Transcript 29-1Bonding in Molecules

29-1Bonding in Molecules
*When atoms cling together as a single unit to
achieve lower energy levels, this is a chemical bond.
*Bonds occur as ionic an covalent bonds.
*Two non-metals typically bond covalently
*A metal and non-metal generally make ionic bonds.
*An attraction between two molecules is a VanDer
Waals Bond. (or weak bond)
29-2 Potential-Energy Diagrams
for Molecules
*See diagram p 891
29-3 Van der Waals Bonds
• Attractions between molecules.
• Determine melting/boiling points and physical
states.
• (strongest) Hydrogen bond is 5% of a
molecular bond.
• Dipole interactions involve +/- ends of
molecules sticking together.
• Dispersion forces are attractions because of
electron movement caused magnetic
attractions.
29-4 Vibrational Energy in
Hydrogen
See practice problem 29-3 p899
29-5 Bonding in Solids
• Some materials are amorphous.
• Many solids are crystalline and have a
interconnected structure called a lattice. See
page 901
• Important Strong Bonds
– Ionic…an electron is stolen giving +/- ions
– Covalent…electrons are shared causing attraction
– Metallic…electrons shared as a free-moving sea of
electrons.
29-6 Band Theory of Solids
• When atoms come together, they form layers we
call bands.
• For a good conductor, the highest or valence
band is only partially filled and the conduction
band is close or connected along side it.
Electrons jump from hole to hole along the
conductor.
• For an insulator, the conduction space or band is
located fare away, separated by an energy gap.
• For a Semiconductor, the separation is smaller
than the insulator.
29-7 Semiconductor Doping
• Germanium and Silicon only acquire useful
properties for electronics when they are treated
with impurities, or doped.
• An arsenic doped silicon crystal is called an n-type
semiconductor because negative charges carry
the electric current.
• If a small amount of a 3-electron valence atom,
such as gallium is added, a p-type semiconductor
is formed, because positive holes seem to carry
the charge.
29-8 Semiconductor Diodes
• In a PN junction diode, a few electrons from
the n-side cross to the p-side causing a
potential difference….
• If the diode is connected forward biased, with
positive to p, current flows. If it connected
reverse biased, then no current flows.
• Since a p-n junction diode only allows
electricity to flow one way, it can serve as a
rectifier. See page 908
• A half-rectifier with a capacitor can change AC
to DC
• Light emitting diodes
• Solar cells
29-9 Transisters and Integrated
Circuits
NPN transistors and PNP transistors
serve as amplifiers see page 909