Chemical Bonding Chapters 8,9 Concepts of Chemical Bonding and Bonding Theories.
Chapter 8 Basic Concepts of Chemical Bonding. 6.3 Describing Chemical Bonding Page: 288 - 330.
-
Upload
remington-furlong -
Category
Documents
-
view
236 -
download
3
Transcript of Chapter 8 Basic Concepts of Chemical Bonding. 6.3 Describing Chemical Bonding Page: 288 - 330.
Chapter 8Basic Concepts
of Chemical Bonding
6.3 Describing Chemical Bonding
Page: 288 - 330
Thermochemistry
CHEMICAL BONDSIONIC
Electrostatic attraction
between ions.
METALLIC
Metal atoms
bonded to several other
atoms.
COVALENT
Sharing of electrons.
Thermochemistry
The Ionic Bonding
Thermochemistry
Na(s) + ½Cl2(g) NaCl(s)
8.2 The Ionic Bonding
Thermochemistry
Na: loss of an electronCl: gain of an electron
HOW CAN YOU KNOW WHETHER THIS ELECTRON
TRANSFER OCCURS READILY?
The Ionic Bonding
Thermochemistry
HOW CAN YOU KNOW WHETHER THIS ELECTRON
TRANSFER OCCURS READILY?
The Ionic Bonding1. Ionization energy
2. Electron Affinity
3. Lattice Energy
4. Electronegativity Difference
Thermochemistry
Na: loss of an electronCl: gain of an electron
1. IONIZATION ENERGY
One species must have very low ionization
energy (Na)
The Ionic Bonding
Thermochemistry
Na: loss of an electronCl: gain of an electron
1. IONIZATION ENERGY2. ELECTRON AFFINITY
One species must have very high electron
affinity (Cl)
The Ionic Bonding
Thermochemistry
Na(s) + ½Cl2(g) NaCl(s) Δ Hf˚= -410.9 kJ
ELECTROSTATIC ATTRACTIONS
The Ionic Bonding
Thermochemistry
Attraction between oppositely charged ions release of energy
formation of lattice
LE is a measure of a
stability of ions arranged within
an ionic solid
3. LATTICE ENERGY
Thermochemistry
If the difference in electronegativity (ΔEN) between
the species is:ΔEN > 1.7
An Ionic Bond would form between these
species
4. ELECTRONEGATIVITY DIFFERENCE
Thermochemistry
The Ionic Bonding SUMMARY
Thermochemistry
Thermochemistry
Thermochemistry
Thermochemistry
6.3 THE COVALENT BONDING
Covalent Bonding
Covalent Bonding
Thermochemistry
ELECTRONEGATIVITYis the ability of atoms in a
molecule to attract electrons to themselves.
Thermochemistry
DIPOLE and ELECTRONEGATIVITY
DIPOLE: a partial separation of charge One end of the molecule has slightly positive
charge The other end of the molecule has slightly
negative charge
Slight excess of a negative
charge
Slight excess of
a positive charge
DIPOLE and ELECTRONEGATIVITY
Slight excess of
a positive charge
Thermochemistry
DIPOLES are caused by ELECTRONEGATIVITY
Chlorine has higher electronegativity
The electrons spend MORE time around
Chlorine
Hydrogen has lower
electronegativity
The electrons spend LESS time around Hydrogen
BOND POLARITYIn different
compounds, electrons are
not shared equally
Homo nuclear diatomic compounds: H2, Cl2, O2,N2 … share electrons equally =
NON POLAR COVALENT BOND
BOND POLARITY
Hetero nuclear diatomic compounds: HF, HCl, NO, NaCl, LiO… DO NOT share electrons
equally
a. POLAR COVALENT BOND(the attraction of one of the atoms for the bonding electrons is LARGE)δ+ = slightly positive
δ- = slightly negative
BOND POLARITY
Hetero nuclear diatomic compounds: HF, HCl, NO, NaCl, LiO… DO NOT share electrons
equally
a. POLAR COVALENT BOND(the attraction of one of the atoms for the bonding electrons is LARGE)
H – Fδ+ δ-
BOND POLARITY
Hetero nuclear diatomic compounds: HF, HCl, NO, NaCl, LiO… DO NOT share electrons
equally
b. MOSTLY COVALENT BOND (the attraction of one of the atoms for the bonding electrons is
SLIGHTLY GREATER)
H – Fδ+ δ-
PeriodicProperties
of the Elements
© 2012 Pearson Education, Inc.
BOND POLARITY
Hetero nuclear diatomic compounds: HF, HCl, NO, NaCl, LiO… DO NOT share electrons equallya. POLAR COVALENT BOND(the
attraction of one of the atoms for the bonding electrons is LARGE)
b. IONIC BONDS (the attraction of one of the atoms for the bonding electrons is VERY LARGE)
HOW DO YOU KNOW IF THE GIVEN BOND IS
POLAR, NONPOLAR, OR IONIC?
ELECTRONEGATIVITY
The greater the difference in
electronegativity, the more polar the bond is
NON POLAR COVALENT BOND
ELECTRONEGATIVITY
MOSTLY COVALENT BOND
POLAR COVALENT BOND
IONIC BOND
ΔE = 0
0 < ΔE < 0.4
0.4 < ΔE < 1.7
ΔE > 1.7
H2, Cl2, O2,N2
MgO, NaCl, LiF
H2O, CO2, HF
H2O, CO2, HF
NON POLAR COVALENT
BONDH2, Cl2, O2,N2
POLAR COVALENT
BONDH2O, CO2, HF
IONIC BONDMgO, NaCl,
LiFelectronegativity
difference > 2.1
ELECTRONEGATIVITY
Thermochemistry
EXAMPLE In each case, which bond is more
polar: a) B – Cl or C – Cl,
b) P – F or P – Cl? Indicate in each case which atom has the partial
negative charge.B – Cl
C – Cl
1.0
0.5
δ+ δ-
δ+ δ-
F – P
P – Cl
1.9
0.9
δ+δ-
δ+δ-
IONIC vs COVALENT Compounds
Metal + nonmetalHigh melting point
Lattice(crystal) structures
Strong electrolytes
Nonmetal + nonmetal
Low melting pointLow boiling pointNon - electrolytes
IONIC Bonding
COVALENT Bonding
HOMEWORKPAGE: 324 - 323
PROBLEMS: all even
37
6.4 LEWIS STRUCTURE DIAGRAMS
LEWIS DIAGRAMS - REVIEWShow only an atom’s valence electrons
and the chemical symbol.
LEWIS DIAGRAMS
Rule # 1Dots representing valence
electrons are placed around the element symbols
Electron dots are placed singly until the fifth electron
is reached then they are paired
Rule # 2
Lewis Diagrams of IONS and IONIC BONDS
• For positive ions, one electron dot is removed from the valence shell for each positive charge.
• For negative ions, one electron dot is added to each valence shell for each negative charge.
• Square brackets are placed around each ion to indicate transfer of electrons.
LEWIS STRUCTURE DIAGRAMS for MOLECULES
Lewis structures are representations of molecules showing all valence
electrons:• bonding and • nonbonding
LEWIS STRUCTURE DIAGRAMS for MOLECULES
Lewis structures are representations of molecules showing all valence
electrons:• bonding and • nonbonding
a lone pair
Lewis Structures and Multiple Bonds
When two electron pairs are shared, two lines are drawn, i.e. double
bond
::O :: C :: O::
or ::O = C = O::
When three electron pairs are shared, three lines are drawn, i.e. triple
bond
:N ::: N: or
:N ≡ N:
Lewis Structures and Multiple Bonds
BOND LENGTHSThe length of the bond between two atoms decreases as the number of
shared electrons increases
N ≡ N
N = N
N – N > >
BOND STRENGTH?
STEPS TO FOLLOW…
1. Find the sum of valence electrons of all atoms in the polyatomic ion or molecule
a. If it is anion add e-b. If it is cation subtract e-
PCl35 + 21
valence electrons= 26 v.e.
2. Write the symbols for the atoms – Make one of the atoms a central atom (usually the least electronegative atom)
– connect it with the other atoms by single bonds
STEPS TO FOLLOW…
PCl35 + 21
valence electrons= 26 v.e.
26 – 6 valence
electrons= 20 v.e. left
STEPS TO FOLLOW…
2. Write the symbols for the atoms – Subtract those electrons from your total
number of valence electrons
3. Fill the octets of the outer atoms
20 valence electrons
STEPS TO FOLLOW…
3. Fill the octets of the outer atoms
20 valence electrons
STEPS TO FOLLOW…
Subtract the added electrons from your total number of valence electrons
20 valence electrons
STEPS TO FOLLOW…
Subtract the added electrons from your total number of valence electrons
STEPS TO FOLLOW…
20 – 18 valence
electrons= 2 v.e. left
4. Fill the octet of the central atom
2 valence electrons
STEPS TO FOLLOW…
4. Fill the octet of the central atom
0 valence electrons
STEPS TO FOLLOW…
5. If you run out of electrons before the central atom has an octet…
…form multiple bonds until it does
STEPS TO FOLLOW…
WORKSHEET EXAMPLE Draw Lewis Structures for
a) CH2Cl2
b) C2H4
c) BrO3-
WORKSHEET EXAMPLE Draw Lewis Structures for
a) NOb) BF3
c) PF5
EXCEPTIONTS TO THE OCTET RULE1. For molecules and polyatomic ions
containing an odd number of valence electrons
2. For molecules and polyatomic ions in which an atom has fewer than an octet of
valence electrons
EXCEPTIONTS TO THE OCTET RULE3. For molecules and polyatomic ions in
which an atom has more than an octet of valence electrons
1. For molecules and polyatomic ions containing an odd number of valence
electronsComplete pairing of valence electrons is impossible due to the odd number of
valence electrons
E.g.: ClO2, NO, NO2, O2-
2. For molecules and polyatomic ions in which an atom has fewer than an octet of
valence electrons• Not very common
• Mostly Boron or Beryllium compounds
3. For molecules and polyatomic ions in which an atom has more than an octet of
valence electrons• Very common• Such molecules/ions are called
HYPERVALENT• Only for atoms of 3rd period or higher
WHY?1. They have available and unfilled d
orbitals for bonding2. Their central atom (P, S, I, Xe…) is large enough to be bonded to even five different
atoms (Cl, F or O)
Thermochemistry
EXAMPLE Draw a Lewis Structure for ion:
ICl4-
Thermochemistry
EXAMPLE Draw a Lewis Structure for the
thiocyanate ion: NCS-
Thermochemistry
WHICH ONE IS CORRECT ?
WHICH ONE IS THE MOST IMPORTANT?
Thermochemistry
Calculate THE FORMAL CHARGE of each ion to find
out…
FORMAL CHARGEThe charge the atom would have if all the
atoms in the molecule had the same electronegativity
1. ALL unshared electrons are assigned to the atom on which
they are found
2. For a single/double/triple bond, half of the bonding electrons is assigned to to
each atom in the bond
FORMAL CHARGE OF AN ATOM =
# of VALENCE ELECTRON – # OF ELECTRONS ASSIGNED
1. ALL unshared electrons are assigned to the atom on which
they are found
2. For a single/double/triple bond, half of the bonding electrons is assigned to to
each atom in the bond
Thermochemistry
EXAMPLE What are the formal charges of C
and N in the cyanide ion: CN-?
[:C ≡ N:]-
1. ALL unshared electrons are
assigned to the atom on which they are found
2. For a single/double/triple bond, half of the bonding electrons is assigned to to each atom in the
bond# of VALENCE ELECTRON – # OF ELECTRONS
ASSIGNED
Thermochemistry
EXAMPLE What are the formal charges of C
and N in the cyanide ion: CN-?
[:C ≡ N:]-
1. ALL unshared electrons are
assigned to the atom on which they are found
2. For a single/double/triple bond, half of the bonding electrons is assigned to to each atom in the
bond# of VALENCE ELECTRON – # OF ELECTRONS
ASSIGNED
C NValence e- 4 5
Assigned e- 5 5
Formal Charge
-1 0
-1
0
Thermochemistry
WORKSHEET EXAMPLE What are the formal charges on
the thiocyanate ions?
WHICH STRUCTURE IS THE MOST IMPORTANT (DOMINANT) ONE?
1. The most important (most dominant) Lewis structure is the one which has its value closest to 0 (the one with the
fewest charges)
2. The most important (most dominant) Lewis structure is the one which has any negative charges reside on the
more electronegative atoms
Thermochemistry
WORKSHEET EXAMPLE What are the formal charges the
thiocyanate ions?
WHICH STRUCTURE IS THE MOST IMPORTANT (DOMINANT) ONE?
1. The most important (most dominant) Lewis structure is the one which has its value closest to 0 (the one with the
fewest charges)
2. The most important (most dominant) Lewis structure is the one which has any negative charges reside on the
more electronegative atoms
Thermochemistry
WORKSHEET EXAMPLE The cyanate ion, NCO-, has three
possible Lewis structures. a) Draw these three structures
assign formal charges in each.b) Which Lewis structure is
dominant?
8.6 RESONANCE STRUCTURESDraw a Lewis Structure of OZONE,
O3
RESONANCE STRUCTURES
OZONE: A mix of different
resonance structures
GREEN PAINT: A mix
of different colors
Not a single color
Not a single resonance structure
RESONANCE STRUCTURES
SAME BOND LENGTHS
Somewhere between single bond and double bond
RESONANCE STRUCTURES• Is the use of two or more Lewis structures to
represent a molecule• this is because that molecule can not be represented by only a single Lewis structure
Basic Concepts of Chemical
Bonding
© 2012 Pearson Education, Inc.
• In truth, the electrons that form the second C—O bond in the double bonds below do not always sit between that C and that O, but rather can move among the two oxygen atoms and the carbon.
• They are not localized; they are delocalized.
RESONANCE STRUCTURES
Basic Concepts of Chemical
Bonding
© 2012 Pearson Education, Inc.
RESONANCE STRUCTURES• The organic
compound benzene, C6H6, has two resonance structures.
• It is a hexagon with a circle inside to signify the delocalized electrons in the ring.
Thermochemistry
EXAMPLE What are the resonance
structures of nitrate ion: NO3-
Thermochemistry
WORKSHEET EXAMPLE Using Formal Charges and the
concept of Resonance, show why the correct Lewis structure of BF3
is the one in which B has an incomplete octet and only single
bonds are present.
8.8 STRENGTHS OF COVALENT BONDSLearn on your own
HOMEWORKPAGE: 325 - 326
PROBLEMS: 49, 51, 53, 55, 57, 61, 67, 69, 71, 73,
87
ANSWERS TO
WORKSHEET
EXAMPLES
Thermochemistry
WORKSHEET EXAMPLE Copy this table and fill out the
missing information.
Thermochemistry
WORKSHEET EXAMPLE Which substance would you expect to have the greatest
lattice energy, MgF2, CaF2, and ZrO2 ?
ZrO2
Thermochemistry
WORKSHEET EXAMPLE Ionizing an H2 molecule to H2
+
changes the strength of the bond. Based on the description of
covalent bonding given previously, do you expect the
bond H—H in H2+ to be weaker or
stronger than the bond in H2?Weaker. In both H2 and H2
+ the two H atoms are principally
held together by the electrostatic attractions between the nuclei and
the electron(s) concentrated between them. H2+ has only one
electron between the nuclei whereas H2 has two and this results
in the H—H bond in being stronger.
Thermochemistry
WORKSHEET EXAMPLE How does the
ELECTRONEGATIVITY of an element differ from its ELECTRON
AFFINITY?Electron affinity measures:a. the energy released when an isolated atom
gains an electron to form a 1- ion.
The electronegativity measures: b. the ability of the atom to hold on to its own
electrons c. the ability of the atom to attract electrons from
other atoms in compounds.
Thermochemistry
WORKSHEET EXAMPLE In each case, which bond is more
polar: a) B – Cl or C – Cl,
b) P – F or ? Indicate in each case which atom has the partial
negative charge.
Thermochemistry
WORKSHEET EXAMPLE The cyanate ion, NCO-, has three
possible Lewis structures. a) Draw these three structures
assign formal charges in each.b) Which lewis structure is
dominant?
Thermochemistry
WORKSHEET EXAMPLE Using Formal Charges and the concept
of Resonance, show why the correct Lewis structure of BF3 is the one in
which B has an incomplete octet and there are only single bonds present.
– Giving boron a filled octet places a negative charge on the boron and a positive charge on fluorine.
– This would not be an accurate picture of the distribution of electrons in BF3.
– Therefore, structures that put a double bond between boron and fluorine are much less important than the one that leaves boron with only 6 valence electrons.