SJTU JI O N = O N
O O N does not
have an octet;
a pair of e- will
move in to
make a double
O O O 3 equivalent resonance structures, each contributing
33.3% to the resonance hybrid. N – O is ~ 1 bond
bond S O SAMPLE PROBLEM 2
NCO- also has 3 possible
resonance forms. Are these equivalent forms? NC
A O NC
B O N C O C The 3 possible NCO- resonance forms are NOT
equivalent to each other…
• N, C, and O are different elements
• Single – double – triple bonds do not occur in
the structures consistently.
• They do not contribute equally to the overall
resonance hybrid. (“Stable” ones contribute
more than “unstable” ones.) 2 Chem 210/ WANG / Chapter 8B
Lewis Structures More than one valid Lewis structure: Use
“Formal Charges” to decide if one is more
stable (preferred) than the other(s).
Is the charge assigned to each atom in a
Lewis structure as if all atoms had the
= no. of valence electrons – sum of all
the electrons assigned to the atom.
• Assign all nonbonding electrons
around the atom to the atom.
• Assign half of the bonding electrons
to each atom joined by the bond . Resonance
Resonance Structures of O3
Explain why the O-to-O bond in O3 is longer than that
in O2, but shorter than that in H2O2. Use “Formal Charges” to decide which
Lewis structure is more stable
A Lewis structure is more stable if it fits
well with the electronegativity & number of
valence electrons each element has,
• • Formal charges on all atoms being zero or
close to zero (low pos. or neg. number)
Negative formal charge found on more
electronegative atom / postive formal
charge on less electronegative atom.
No like-charges (+,+ or -,-) appearing on
likeadjacent atoms. O H O The O-to-O bond in O3
O-tois between the double
and the single. O Boron only has 3e … makes 3 bonds only.
BCl2F = B lacks octet. B
O O O formal charges:
O (left) = 6 – (6+1) = -1
O (middle) = 6 – (2+3) = +1
O (right) = 6 – (4+2) = 0 -1 +1
O O O O H O OO PCl5 = P exceeds octet.
Sulfur makes 6 bonds sometimes.
SF6 = S exceeds octet. Fall 2009 / UM-SJTU JI A 0 -1 0 0 0 O N C O formal charges:
N = 5 – (6+1) = -2 B
N = 5 – (4+2) = -1 C
N = 5 – (2+3) = 0 C = 4 – (4) = 0
O = 6 – (3+2) = +1 C = 4 – (4) = 0
O = 6 – (2+4) = 0 C = 4 – (4) = 0
O = 6 – (1+6) = -1 “A” has +1 formal charge on O, the most electronegative
atom, making it least preferred of the 3 forms.
“C” contributes the most to the resonance hybrids because
O has -1 formal charge and both C & N has 0 charge.
has “C” is the preferred Lewis Structure! O2
H2O2 +1 -1
OO Exceptions to the octet rule
Expanded octet occur with elements of the
3rd period and beyond: P, S and Cl, etc.
– Extra electrons occupy the empty d
orbitals (3d) in these atoms. Cl Phosphorus makes 5 bonds sometimes. NC O the double bond in O2 but shorter than the
single bond in H2O2. H Exceptions to the octet rule -1 C Resonance Structures of O3 O2 has O=O
double bond. H2O2 has O-O
has Osingle bond. N 0 +1 has equivalent
O-toANSWER O3 has 2 equivalent Lewis structures. O-to-O
Conti. bond order being 1 , the bond is longer than ANSWER
O SAMPLE PROBLEM 3 ANSWER
These 5 orbitals can share electrons
with five Cl atoms to make PCl5. Expanded octet also occurs more often
with larger atoms: Br, I or Xe
– More room around these large atoms to
accommodate extra electrons. (1.48+1.21)÷ 2 = 1.34
O-to-O bond in O3 = 1.278
Agree within 2 sig. figs = 1.3 Å
1.3 An example: ICl41. Total the electrons.
2. Write a possible
3. Spread the electrons
so all get octet…expand
Octet as necessary. -1
I Cl Cl 3 Cl Chem 210/ WANG / Chapter 8B
Lewis Structures 0 P P 5 – 4 = +1 0HO 0 6-(6) = 0 0 0 More important (preferred)
Lewis Structure 0 0 O H0 O 0 0 more stable 0 0 O -1 O S O : : -1 O O 0
forms possible :
: O: -1 lower formal charges -1 O S
O -1 How many resonance structures of PO4-3 are shown
Which is the preferred Lewis Structure? Why? -2 : More.. O: : O: : -2 : :O:
:O S O: : :
: O: : These are equivalent
Better Lewis Structure
than the one drew earlier! Fall 2009 / UM-SJTU JI O: : 0 S : : -1 O 0
0HO -2 :O :
:O S : :
:O S : : :
: O: O: : :O S :O : : -2 :O : : :
: O: O: O +2
O H0 O: SO4 -2 : :
: :O S S lower formal charges Answer (continued): Re-consider the Lewis structure
Regiven for the sulfate ion earlier. Is there another Lewis
structure existing in resonance with it? If so, which is the
better one? Why?
2:O: O 0 HO 0 : O H0 0
lower formal charges
more stable 0
O 0 P O
H 0 -1 O 0 O H0 O
H 0 : -1
O 0 :O
:O ANSWER 6-(6+1) = -1 0 HO -2 : ANSWER
O Answer to: Re-consider the Lewis structure given for the
Resulfate ion earlier. Is there another Lewis structure
existing in resonance with it? If so, which is the more
important one? Why?
: Draw two Lewis structures for H2SO4. Identify the
more important form based on formal charges. : SAMPLE PROBLEM 5 : Write two Lewis structures for H3PO4.
Indicate the more important form based
on formal charges. SAMPLE
PROBLEM 4 http://wb.chem.lsu.edu/htdocs/people/sfwatkins/MERLOT/drawlewis/dls.html 4 +2
O -1 ...
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