chapter 8 basic concepts in chemical bonding. introduction attractive forces that hold atoms...
DESCRIPTION
Introduction Chemical bonds are classified into two types: Ionic bonding results from electrostatic attractions among ions, which are formed by the transfer of one or more electrons from one atom to another. Covalent bonding results from sharing one or more electron pairs between two atoms.TRANSCRIPT
CHAPTER 8
Basic Concepts in Chemical Bonding
Introduction
Attractive forces that hold atoms together in compounds are called chemical bonds. The electrons involved in bonding are usually those in the outermost (valence) shell.
Introduction
Chemical bonds are classified into two types:
Ionic bonding results from electrostatic attractions among ions, which are formed by the transfer of one or more electrons from one atom to another.
Covalent bonding results from sharing one or more electron pairs between two atoms.
Lewis Dot Representations
of AtomsThe Lewis dot representation (or Lewis dot formulas) convenient bookkeeping method for
valence electrons electrons that are transferred or
involved in chemical bonding
Lewis Dot Representations
of Atoms
Li Be B C N O F Ne
H.
He
Lewis Dot Representations
of Atoms
elements in same group have same Lewis dot structures
Li Be B C N O F Ne.... .. ..
..HeH
.
.. . .
.. ....
...
..
.. .. .. .... ..
.
Lewis Dot Representations
of Atoms
elements in same group have same Lewis dot structures
Li Be B C N O F Ne.... .. ..
..HeH
.
.. . .
.. ....
...
..
.. .. .. .... ..
.
Li & Na. .
N & P.. ..
..
. ..
. F & Cl...
....
.
... ..
.
Ionic Bonding
We can also use Lewis formulas to represent the neutral atoms and the ions they form.
Li + F.....
.. . Li+ F[ ]..
.... ..
Ionic Bonding
Li+ ions contain two electrons same number as heliumF- ions contain ten electrons same number as neonLi+ ions are isoelectronic with heliumF- ions are isoelectronic with neon Isoelectronic species contain the same number of electrons.
Ionic Bonding
potassium reacts with bromine equation for the reaction
Ionic Bonding
potassium reacts with bromine2 K(s) + Br2 (l) + Br-(s)
electronically this is occurring 4s 4pK [Ar] Br [Ar] becomes
Ionic Bonding
4s 4pK+ [Ar]Br- [Kr]Lewis dot formulas
Ionic Bonding
cations become isoelectronic with preceding noble gasanions become isoelectronic with following noble gas
K + Br.....
.. . K+ Br[ ]..
.... ..
Covalent Bondingcovalent bonds form when atoms share electronsshare 2 electrons - single covalent bondshare 4 electrons - double covalent bondshare 6 electrons - triple covalent bondattraction is electrostatic in nature lower potential energy when bound
Covalent Bonding
Covalent Bonding
Covalent Bonding
Lewis dot representation H molecule formation
HCl molecule formation+H. H . H H.. or H2
H Cl H Cl+... ..
.. ....
..
... or HCl
Covalent Bonding
extremes in bondingpure covalent bonds electrons equally shared by the atomspure ionic bonds electrons are completely lost or
gained by one of the atoms most compounds fall somewhere between these two extremes
Lewis Dot Formulas for Molecules and Polyatomic Ions
homonuclear diatomic molecules hydrogen, H2
fluorine, F2
nitrogen, N2
H HorH H..
F F.. .. ....
..
.. .. F F.. .... ..
.. ..or
N N········ ·· N N·· ··or
Lewis Dot Formulas for Molecules and Polyatomic Ions
heteronuclear diatomic moleculeshydrogen halides hydrogen fluoride, HF
hydrogen chloride, HCl
hydrogen bromide, HBr
or ··H F··
··H F..
·· ····
or ··H Cl··
··H Cl..
·· ····
or ··H Br··
··H Br..
·· ····
Lewis Dot Formulas for Molecules and Polyatomic Ions
water, H2O
H
H
O···· ··
··
Lewis Dot Formulas for Molecules and Polyatomic Ions
ammonia molecule , NH3
H
H
N···· ··
·· H
Lewis Dot Formulas for Molecules and Polyatomic Ions
ammonium ion , NH4+
H
H
N···· ··
·· H
H +
The Octet Rule
Elements try to a achieve noble gas configuration in most of their compounds.Lewis dot formulas are based on the octet rule.
N - A = S rule
N = # of electrons needed to be noble gas usually 8 2 for HA = # of electrons available in outer shells equal to group # 8 for noble gases
N - A = S rule
for ions add one e- for each negative charge subtract one e- for each positive chargecentral atom in a molecule or polyatomic ion is determined by: atom that requires largest number of
electrons for two atoms in same group - less
electronegative element is central
Drawing Lewis Dot Formulas
Write Lewis dot and dash formulas for hydrogen cyanide, HCN.
N = 2 + 8 + 8 = 18A = 1 + 4 + 5 = 10S = 8
H C N·· ·· ···· H C N ··or··
Drawing Lewis Dot Formulas
Write Lewis dot and dash formulas for the sulfite ion, SO3
2-. N = 8 + 3(8) = 32A = 6 + 3(6) + 2 = 26S = 6
O S O
O··
····
····
··
··
··
····
··
····
2-O S
O
O·· ·· ·· ···· ·· ··
······
2-or
Drawing Lewis Dot Formulas
Write Lewis dot and dash formulas for sulfur trioxide, SO3.
N = 8 + 3(8) = 32A = 6 + 3(6) = 24S =8
orO S O
O··
····
····
··
····
····
·· ·· O S
O
O·· ···· ·· ··
······
Resonance
three possible structures for SO3
two or more dot structures necessary to show bondinginvoke resonance Double-headed arrows are used to indicate
resonance formulas.
O S
O
O·· ···· ·· ··
······
OS
O
O·· ···· ·· ··
··
······
O S
O
O·· ···· ·· ··
····
Resonance Structures
Some molecules are not described by Lewis Structures very well.
Typically, structures with multiple bonds can have similar structures with the multiple bonds between different pairs of atoms.
Resonance
flaw in our representations of moleculesno single or double bonds in SO3
all bonds are the samebest picture
incorrect shape
OSO
O
--- ---
Limitations of the Octet Rule for Lewis Formulas
species that violate the octet rule N - A = S rule does not apply
covalent compounds of Be, B, Ncovalent compounds with expanded octetsspecies containing an odd number of electrons
Limitations of the Octet Rule for Lewis Formulas
In cases where the octet rule does not apply, the substituents attached to the central atom nearly always attain noble gas configurations. The central atom does not.
Limitations of the Octet Rule for Lewis Formulas
Write dot and dash formulas for BBr3.
B··. Br··
··
··.
BBr Br
Br
········
····
····
····
····
Br B
Br
Br····
··
·· ····
·· ····
or
Limitations of the Octet Rule for Lewis Formulas
Write dot and dash formulas for AsF5.
As··..
. F····
··.
··
As
F
F F
F F
····
··
·· ······
······
····
·· ··or
····
····
··
·· ······
······
···· AsF
F F
FF
······ ··
··
··
Formal Charges
It is possible to draw more than one Lewis structure with the octet rule obeyed for all the atoms.To determine which structure is most reasonable, we use formal charge.
Formal Charges
The charge on an atom that it would have if all the atoms had the same electronegativity.To calculate formal charge, electrons are assigned as follows:
•All nonbonding electrons are assigned to the atom on which they are found.•Half the bonding electrons are assigned to each atom in a bond.
Formal Charges
valence electrons – (½ number of bonds + lone pair electrons)
Consider:
C N
Formal Charges
For C: There are 4 valence electrons.In the Lewis structure there are 2 nonbonding electrons and 3 from the triple bond. There are 5 electrons from the Lewis structure.
Formal charge: 4 - 5 = -1
C N
Formal ChargesConsider:Consider:
For For NN::There are 5 valence electrons.There are 5 valence electrons.In the Lewis structure there are 2 nonbonding In the Lewis structure there are 2 nonbonding electrons and 3 from the triple bond. There electrons and 3 from the triple bond. There are 5 electrons from the Lewis structure.are 5 electrons from the Lewis structure.
Formal charge = 5 - 5 = 0Formal charge = 5 - 5 = 0We write:We write:
C N
C N
Formal Charges
The most stable structure has:•the smallest formal charge on each atom,•the most negative formal charge on the most electronegative atoms.
Polar and Nonpolar Covalent Bonds
polar covalent bonds unequally shared electrons assymmetrical charge distribution different electronegativities
ElectronegativityElectronegativity
Polar and Nonpolar Covalent Bonds
bondpolar very 1.9 Difference
4.0 2.1 ativitiesElectroneg F H
1.9
Polar and Nonpolar Covalent Bonds
Electron density map of HF blue areas - low
electron density red areas - high
electron densitypolar molecules have separation of centers of negative and positive charge
Polar and Nonpolar Covalent Bonds
bondpolar slightly 0.4 Difference
2.5 2.1 ativitiesElectronegI H
0.4
Polar and Nonpolar Covalent Bonds
Electron density map of HI blue areas - low
electron density red areas - high
electron densitynotice that the charge separation is not as big as for HF HI is only slightly polar
Polar and Nonpolar Covalent Bonds
Continuous Range of Bonding Types
all bonds have some ionic and some covalent character HI is about 17% ionicgreater the electronegativity differences the more polar the bond
Continuous Range of Bonding Types
molecules whose centers of positive and negative charge do not coincide are polar have a dipole momentdipole moment has the symbol is the product of the distance,d, separating charges of equal magnitude and opposite sign, and the magnitude of the charge, q
Dipole Moments
molecules that have a small separation of charge have small molecules that have a large separation of charge have a large example HF and HI
Dipole Moments
in molecules some nonpolar molecules have polar bonds
2 conditions must hold for a molecule to be polar
units Debye0.38 units Debye1.91 I- H F- H
--
Dipole Moments
There must be at least one polar bond present or one lone pair of electrons.
The polar bonds, if there are more than one, and lone pairs must be arranged so that their dipole moments do not cancel one another.
Polar and Nonpolar Covalent Bonds
nonpolar covalent bonds electrons are shared equally symmetrical charge distributionmust be the same element to share exactly equallyH2
N2
H HorH H..
N N········ ·· N N·· ··or
Bond Energies
Bond energy - amount of energy required to break the bond and separate the atoms in the gas phase
A - B bond energy A + B
H - Cl H + Clg g g
gkJ
mol g g
432
Bond EnergiesTable of average bond energiesMoleculeBond Energy (kJ/mol)
F2 159 Cl2 243
Br2 192O2 (double bond) 498N2 (triple bond) 946
Bond EnergiesBond energies can be calculated from otherHo
298values
Bond Energies
Example: Calculate the bond energy for hydrogen fluoride, HF.
Bond Energies
Example: Calculate the bond energy for hydrogen fluoride, HF.
H - F BE H F atoms NOT ions
H - F H F H BE
H H H H
H kJ + 78.99 kJ kJ
H kJ BE for HF
g HF g g
g g g 298o
HF
298o
f Ho
f Fo
f HFo
298o
298o
g g g
or
218 0 271
568 0
.
.
Bond Energies
Example: Calculate the average N-H bond energy in ammonia, NH3.
Bond Energies
bonds H-N molkJ
H-N
o298
o298
oNH f
oH f
oN f
o298
H-No298ggg3
3913
kJ 1173BE average
kJ 1173H
kJ 11.46)218(3)7.472(H
HH 3HH
BE 3H H 3 + NNH
g3gg
Bond Energies
In gas phase reactions Ho values may be related to bond energies of all species in the reaction.
formed bondsbroken bonds DDH rxn
Bond Energies
Example: Use the bond energies listed in Table 8.4 to estimate the heat of reaction at 25oC for the reaction below.
CH O CO 2 H O
H BE BE BE 4 BE4 g 2 g 2 g 2 g
298o
C-H O=O C=O O-H
2
4 2 2
Bond EnergiesExample: Use the bond energies listed in Table 8.4 to estimate the heat of reaction at 25oC for the reaction below.
CH O CO 2 H O
H BE BE BE 4 BE
H kJ
H kJ
4 g 2 g 2 g 2 g
298o
C-H O=O C=O O-H
298o
298o
2
4 2 2
4 414 2 498 2 741 4 464
686
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