Chemical Bonding

Chemical Bonding
Chapters 8 & 9
Bonding occurs to lower the energy of the system.
ionic bonding - transfer of electrons; bonding
occurs due to the attraction of oppositely charged
covalent bonding - sharing of electrons; If the
electrons are shared unequally it is a polar
covalent bond.
polarity of water:
Electronegativity - the ability of an atom in a
molecule to attract shared electrons to itself.
Periodic trend: increases across a period
(excluding noble gases), decreases down a group
Polar molecules are called dipoles. They have a
dipole moment.
Bond Energy - the enthalpy change required to
break a bond in a mole of a gaseous substance.
Breaking bonds is ALWAYS endothermic.
Cl2(g) --> 2 Cl(g)
ΔH = 242 kJ
As the number of bonds between two atoms
increases, the bond grows shorter and stronger.
Reaction enthalpy can be ESTIMATED from bond
energy data.
ΔH = sum of the bonds broken - sum of the
bonds formed
(chart p. 326)
ex - Estimate ΔH for the following reaction.
N2H4(g) --> N2(g) + 2 H2(g)
Writing Lewis Structures:
(see rules handout)
Write the Lewis structure of NO2-.
Resonance forms - equivalent Lewis structure.
The true structure is an average of the Lewis
Formal Charge - helps to decide between two
possible, nonequivalent Lewis structures.
Remember: formal charges are not real charges.
Assign electrons:
1. All bonding electrons are divided equally
between the atoms that form bonds.
2. All nonbonding electrons are assigned
entirely to the atom on which they are
3. The structure with the lowest formal
charge on each atom is the best structure.
ex NCO-
VSEPR- Valence Shell Electron Pair Repulsion
The repulsion of the electron pairs - both
bonding and nonbonding - determines the shape
of the molecule.
return to worksheet - 2nd and 3rd
columns - use handouts on molecular
geometry. Then build and look at the
symmetry to complete column 4
When s orbitals overlap with s or p orbitals
a sigma bond is formed.
When p orbitals overlap with each other a
pi bond is formed.
Single bonds are always sigma bonds.
Double bonds are 1 sigma and 1 pi.
Triple bonds are one sigma and 2 pi.
Hybridization counts the orbitals holding
electrons (electron domains).
2 electron domains = sp
3 electron domains = sp2
4 electron domains = sp3
5 electron domains = dsp3
6 electron domains = d2sp3
Watch this simulation of hybridization
ex Acetonitrile, CH3CN
a. Draw the Lewis structure.
b. Predict the bond angles around each carbon
c. Give the hybridization of each carbon atom.
d. Determine the total sigma and pi bonds
i. on each carbon atom.
ii. in the molecule
Lattice energy: The attractions that hold ionic
compounds together. They are governed by
Coulomb's Law:
The greater the charges of the ions, the greater
the attraction and the stronger the lattice (more
negative lattice energy).
The smaller the atoms, the greater the attraction
and the stronger the lattice energy.
Notes Quiz