18, 20 oct 97bonding and structure1 chemical bonding and molecular structure (chapter 9) ionic vs....
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18, 20 Oct 97 Bonding and Structure 1
Chemical Bonding and
Molecular Structure (Chapter 9)
• Ionic vs. covalent bonding• Molecular orbitals and the covalent bond (Ch. 10)• Valence electron Lewis dot structures
octet vs. non-octetresonance structuresformal charges
• VSEPR - predicting shapes of molecules• Bond properties
polarity, bond order, bond strength
18, 20 Oct 97 Bonding and Structure 2
Chemical Bonding
Problems and questions —
• How is a molecule or polyatomic ion held together?
• Why are atoms distributed at strange angles?
• Why are molecules not flat?
• Can we predict the structure?
• How is structure related to chemical and physical properties?
18, 20 Oct 97 Bonding and Structure 3
Most bonds are somewhere in between.
Forms of Chemical Bonds• There are 2 extreme forms of connecting
or bonding atoms:
• Ionic—complete transfer of electrons from one atom to another
•Covalent—electrons shared between atoms
18, 20 Oct 97 Bonding and Structure 4
Ionic Ionic BondsBonds
Ionic compounds
- essentially complete electron transfer from an element of low IE (metal) to an element of high electron affinity (EA) (nonmetal)
Na(s) + 1/2 Cl2(g) Na+ + Cl-
NaCl (s)
- NON-DIRECTIONAL bonding via Coulomb (charge) interaction
- primarily between metals (Grps 1A, 2A and transition metals) and nonmetals (esp O and halogens)
18, 20 Oct 97 Bonding and Structure 5
Covalent Bonding
Covalent bond is the sharing of the VALENCE ELECTRONS of each atom in a bond
Recall: Electrons are divided between core
and valence electrons.
ATOM core valenceNa 1s2 2s2 2p6 3s1 [Ne] 3s1
Br [Ar] 3d10 4s2 4p5 [Ar] 3d10 4s2 4p5
Br Br
18, 20 Oct 97 Bonding and Structure 6
Valence Electrons1A1A
2A2A 3A3A 4A4A 5A5A 6A6A 7A7A
8A8A
Number of valence electrons is equal to the Group number.
18, 20 Oct 97 Bonding and Structure 7
Covalent BondingThe bond arises from the mutual attraction of
2 nuclei for the same electrons.
HB+ HAHBHA
A covalent bond is a balanceof attractive and repulsive forces.
6_H2bond.mov
18, 20 Oct 97 Bonding and Structure 8
Bond FormationA bond can result from a “head-to-head” overlap
of atomic orbitals on neighboring atoms.
H H Cl••
••
•• Cl
••
••
••
+
Overlap of H (1s) and Cl (2p)
This type of overlap places bonding electrons in a
MOLECULAR ORBITAL along the line between
the two atoms and forms a SIGMA BOND ().
18, 20 Oct 97 Bonding and Structure 9
Sigma Bond Formation by Orbital Overlap
•• ••
sigma bond ( )
+HH
Two s Atomic Orbitals (A.O.s) overlap to form an s (sigma) Molecular Orbital (M.O.)
6_H2pot.mov
18, 20 Oct 97 Bonding and Structure 10
Sigma Bond Formation by Orbital Overlap
•• ••
sigma bond ( )
+HH
Two s A.O.s overlap to from an s M.O.
Similarly, two p A.O.s can overlap end-on to from a p M.O.
e.g.F2
18, 20 Oct 97 Bonding and Structure 11
Electron Electron Distribution in Distribution in
MoleculesMolecules• Electron distribution
is depicted with
Lewis electron dot structures
• Electrons are distributed as:
• shared or BOND PAIRS and
• unshared or LONE PAIRS.
G. N. Lewis 1875 - 1946
18, 20 Oct 97 Bonding and Structure 12
Bond and Lone Pairs• Electrons are distributed as shared or BOND
PAIRS and unshared or LONE PAIRS.
••H Cl
••••
This is a LEWIS ELECTRON DOT structure.
shared or bond pair
Unshared orlone pair (LP)
18, 20 Oct 97 Bonding and Structure 13
• This observation is called the OCTET RULE
Rules of Lewis StructuresRules of Lewis Structures• No. of valence electrons of an atom =
Group number
• Except for H (and atoms of 3rd and higher periods),
#Bond Pairs + #Lone Pairs = 4
• For Groups 5A-7A (N - F), no. of BOND PAIRS = 8 - group No.
• For Groups 1A-4A (Li - C), no. of BOND PAIRS = group number
18, 20 Oct 97 Bonding and Structure 14
2. Count valence electrons H = 1 and N = 5 Total = (3 x 1) + 5 = 8 electrons or
1. Decide on the central atom; never H. Central atom is atom of lowest affinity for electrons. In ammonia, N is central
Building a Dot Structure
Ammonia, NH3
4 pairs
18, 20 Oct 97 Bonding and Structure 15
4. Remaining electrons form LONE PAIRS to complete octet as needed.
3. Form a sigma bond between the central atom and surrounding atoms.
H H
H
N
Building a Dot Structure
H••
H
H
N
3 BOND PAIRS and 1 LONE PAIR.
Note that N has a share in 4 pairs (8 electrons), while each H shares 1 pair.
18, 20 Oct 97 Bonding and Structure 16
Step 2. Count valence electrons S = 6 3 x O = 3 x 6 = 18 Negative charge = 2
TOTAL = 6 + 18 + 2 = 26 e- or 13 pairs
Step 1. Central atom = S
10 pairs of electrons are left.
Sulfite ion, SO32-
Step 3. Form sigma bonds
O O
O
S
18, 20 Oct 97 Bonding and Structure 17
Remaining pairs become lone pairs, first on outside atoms
then on central atom.
Sulfite ion, SO32- (2)
Each atom is surrounded by an octet of electrons.
••O O
O
S
••
••
•• ••
••
••
••
••
••
NOTE - must add formal charges (O-, S+) for complete dot diagram
18, 20 Oct 97 Bonding and Structure 18
Carbon Dioxide, CO2
1. Central atom = __C____
2. Valence electrons = _16_ or _8_ pairs
3. Form sigma bonds.
O OC
••O OC
•• ••
••••••
This leaves __6__ pairs.4. Place lone pairs on outer atoms.
18, 20 Oct 97 Bonding and Structure 19
••O OC
•• ••
••••••
Carbon Dioxide, CO2 (2)4. Place lone pairs on outer atoms.
••O OC
•• ••
••••••
••O OC
•• ••
••
The second bonding pair forms a pi () bond.
5. To give C an octet, form DOUBLE BONDS between C and O.
18, 20 Oct 97 Bonding and Structure 20
SO3
H2CODouble and even triple bonds are commonly observed for C, N, P, O, and S
••O OC
•• ••
••
C2F4
18, 20 Oct 97 Bonding and Structure 21
Sulfur Dioxide, SO2
1. Central atom = S
2. Valence electrons = 6 + 2*6 = 18 electrons
or 9 pairs••O OS
••
••
••
••••••
••O OS
••
••
••
••••••
bring inleft pair
OR bring inright pair
3. Form pi () bond so that S has an octet — note that there are two ways of doing this.
18, 20 Oct 97 Bonding and Structure 22
Sulfur Dioxide, SO2
••O OS
••
••
••
••••••
bring inleft pair
OR bring inright pair
••O OS
••
••
••
••••
••O OS••
••
••
••
••
Equivalent structurescalled:
RESONANCE STRUCTURES
The proper Lewis structure is a HYBRID of the two.
A BETTER representation of SO2 is made by forming 2 double bonds
O = S = OEach atom has - OCTET - formal charge = 0
18, 20 Oct 97 Bonding and Structure 23
Urea (NHUrea (NH22))22COCO1. Number of valence electrons = 24 e-
2. Draw sigma bonds.CN N H
HH
H
O
4. Place remaining electron pairs on oxygen
3. Complete C atom octet with double bond.
CN N H
HH
H
O
Leaves 24 - 14 = 10 e- pairs.
and nitrogen atoms.
18, 20 Oct 97 Bonding and Structure 24
Violations of the Octet RuleUsually occurs with:
Boron
BF3 SF4
elements of higher periods.
18, 20 Oct 97 Bonding and Structure 25
Boron Trifluoride
• Central atom = B
• Valence electrons = 3 + 3*7 = 24
or electron pairs = 12
• Assemble dot structure
F••
••
••
F
F
B••
••
••
••
••
••
The B atom has a share in only 6 electrons (or 3 pairs). B atom in many molecules is electron deficient.
18, 20 Oct 97 Bonding and Structure 26
Sulfur Tetrafluoride, SF4
• Central atom = S
• Valence electrons = 6 + 4*7 = 34 e-
or 17 pairs.
• Form sigma bonds and distribute electron pairs.
F
••
••
••
F
F
S••
••
••
••
•• F
••
••
••
••
••5 pairs around the S 5 pairs around the S atom. A common atom. A common occurrence outside the occurrence outside the 2nd period. 2nd period.
18, 20 Oct 97 Bonding and Structure 27
Formal charge = Group no. - 1/2 (no. bond electrons)
- (no. of LP electrons)
Formal Atom Charges
• Atoms in molecules often bear a charge (+ or -).
• The most important dominant resonance structure of a molecule is the one with formal charges as close to 0 as possible.
18, 20 Oct 97 Bonding and Structure 28
04 - (1/2)(8) - 0 =
6 - (1/2)(4) - 4 = 0
Carbon Dioxide, CO2
At OXYGEN
O C O••
• •••
• •
At CARBON
18, 20 Oct 97 Bonding and Structure 29
C atom charge is 0.
6 - (1/2)(6) - 2 = +1
6 - (1/2)(2) - 6 = -1
Carbon Dioxide, CO2 (2)
O C O••
• •••
• •
An alternate Lewis structure is:
AND the corresponding resonance form
+
O C O••
• •••
• •
+
18, 20 Oct 97 Bonding and Structure 30
• REALITY: Partial charges calculated
by CAChe molecular modeling system (on CD-ROM).
+1.46-0.73 -0.73
Carbon Dioxide, CO2 (3)
Which is the predominant resonance structure?
O C O••
• •••
• •
ORO C O•
•
• •••
• •
O C O••
• •••
• •
+
+
Answer ?Form without formal charges isBETTER - no +ve charge on O
18, 20 Oct 97 Bonding and Structure 31
Boron Trifluoride, BF3
F••
••
••
F
F
B••
••
••
••
••
••
What if we form a B—F double bond to satisfy the B atom octet?
18, 20 Oct 97 Bonding and Structure 32
Boron Trifluoride, BF3 (2)
• To have +1 charge on F, with its very high electron affinity is not good. -ve charges best placed on atoms with high EA.
• Similarly -1 charge on B is bad
• NOT important Lewis structure
fc = 7 - 2 - 4 = +1 Fluorine
F••
••
F
F
B••
••
••
••
••
••
fc = 3 - 4 - 0 = -1 Boron
+