Chapter 29:
Molecular bonding
Aileen, Jamie,
Madeleine, and Ross
Bonding in molecules
• A molecule is two or more atoms that are
held together so as to function as a single
unit. When atoms make this attachment, it
is called a chemical bond.
• Two types of bond: covalent and ionic.
Covalent bonding
• Also called “sharing bond”.
• When two atoms approach each other, the
electron clouds begin to overlap, and the
electrons from each atom orbits both
nuclei.
• For example, when using two hydrogen
atoms to form a hydrogen molecule, there
are two possibilities. Parallel with a spin of
1 and opposite with a spin of 0.
• The exclusion principle tells us that since
no two electrons can occupy the same
state, if two electrons are the same they
must be different is some other way.
• The exclusive principle requires that when
the spins are the same, there is
destructive interference of the electrons
wave functions between the two atoms.
• A covalent bond results from constructive
interference of the electron wave function.
• A bond can be understood from the
energy point of view.
• If a H atom spin is opposite, electrons
move freely, the wavelength is longer,
momentum is less and energy is less.
Ionic Bonding
• Ionic bonds are a special type of covalent
bonds
• Instead of equal sharing of electrons, they
are transferred or unequally shared
• The electrostatic attraction between two
atoms, usually a metal and a non-metal,
keeps them together
Ionic Bonding
• Na+ + Cl- = NaCl
• The Sodium loses an electron, forming a
cation, and the Chlorine accepts the
electron, forming an anion
• These positive and negative charged ions
then combine
• When formed together they make the table
salt Sodium Chloride
• Reaction is endothermic
• Only occurs if energy change is favorable
– Bonded atoms have a lower energy than the
free atoms
• The larger the energy change indicates how
strong the bond will be
• The low electronegativity of metals and high
electronegativity of non-metals means the
energy change is favorable when metals lose an
electron and non-metals gain them
Weak bonds
 Intermolecular forces
 Forms between molecules due to electrostatic
attraction (positive/negative charges)
 Energy required for bonding to occur is called the
bond energy
 Typical weak bond energy = 0.04 to 0.3 eV
 To put things into perspective, a typical strong
bond energy = 2 to 5 eV
 Avg. 12 times more energy than the strongest of the
weak bonds (Hydrogen bonding)
Types of weak bonds
 Dipole-dipole interaction: the (+) side of a dipole
molecule attracts the (-) side of another dipole
molecule.
 Dipole molecule : two elements with different
amounts of electronegativity (attraction)
 NaCl – table salt
 Na: EN value = 0.9
 Cl: EN value = 3.0
 Chlorine has a higher EN value, thus pulling more
electrons toward it and resulting in an partial (-) charge
on Cl and a partial (+) charge on Na.
 This slight separation of charges is known as a dipole
moment.
Dipole-Dipole bonds
www.sparknotes.com
Stezlab1.unl.edu
Hydrogen Bonding
• A dipole-dipole bond that involves hydrogen and
oxygen, nitrogen, or fluorine.
• The strongest of the weak bonds
• Simplest example: water molecules
• Very important in the double helix shape of DNA
Geneticengineering.org
Junction transistors
• Junction transistor consists of a crystal of one
type of doped semiconductor sandwiched
between two crystals of opposite type.
• Figure 29-31
•npn arrow placed on emitter
comes away from base while in
pnp arrow goes toward the base
•Arrow indicates the direction of
current flow in normal operation
• Transistors are the basic elements in modern
electronic amplifiers
• They also can act like a “gate” or switch in digital
circuits. That is, they let a current pass or they
block it off.
• Transistors were a great advance in
miniaturization of electronic circuits.
• Individual transistors are very small compared to
ones used in vacuum tubes but huge compared
to integrated circuits or chips.
• A tiny chip can contain millions of transistors
and other circuit elements.
• Integrated circuits are the main part of
computers, televisions, calculators, cameras
and electronic instruments in aircraft space
vehicles and autos.
• The miniaturization made by integrated circuits
not only allows extremely complicated circuits to
be placed in small places, but they do allow a
great increase in speed of operation.
Summary
• Molecular bonding due to electrostatic forces
• Strong bonds within molecules:
– Covalent bonding is the sharing of electrons by two or more atoms
while Ionic bonding is the donation of electrons by two or more
atoms.
• Weak bonds between molecules:
– Hydrogen bonding: type of intermolecular bonding that occurs
between H and O, N, or F.
• An example of bonding in solids is a junction transistor,
where impure crystals of silicon can be used as a
semiconductor.
• Junction transistors have lead to advances in electronics
by creating a more compact circuit.
Problems
Concepts:
1. What is the main difference between ionic and
covalent bonding?
2. How is hydrogen bonding exclusive? What elements
are involved in this type of bond?
Problems:
1. What is the energy range of a strong bond? A weak
bond?
2. Explain how a transistor can behave like a switch in
digital circuits.