Ch 9 Bonding and Molecular Structure:
Fundamental Concepts
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Valence Electrons
Chemical Bond Formation
Bonding in Ionic Compounds
Covalent Bonding and Lewis Structures
Resonance
Exceptions to the Octet Rule
Charge Distribution in Covalent Molecules
Bond Properties
Molecular Shapes
Molecular Polarity
Valence Electrons
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There are core electrons and valence electrons
Valence electrons, outermost shell,determine
chemical properties
Main group elements - valance electrons
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are the s and p,
Same as the group number
Transition elements - valance electrons are the
ns and the (n-1) d orbitals.
Valence Electrons
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Lewis electron dot diagrams
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The element’s symbol represents the atomic nucleus
and the core electrons.
Up to 4 valence electrons, dots, are placed around the
symbol
Any remaining electrons are paired with those 4
A full set of 8 electrons is an octet and stable
Lewis electron dot diagrams
Covalent bonding involves sharing of valence electrons
Between atoms.
Bonding in Ionic Compounds
Formation of these compounds is exothermic in part ionization
Of Na and Ca is low.
Bonding in Ionic Compounds
Bonding in Ionic Compounds
Bonding in Ionic Compounds
Lattice Energy
Lattice Energy
F-
Cl-
Br-
I-
Li+
1036
853
807
757
Na+
923
787
747
704
K+
821
715
682
649
Rb+
785
689
660
630
Cs+
740
659
631
604
The bond between ions of opposite charge is
strongest when the ions are small.
Lattice Energy
The ionic bond should also become stronger as the charge on
the ions becomes larger. The data in the table below show that
the lattice energies for salts of the OH- and O2- ions increase
rapidly as the charge on the ion becomes larger.
Na+
OH900
O22481
Mg2+
3006
3791
Al 3+
5627
15,916
Lattice Energy
Lattice Energy
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Why do NaCl2 or Na2Cl not exist?
The energy for Na+ to lose a 2nd electron is very
high and endothermic.
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So the energy of the reaction would not be
negative
The reaction would not be product favored
Cl- is not favorable to add a second electron
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The amount of energy required is very large
Covalent Bonding and Lewis Structure
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Molecules or polyatomic ions made of
nonmetal atoms bonding covalently.
Bonding Pair Shared Pair each 2 shared dots is one stick bond
Nonbonding Pair, Unshared Pair, Lone Pair
Double Bonds 2 shared pair 4 shared electrons 2 stick bonds
Triple Bonds 3 shared pair 6 shared electrons 3 stick bonds
Drawing Lewis Structures
http://chemed.chem.purdue.edu/genchem/topicreview/
bp/ch8/lewis.html
misterguch.brinkster.net/lewisstructures.html
http://pages.towson.edu/ladon/lewis.html
Drawing Lewis Structures
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5.
Decide on the Central Atom the one with the
lowest electron affinity (or most bonds)
Determine the total number of valence electrons
in the molecule or ion -add up all electron dots
and charges
Draw skeletal structure with single bonds
Use remaining electrons to surround terminal
atoms with 8 dots (except H)
If central atom has fewer than 8 electrons, move
lone pair to create multiple bonds and allow all
atoms to have 8 electrons.
Drawing Lewis Structures
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Practice drawing structures for
NH4+
CO
NO+
SO4 2-
Drawing Lewis Structures
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Practice drawing structures for
CH4
C 2 H4
NH2-
C 2 H6
C 2 H2
H3O +
Lewis Structures of Acids and their
Anions
Lewis Structures of Acids and their Anions
Isoelectronic Species
Isoelectronic Species
Resonance Structure
When two equivalent structures can be
drawn for one molecule, the extra
bond was believed to resonate (flip)
between the two.
If that were occurring,t he lengths of the
bonds would be different on each side.
This difference in bond length is not
observed.
Resonance theory was proposed by
Linus Pauling as a composite. The resonance
Hybrid will provide two equal bonds with
bond lengths
Resonance Structure
Resonance Structure
Exceptions to the Octet Rule
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Atoms that have fewer than four pair of electrons
on a central atom
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Generally Group 2A or 3A
Classic example BF3
Non octet structures
Atoms that have more than four pair of electrons on
a central atom.
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Generally Group 4A, 5A, 6A, 7A, or 8
(Yes, Group 8 can form bonds)
Expanded Octet
Expanded Octet
Only elements of the third or higher periods in the periodic
Table form compounds and ions in which an octet is exceeded.
Second period elements have only s and p orbitals
Elements win the third and higher periods, the d orbitals in the
Outer shell are traditionally included among valence orbitals
For the elements.
The extra orbitals provide the elements with an opportunity to
Accommodate up to 12 electrons.
Expanded Octet
Molecules with an Odd Number of Electrons
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There is a small group of molecules with an odd
number of electrons
Molecules with an odd number of electrons do not
obey the Octet
The single electron classifies the molecules as a
free radical.
Free radicals are more reactive than molecules
with paired electrons
Charge Distribution in the Bonds &Molecules
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Lewis structures indicate that electrons are evenly
distributed along bonds.
Valence electrons are not distributed among the
atoms as evenly as suggested.
Some atoms may have a slight negative charge or a
slight positive charge
Electrons are drawn more strongly to one atom in
the bond.
Electron distribution defines the uneven
distribution of electrons.
Charge Distribution in the Covalent Bonds &Molecules
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Electrons in covalent bonds are not necessarily
evenly distributed.
Charge distribution defines the uneven
distribution of electrons.
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Polar diatomic molecules will line up by polarity
Determines the site at which reactions occur (like the
attachment of an H+), will the H attach to the O or Cl
end of a OCl-
Formal Charges on Atoms
Formal Charges on Atoms
Formal Charges on Atoms
Calculate formal charges for the atoms in
NH4+
CO32-
CN -
SO3
Bond Polarity and Electronegativity
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Pure covalent equal sharing-few molecules
Polar covalent bonds - electron pair shared unequally
molecule acts as a dipole
1930 Linus Pauling determined Electronegativity
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The ability of an atom in a molecule to attract electrons
to itself (in a bond) F = 4.0
EN differences: Covalent < 1.67 < Ionic
Bond Polarity and Electronegativity
Assign a positive and negative pole to each of the
following pair, decide which is ore polar
B-F
B - Cl
Si - O
P-P
C=O
C=S
Combining Formal Charge and Bond Polarity
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Electroneutrality Principle electrons in a
molecule are distributed in such a way that the
charges on the atoms are as close to 0 as
possible.
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When a negative charge occurs, it is on the most
electronegative element.
BF4- formal charge gives B a (-) but actually the () is distributed over the F
Combining Formal Charge and Bond Polarity
A is more satisfactory
Combining Formal Charge and Bond Polarity
Bond Properties- Bond Order
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The Order of a Bond is the number of
bonding electron pair shared by two atoms.
Bond order = 1 single H2 CH4
Bond order = 2 double H2 C = C H2
Bond Order = 3 triple H C C H
Bond Properties Bond Order for Resonance Structures
Bond Order = number of shared pair
number of bonding sites
Bond Length and Bond Order
Bond Length is the distance
Between the nuclei of two
Bonded atoms.
Single bonds are determined
By the size of atoms
Double bonds are shorter
Triple bonds, shorter still.
Bond Length and Bond Order
Bond Length and Bond Order
Bond Energy: Bond Dissociation Energy
Bond Energy: Bond Dissociation Energy
Bonds broken
Bonds formed
C=C 610 kJ / mol and the H-H 1046 kJ / mol
C-C 346 kJ and 2 C-H 2 x 413 kJ / mol
D Horxn = 1046 kJ - 1172 kJ = -126 kJ
Molecular Shapes
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are needed to see this picture.
Molecular Shapes
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VSEPR valence shell electron-pair repulsion theory
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Lewis dot diagrams can predict the 3-d geometry of a
molecule - which determines its properties
Bond and lone electron pair in the valence shell of an
element repel each other and seek to be as far apart as
possible.
Central Atoms Surrounded Only by Single -Bond Pairs
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Linear
- trigonal planar
Trigonal-bipyrimadal
-octahedral
-Tetrahedral
Molecular Shapes single bond -Pair
Molecular Shapes single bond -Pair
Expanded Octet
Effect of Lone Pair on Bond Angles
Relative strengths of repulsion are in the order
Lone pair-lone pair < Lone pair-bond pair < bond pair-pond pair
Central Atoms with More Than Four
Valence Electron Pairs
Multiple Bonds and Molecular Geometry
Multiple Bonds and Molecular Geometry
Bonding and Polarity
http://www.bsc2.ehbschweiz2.ch/Chemie/Simulationen%20Chemie/Bindu
ng/Bindungstypen%20Animation.htm
Molecular Polarity
Molecular Polarity
QuickTime™ and a
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are needed to see this picture.
Figure 1: A model of
ethanol showing the
volume that is occupied by
its electrons. The distribution
of the electrons in an ethanol
molecule is skewed relative to
the protons to give a region
having a partial negative
charge, which is shown in
red, and a corresponding
region having a partial
positive charge, which is
shown in blue.
Molecular Polarity
Molecular Polarity
Molecular Shapes and Molecular Polarity
Homework Ch 9
32. Which compound has the most negative energy of ion pair formation?
Which has the least negative value? (a) NaCl
(b) MgS
(c) MgF 2
34. Place the following compounds in order of increasing lattice energy
(from least negative to most negative): LiI, LiF, CaO, RbI.
37. Which compound in each of the following pairs should require the
higher temperature to melt? (See Study Question 36.)
(a) NaCI or RbCI
(b) BaO or MgO
(c) NaCI or MgS
39. Draw Lewis structures for the following molecules or ions:
(a) CS2
(b) NO2(c) BF 4 (d) Cl2SO
Homework Ch 9
41. Draw Lewis structures for each of the following molecules:
(a) Methanol, CH30H (C is the central atom)
(b) Vinyl chloride, H2C = CHCI, the molecule from which PVC
plastics are made. Its
structure resembles that of C2F4 in Study Question 40.
(c) Acrylonitrile, H2C= CHCN, the molecule from which materials
such as Orlan are
made
42. Show all possible resonance structures for each of the following
molecules or ions:
(a) S02
(b) N02 (c) SCN45. Draw Lewis structures for each of the following molecules or ions:
(a) BrF5
(b) IF3
(c)IBr2
(d) BrF2 +
Homework Ch 9
46. Determine the formal charge on each atom in each of the
following molecules or ions:
(a) N2H4
(b) P04 3(c) BH4 (d) NH20H
48. Determine the formal charge on each atom in the following
molecules and ions:
(a) N02 +
(b) N02 (c) NF3
(d) HN03
51. For each pair of bonds, indicate the more polar bond and use
an arrow to show the direction of polarity in each bond.
(a) C-O and C-N
(b) P-Br andP-CI (c) B-O and B-S
(d) B-F andB-I
Homework Ch 9
53. Urea, (NH2)2CO, is used in plastics and fertilizers. It is also the
primary nitrogen-containing substance excreted by humans. Draw
the molecule with C=O as the center
(a) Which bonds in the molecule are polar and which are nonpolar?
(b) Which is the most polar bond in the molecule? Which atom is the
negative end of the bond dipole?
54. Considering both formal charges and bond polarities, predict on
which atom or atoms the negative charge resides in the following
anions:
(a) BF4 (b) BH4 (c) OH(d) CH3C02-
Homework Ch 9
57. Compare the electron dot structures of the carbonate (C03 2- )
and borate (B03 3- ) ions.
(a) Are these ions isoelectronic?
(b) How many resonance structures does each ion have?
(c) What are the formal atom charges in each ion?
(d) If an H+ ion attaches to C03 2- to form the bicarbonate ion,
HC03 -,
does it attach to an 0 atom or to the C atom?
61. Specify the number of bonds for each of the following
molecules or ions.
Give the bond order for each bond.
(a) CN(b) CH3CN (c) S03
(d) CH3CH=CH2
Homework Ch 9
64. Consider the nitrogen-oxygen bond lengths in N02 +, N02 -, and
N03 -. In which ion is the bond predicted to be longest? Which is
predicted to be the shortest? Explain briefly.
69. Phosgene, Cl2CO, is a highly toxic gas that was used as a
weapon in World War I. Using the bond energies of Table 9.9,
estimate the enthalpy change for the reaction of carbon monoxide
and chlorine to produce phosgene. (Hint: First draw the electron
dot structures of the reactants and products so you know the types
of bonds involved.)
CO(g) + Cl2(g) -> Cl2CO(g)
Homework Ch 9
74. The following molecules or ions all have two oxygen atoms
attached to a central atom. Draw the Lewis structure for each
one and then describe the electron-pair geometry and the
molecular geometry. Comment on similarities and differences
in the series.
(a) CO2
(b) N02 (c) 03
(d) CI02 75. The following molecules or ions all have three oxygen atoms
attached to a central atom. Draw the Lewis structure for each
one and then describe the electron-pair geometry and the
molecular geometry. Comment on similarities and differences
in the series.
(a) C03 2(b) N03 (c) S03 2(d) CI03 -
Homework Ch 9
Homework Ch 9
78. Give approximate values for the indicated bond angles.
(a) O-S-O in S02
(b) F-B-F angle in BF3
(b) (c) Cl- C - Cl angle in Cl2CO
(d) H -C- H (angle 1) and C-C=N (angle 2) in acetonitrile
CH3CN
83. Consider the following molecules:
(a) CH4
(b) NCl3
(c) BF3
(d) CS2
(i) Which compound has bonds with the greatest degree of
polarity?
(ii) Which compounds in the list are not polar?
Homework Ch 9
85. Which of the following molecules is (are) polar? For each
polar molecule indicate the direction of polarity, that is,
which is the negative and which is the positive end of the
molecule.
(a) BeCl2
(b) HBF2
(c) CH3Cl
(d) S03