Title: Chemical Bonding and Molecular Structure (Chapter 9)
1Chemical 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-octet
- resonance structures
- formal charges
- VSEPR - predicting shapes of molecules
- Bond properties
- polarity, bond order, bond strength
2Chemical 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?
3Forms of Chemical Bonds
- There are 2 extreme forms of connecting or
bonding atoms - Ioniccomplete transfer of electrons from one
atom to another - Covalentelectrons shared between atoms
Most bonds are somewhere in between.
4Ionic Bonds
- 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)
- primarily between metals (Grps 1A, 2A and
transition metals) and nonmetals (esp O and
halogens)
- NON-DIRECTIONAL bonding via Coulomb
(charge) interaction
5Covalent 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 valence Na
1s2 2s2 2p6 3s1 Ne 3s1
Br Ar 3d10 4s2 4p5 Ar 3d10 4s2 4p5
6Valence Electrons
8A
1A
2A
3A
4A
5A
6A
7A
Number of valence electrons is equal to the Group
number.
7Covalent Bonding
- The bond arises from the mutual attraction of 2
nuclei for the same electrons.
A covalent bond is a balance of attractive and
repulsive forces.
6_H2bond.mov
8Bond Formation
- A bond can result from a head-to-head overlap
of atomic orbitals on neighboring atoms.
This type of overlap places bonding electrons in
a MOLECULAR ORBITAL along the line between the
two atoms and forms a SIGMA BOND (s).
9Sigma Bond Formation by Orbital Overlap
Two s Atomic Orbitals (A.O.s) overlap to form an
s? (sigma) Molecular Orbital (M.O.)
10Sigma Bond Formation by Orbital Overlap
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
11Electron Distribution in Molecules
- 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
12Bond and Lone Pairs
- Electrons are distributed as shared or BOND PAIRS
and unshared or LONE PAIRS.
This is a LEWIS ELECTRON DOT structure.
13Rules of Lewis Structures
- No. of valence electrons of an atom Group
number
- For Groups 1A-4A (Li - C),
- no. of BOND PAIRS group number
- For Groups 5A-7A (N - F),
- no. of BOND PAIRS 8 - group No.
- Except for H
- (and atoms of 3rd and higher periods),
-
- Bond Pairs Lone Pairs 4
14Building a Dot Structure
1. Decide on the central atom never H.
Central atom is atom of lowest affinity for
electrons. In ammonia, N is central
2. Count valence electrons H 1 and N
5 Total (3 x 1) 5 8 electrons
or
4 pairs
15Building a Dot Structure
3. Form a sigma bond between the central
atom and surrounding atoms.
4. Remaining electrons form LONE PAIRS to
complete octet as needed.
3 BOND PAIRS and 1 LONE PAIR.
Note that N has a share in 4 pairs (8 electrons),
while each H shares 1 pair.
16Sulfite ion, SO32-
Step 1. Central atom S
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
10 pairs of electrons are left.
17Sulfite ion, SO32- (2)
Remaining pairs become lone pairs, first on
outside atoms then on central atom.
- Each atom is surrounded by an octet of electrons.
NOTE - must add formal charges (O-, S) for
complete dot diagram
18Carbon Dioxide, CO2
- 1. Central atom __C____
- 2. Valence electrons _16_ or _8_ pairs
- 3. Form sigma bonds.
This leaves __6__ pairs. 4. Place lone pairs
on outer atoms.
19Carbon Dioxide, CO2 (2)
- 4. Place lone pairs on outer atoms.
5. To give C an octet, form DOUBLE BONDS
between C and O.
The second bonding pair forms a pi (p) bond.
20Double and even triple bonds are commonly
observed for C, N, P, O, and S
21Sulfur Dioxide, SO2
- 1. Central atom S
- 2. Valence electrons 6 26 18 electrons
- or 9 pairs
3. Form pi (?) bond so that S has an octet
note that there are two ways of doing this.
22Sulfur Dioxide, SO2
Equivalent structures called
RESONANCE STRUCTURES
The proper Lewis structure is a HYBRID of the
two.
A BETTER representation of SO2 is made by
forming 2 double bonds
Each atom has - OCTET - formal charge 0
O S O
23Urea (NH2)2CO
- 1. Number of valence electrons 24 e-
- 2. Draw sigma bonds.
Leaves 24 - 14 10 e- pairs.
3. Complete C atom octet with double bond.
4. Place remaining electron pairs on oxygen
and nitrogen atoms.
24Violations of the Octet Rule
- Usually occurs with
- Boron
elements of higher periods.
25Boron Trifluoride
- Central atom B
- Valence electrons 3 37 24
- or electron pairs 12
- Assemble dot structure
The B atom has a share in only 6 electrons (or 3
pairs). B atom in many molecules is electron
deficient.
26Sulfur Tetrafluoride, SF4
- Central atom S
- Valence electrons 6 47 34 e-
- or 17 pairs.
- Form sigma bonds and distribute electron pairs.
5 pairs around the S atom. A common occurrence
outside the 2nd period.
27Formal Atom Charges
- Atoms in molecules often bear a charge ( or -).
Formal charge Group no. - 1/2 (no. bond
electrons) - (no. of LP electrons)
- The most important dominant resonance structure
- of a molecule is the one with formal charges
- as close to 0 as possible.
28Carbon Dioxide, CO2
At OXYGEN
At CARBON
29Carbon Dioxide, CO2 (2)
An alternate Lewis structure is
C atom charge is 0.
AND the corresponding resonance form
30Carbon Dioxide, CO2 (3)
Which is the predominant resonance structure?
OR
Answer ? Form without formal charges is BETTER -
no ve charge on O
- REALITY Partial charges calculated
- by CAChe molecular modeling
- system (on CD-ROM).
31Boron Trifluoride, BF3
What if we form a BF double bond to satisfy the
B atom octet?
32Boron Trifluoride, BF3 (2)
fc 7 - 2 - 4 1 Fluorine
fc 3 - 4 - 0 -1 Boron
- 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
33Thiocyanate ion, (SCN)-
Which of three possible resonance structures is
most important?
ANSWER C gt A gt B