Chapter 9: Basic Concepts of Chemical Bonding NaCl versus C 12
H 22 O 11.
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Types of Bonds Types of AtomsType of Bond Bond Characteristic
metals to nonmetals Ionic electrons transferred nonmetals to
nonmetals Covalent electrons shared metals to metals Metallic
electrons pooled We can classify bonds based on the kinds of atoms
that are bonded together. 2 Tro: Chemistry: A Molecular Approach,
2/e
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Types of Bonding 3 Tro: Chemistry: A Molecular Approach,
2/e
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Lewis Dot Symbols Combines the element symbol plus the valence
electrons as dots placed around symbol. Dots are first placed on
each of the four sides (N-S-E- W), then paired up after that.
Symbols for period 2.
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Octet Rule Atoms tend to gain, lose, or share electrons until
they are surrounded by eight electrons. Ionic = gain or lose
Covalent = share While there are exceptions to this rule, it is the
important driving force for the formation of compounds.
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Ionic Bonding When Na(s) and Cl 2 (g) are combined, a very
violent, exothermic reaction results. LEP #1
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Ionic Bonding
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The energetics of ionic bond formation can be explained by a
series of steps. Step 1: Loss of electron by Na Na (s) Na (g) ; H =
+108 kJ Na (g) Na + (g) + 1e - ; H = +496 kJ Step 2: Gain of
electron by Cl Cl 2(g) Cl (g) ; H = +122 kJ Cl (g) + 1e - Cl - (g)
; H = -349 kJ
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Ionic Bonding Lattice energy is the energy required to
completely separate a mole of a solid ionic compound into gaseous
ions. NaCl (s) Na + (g) + Cl - (g) ; H = 788 kJ Reverse this AND
add it to the four previous reactions yields: Na (s) + Cl 2(g) NaCl
(s) ; H = -411 kJ
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Lattice Energy The electrostatic attraction of two charged
particles is ruled by the equation: Q 1, Q 2 are the magnitudes of
the charges and d is the distance between the two nuclei.
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Lattice Energy As the magnitude of the charge increases, the
lattice energy will increase. Na +1 Cl -1 U = +788 kJ Sr +2 Cl -1 U
= +2127 kJ Sr +2 O -2 U = +3217 kJ
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Lattice Energy As the distance between the ions increases, the
lattice energy decreases.
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Summary of Lattice Energies The force of attraction between
oppositely charged particles is directly proportional to the
product of the charges Larger charge means the ions are more
strongly attracted larger charge = stronger attraction stronger
attraction = larger lattice energy Of the two factors, ion charge
is generally more important LEP #2 Lattice Energy = 910 kJ/mol
Lattice Energy = 3414 kJ/mol 13 Tro: Chemistry: A Molecular
Approach, 2/e
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Covalent Bonding When two electrons are shared by two atoms,
this is known as a covalent bond. Formation of H 2
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Covalent Bonding Formation of Cl 2 each Cl atom has seven
valence electrons. Each has one unpaired electron that can pair up
to make a bond.
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Lewis Structures For molecules or ions containing three or more
atoms, we can follow a set of rules to guide in the process. 1. Add
up the total valence electrons from all the atoms in the compound.
Ex) CF 4 2. Make a skeleton structure the first element in the
formula is usually the central atom all others are then placed
around this atom and connected with a bond.
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Lewis Structures 3. Fill the external atoms until they have an
octet. 4. Compare total number of electrons used to step 1. If all
are used, then go to step 5. If some are left, place on central
atom as lone pair(s). 5. Check the central atom for an octet. If no
octet, may need multiple bond(s) by moving non- bonding
pair(s).
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Lewis Structures General Guidelines Group 7A as an external
atom will NOT do multiple bonds. Carbon will almost always have
four bonds. Group 2A, 3A as a central atom may be deficient of
octet. Group 5A, 6A, 7A, and Xe as a central atom may exceed the
octet. Oxygen will do up to two bonds.
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Electronegativity Non-polar covalent bond the electrons are
shared equally. Ex) F 2, Br 2, I 2, O 2, etc. Polar covalent bond
electrons are not shared equally one atom has a greater desire for
the electron pair. Electronegativity Scale
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Electronegativity
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Bond Polarity If the electronegativity difference is zero, then
the bond is non-polar covalent. If there is a difference AND the
two elements are non-metals, then the bond is polar covalent. If
there is a difference AND one element is a metal and the other is a
non-metal, then the bond is ionic.
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Bond Polarity Simple molecules like HCl have a polar covalent
bond. The more electronegative element will have a partial negative
charge and the less electronegative element will have a partial
positive charge.
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Dipole Moment This is the quantitative measurement of the polar
bond. = Q x d is measured in a unit called the Debye or Coulomb x
meter. LEP #4
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Resonance Structures In some cases, a molecule or ion may be
described by more than one Lewis Structure. Ex) O 3 LEP #5
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Bond Strength The energy required to break a covalent bond is
its strength. Some are fairly simple. Cl 2(g) 2 Cl (g) ; H = 242 kJ
Others are more complicated. CH 4(g) C (g) + 4 H (g) ; H = 1660 kJ
Note: Energy is ALWAYS required to break a bond.
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Slide 27
Using Bond Enthalpies One method for estimating the enthalpy of
a reaction is: H = (Bonds Broken) (Bonds Made) LEP #6
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Chemistry of Explosives Many explosives are solids or liquids
that contain the nitro (NO 2 ) or nitrate (NO 3 ) group. The
nitrogen atoms are typically weak (singly) bonded to carbon (293
kJ). They then become N-N strong triple bonds (941 kJ).
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Chemistry of Explosives
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Bond Order Bond order is the number of bonds connecting two
atoms. Can be 1, 2, or 3. C2H2C2H2
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Bond Length As bond order increases, the bond lengths decrease.
Ex) N-N N=NN N 1.47 1.241.10 Ex) C-CC=CC C 1.54 1.34 1.20
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Resonance Structures The bond orders and lengths for resonance
structures must be averaged. Ex) SO 2, NO 3 -1, C 6 H 6
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Formal Charges Formal Charge is a fictitious charge assigned to
each atom in a Lewis Structure. It helps to evaluate the validity
of competing structures. Formal Charge = #Valence Electrons
#Nonbonding electrons #Bonding Electrons
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Formal Charges General Rules: 1. The sum of all formal charges
in a neutral molecule will equal zero. 2. The sum of all formal
charges in an ion must equal the charge of the ion. 3. Small (+1 or
-1 or 0) formal charges are preferred over larger ones. 4. When
formal charges cannot be avoided, a negative formal charge will
preferentially go on the more electronegative atom.
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Formal Charges HCN molecule = 10 electrons. Is skeleton
structure H C N or H N C? Which Lewis Structure for the cyanate
(OCN - ) ion is best?
