Lecture 13 - Nassau Community College 151/151 lecture 13...2 Draw the skeletal structure for the...
Transcript of Lecture 13 - Nassau Community College 151/151 lecture 13...2 Draw the skeletal structure for the...
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Lecture 13Professor Hicks
Inorganic Chemistry (CHE151)
Lewis symbols aka Lewis Structures of atoms
• Visual representation of electron configuration
• 1 electron = dot 2 paired electrons = line
• Nucleus + core electrons = element symbol
C
Element symbol =
nucleus + core electrons
Valence electron =
C2 valence
electrons
All three represent
a carbon atom G. N Lewis=
or
Electron
Configuration
C = 1s22s22p2
Lewis symbol notation
4 valence electrons
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Octet rule
N
O
F
• Main group atoms ions
• Gain/lose electrons noble gas electron configuration
• Noble gases all have 8 valence electrons
• Ions of main group elements have an octet of valence
electrons obey Octet Rule
N
O
F
[He] 2s22p6
all their ions
all have the 8 valence e-’s
electron configuration
8 = octet of
valence electrons
Lewis symbols
of three atoms
O O2- N N3- F F-
Lewis Structures
• Lewis symbols• Atoms and monatomic ions- Atoms +/- electrons ions - Octet rule
• Lewis structures• Molecules and polyatomic ions- Show bonding atom to atom
(the skeletal structure of the molecule/ion)- Atoms molecules/polyatomic ions- Extend concept of Octet Rule from atoms
forming monatomic ions
C H
H
H
H
C
H
H
H
H
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Lewis structures
Can be used to predict
1) If a compound / polyatomic ion will exist
2) Its solubility in oil or water (non-polar or
polar liquids)
3) Three-dimensional shape
4) Intermolecular forces (tendency to stick
to other molecules)
Molecular compounds
and polyatomic ions
• More numerous than ionic compounds
• Multiple compounds with same elements
- CO, CO2, NO, NO2, N2O, N2O4,
- ClO- , ClO2- , ClO3
- , ClO4-
• No crossing over rule to determine formulas
• If a valid Lewis structure can be drawn the compound/ion probably exists
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Steps for Drawing Lewis Structures
1 Calculate the total the number of valence electrons in the
molecule or polyatomic ion.
2 Draw the skeletal structure for the molecule/polyatomic ion.
3 Give octets to all atoms in the skeletal structure (except
hydrogen) by placing lone pairs on them.
4 Count the total number of valence electrons in this structure.
5 If the Lewis structure contains the correct number of valence
electrons it is complete and has no resonance structures that
obey the octet rule.
6 If the structure contains more valence electrons than the
molecule or polyatomic ion has then reduce the number of
valence electrons in the structure by erasing lone pairs on
adjacent atoms and replacing them with a bonding pair
between those atoms.
Bonding in molecular compounds
and polyatomic ions• Based on attraction of nuclei for electrons
• Electrons act like glue to hold nuclei together
nucleus
z+e-e-
e-e-nucleus
z+
repulsion
attractionattraction
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Two kinds of electron pairs
O
H H
O
H H
Bonding electron pair
Non-bonding or
Lone electron pair
Hydrogen
• Does not obey octet rule
• Noble gas it becomes like is [He] = 1s2
• Obeys duet rule
• Like H- ion
• Only 1 bond H at end like a cap
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O
F
F
O
H H
O
O
C H
H
H
H
O
C
C
O
N H
C O
NH
C
O
O
N
O
N
O
O
Circle the octets on all the atoms that have octets.
B
F
F
F
O
F
F
O
H H
O
O
C H
H
H
H
O
C
C
O
N H
C O
NH
C
O
O
N
O
N
O
O
Circle the octets on all the atoms that have octets.
B
F
F
F
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Draw Lewis structures that obey
the Octet/Duet rule for
• H2
• H2S
• Cl2
• HCN (skeletal structure H-C-N)
Resonance structures
• More than one possible way to arrange
electrons = resonance structures
• Atoms connected in the same way
• not different substances
• Molecules/ion is one structure that is a weighted
average of all resonance structures
Example: Draw the Lewis structure and
resonance structures for CO2.
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C
O
O
C O
O
C
O O
Draw a Lewis structure and its resonance
structures for CO2 (skeletal structure O-C-O)
• Double ended arrows are placed between
resonance structures
• Resonance structures are shown in brackets
Isomers
• Different compounds with same formula
• Atoms arranged differently
• NOT resonance structures
cyanic acid isocyanic acid
C
O
N H C O
NH
same atoms arranged differently
H attached to N H attached to O
one structure cannot be changed into the other by moving electrons/double bonds
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Draw Lewis structures and
resonance structures for
• O3
• SO2 (skeletal structure O-S-O)
• BO33- (skeletal structure has B in center)
• CO32- (skeletal structure has C in center)
Isoelectronic structures
• Same arrangement of same # valence
electrons on different atoms
O
C
N
N
N
C
-
S
H H
O
H H
Se
H H
substitute
elements in
same group
substitute
elements from
different groupsthese three are
isoelectronicthese three are
isoelectronic
same # e-’s
C has 1 less proton
structure has
negative charge
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Normal Valence
• The number of bonds an atom forms in order to
achieve an octet is called its Valence
• The Normal Valence of an atom is the number
of bonds it most often forms to obey the octet
rule
• Normal valence is a periodic property
Normal valence = 4 3 2 1
Formal charge
• Charge atoms would have if they got
divorced (equitably)
• Each bond split 1 electron to each atom
• Formal charges add up to total charge
If the octet rule is obeyed
1) Each bond over normal valence +1 in FC
2) Each bond under normal valence -1 in FC
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Assigning formal charge
1) Draw circles around each atom
- split bonding pairs in half
- give lone pairs to the atom they are on
2) Evaluate the charges of the circles as if
they were Lewis symbols
N
O
CH
C O
NH
H atom
FC = 0
O with 5 valence
e-’s = O+1
FC= +1
C atom
FC = 0
N with 6 valence
e-’s = N-1
FC= -1
9.42
Write Lewis structures for these ions: (a) O22- , (b)
C22- , (c) NO+, (d) NH4
+ . Show formal charges.
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Formal charge
• Resonance structures with lower formal
charges dominate the observed structure
C
S
N
C S
N
C
S N
+1 0-2 0 0
-1-1 0 0
Best structure b/c
1) Minimal charges and
2) Charges consistent with
electronegativity of atoms
Formal charge
• Resonance structures with lower formal
charges dominate the observed structure
C
F
O
C F
O
C
F O
+2 0-1 +1 0
00 0 +1
Best structure b/c
1) Minimal charges and
2) Charges consistent with
electronegativity of atoms
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Exceptions to Octet rule
• Group three elements sometimes are
stable with 6 electrons (sextet)
• Molecules with odd numbers of electrons
cannot have octets
- Some (especially neutral species) are
stable enough to exist with less than an
octet
9.59
The BCl3 molecule has an incomplete octet around
B. Draw three resonance structures of the molecule
in which the octet rule is satisfied for both the B and
the Cl atoms. Also, draw one where the B has a
sextet. Show formal charges on all structures.
Based upon the formal charges which is the best
structure?
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Exceptions to the octet rule
• Octet = 8 = full s + p orbitals
• Elements with d orbitals can have more
than octet
• 18 electrons = full s + p + d orbitals of
same n value
• Many elements can lower FC’s by
exceeding octet
• More than 8 electrons = expanded octet
Rarely less than octet
never exceed octet
Rarely less than octet
sometimes exceed octet
Octet rule/expanded octets
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Draw a Lewis structure for H2SO4 that
obeys the octet rule and assign all formal
charges. The skeletal structure has all O
bound to S and the H’s each bound to
different O’s.
Scramble these three maybe
9.43 The following Lewis structures for (a) HCN, (b)
C2H2, (c) SnO2, (d) BF3, (e) HOF, (f) HCOF, and (g) NF3
are incorrect. Explain what is wrong with each one and
give a correct structure for the molecule. (Relative
positions of atoms are shown correctly.)
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9.51
Write three resonance structures for hydrazoic acid,
HN3. The atomic arrangement is HNNN. Show
formal charges.
Based upon the FC predict which resonance
structure or structures dominates the observed
structure.
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9.53
Draw three reasonable resonance structures for the
OCN− ion. Show formal charges.
9.63
Write Lewis structures for SeF4 and SeF6. Is the
octet rule satisfied for Se?
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Draw Lewis Structures and any resonance
structures for NCCN
Draw Lewis Structures and any resonance
structures for N-N-NO2
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Draw Lewis Structures and any resonance
structures for NO2
Draw Lewis Structures and any resonance
structures for IO2
Bases upon this structure predict the
charge of an ion formed from IO2.
Steps for Drawing Lewis Structures
1 Calculate the total the number of valence electrons in the molecule
or polyatomic ion.
2 Draw the skeletal structure for the molecule/polyatomic ion.
3 Give octets to all atoms in the skeletal structure (except hydrogen)
by placing lone pairs on them.
4 Count the total number of valence electrons in this structure.
5 If the Lewis structure contains the correct number of valence
electrons it is complete and has no resonance structures that obey the
octet rule.
6 If the structure contains more valence electrons than the molecule or
polyatomic ion has then reduce the number of valence electrons in
the structure by erasing lone pairs on adjacent atoms and replacing
them with a bonding pair between those atoms.
7 If the molecule or polyatomic ion contains less valence
electrons then add electrons to the structure on any atom that
can exceed the octet rule.