Chapter 7.
7.1
7.2
7.3
7.4
7.5
7.6
7.7
7.9
7.11
7.12
7.1
Covalent Bonds and Molecular Structure
The Covalent Bond
Strengths of Covalent bonds
A Comparison of Ionic and Covalent Compounds
Polar Covalent Bonds: Electronegativity
Electron-Dot Structures
Electron-Dot Structures of Polyatomic Molecules
Electron-Dot Structures and Resonance
Molecular shapes: The VSEPR Model
Hybridization and Sp3 Hybrid Orbitals
Other Kinds of Hybrid Orbitals
Covalent Bond
A. Formation of a covalent bond is the
sharing of electrons between atoms forming a bond
Two nuclei come together and electrostatic interactions begin to develop
a. Two nuclei repel each other; electrons repel each other
b. Each nucleus attracts the electrons from other atom; electrons attract
both nuclei
c. If attractive forces > repulsive forces, then covalent bond is formed.
1
B. Distance between the two atoms affects the magnitude of the various
electrostatic forces
Bond length = the optimum point where net attraction forces are
maximized
a.
Each covalent bond has a characteristic length that leads to max.
stability. The is the bond length.
b.
Can predict the atomic radii
7.2
Strengths of Covalent Bonds
A. Formation of covalent bonds leads to lower energy in the resulting compound
B. Bond dissociation energy- the amount of energy necessary to break a covalent
chemical bond in an isolated molecule in the gas state (positive value)
a negative value equals the amount of energy released when the bond forms
C. Each bonds in every molecule has its own specific bond dissociation energy
- bonds between the same pairs of atoms usually have similar bond
dissociation energies
Energies range from as low as 151 kJ/ mole (I-I) to as high as 570 kJ/ mole (H-F)
for multiple bonds , examples are C=C (611); O=O (498); N N (945)
2
7.3
A Comparison of Ionic and Covalent Compounds
Some physical Properties of NaCl and HCl
Properties
NaCl
HCl
appearance
white solid
colorous gas
bond type
ionic
covalent
melting point
801 oC
-115 oC
o
boiling point
1413 C
-84.9 oC
WHY major differences?
For an ionic solid to melt, each ionic attractive bond within the 3 -D solid lattice
must break, a process that requires a large amount of energy
For covalent compounds, the attractive forces
between the different molecules are fairly weak.
As a result, less energy is required to overcome
these intermolecular forces.
7.4
A.
B.
Polar Covalent Bonds:
Electronegativity
Ionic and covalent bonds represent extremes of a continuous spectrum of
possibilities
Polar covalent bonds- the bonding electrons are attracted somewhat more
strongly by one atom in a covalent bond.
1. Electrons are not completely transferred (are still shared, but not equally)
2. More electronegative atom has symbol
3. Less electronegative atom has symbol +
-
3
C. Electronegativity (EN) - the ability of an atom in an molecule to attract the shared
electrons in a covalent bond
1. low electronegativities - Metallic elements
2. high electronegativities halogens and other elements in upper-right-hand
corner of periodic table
D. Predicting bond polarity
1.
Atoms with similar electronegativities ( Diff. In EN < 0.4)
form nonpolar bond and are non-polar molecules
2.
Atoms whose electronegativities differ by more than 2 units
form Ionic bond and are ionic compounds
3.
Atoms whose electronegativities differ by less than 2 units
from polar covalent bonds and are polar covalent molecules
4
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