Release only ONE color of light
Produce very little heat
What is an LED?
They are semiconductor devices that can convert electrical energy directly into light due to the nature of the bonding that occurs in the semiconductor solid.
Type of bonding is directly related to conductivity of solid.
Bonding in Elemental Solids
Electronegativity – the ability of an atom to attract electrons to itself.
Atoms with low e.n. (metals) don’t hold valence electrons tightly valence electron orbitals are diffuse (large distances away from nucleus)
Atoms with high e.n. (nonmetals) hold valence electrons tightly valence electron orbitals are less diffuse (smaller)
When atoms interact together, their valence orbitals “overlap” to form bonds.
Metals: Weak Covalent Bonds (Metallic Bonding)
Atomic orbitals mix to form a continuous band of orbitals that are very close in energy valence band Low electronegativity metal atoms give up valence electrons (electrons are found in a sea)
Nonmetals: Strong Covalent Bonds
Atomic orbitals mix to form two bands
Bands are separated by an energy gap called the band gap.
Lower-energy band (consists of filled orbitals) valence band Higher-energy band (unfilled orbitals) conduction band
Band gap increases as electronegativity increases.
Band gap for semimetals is smaller than for nonmetals
Nonmetals are poor conductors because their atomic orbitals mix to form two orbital bands with a large band gap, which means a large amount of energy input is necessary to promote an electron from the valence band to the conduction band.
In a complete circuit containing a power source, resistor and an LED, electrons flow from the negative end of the battery to the n-type semiconductor in the diode, then the conduction band of the p-type semiconductor. Light is emitted when electrons drop from the conduction band to the valence band of the p-type semiconductor.
When semimetal atoms interacts to form a solid, their atomic orbitals mix to form two bands of orbitals that are separated by a small band gap while when nonmetal atoms interact to form a solid, their atomic orbitals mix to form two bands of orbitals that are separated by a large band gap.
Steps to Treat Water:
Removal of large debris by passing surface water through a screen. Screens must be cleaned periodically.
Problem: zebra mussels – congregate at water intake and clog screen. Can be removed by using potassium permanganate (KMnO4) Sedimentation
Some suspended (insoluble) particles are small enough to pass through the screen. Water is allowed to sit, so heavy suspended particles settle at the bottom over time (because they are denser than water). Sometimes insoluble particles are too small to settle during sedimentation; two processes known as flocculation and coagulation are used to create larger particles that will settle quickly Flocculation = mixing of water causes agglomeration
Common coagulant: aluminium sulfate (Al2(SO4)3) and other Al or Fe salts Precipitation
Reacting ions with other ions to form insoluble solids which can then be removed by sedimentation or filtration. Major classes of ions removed by precipitation:
Calcium (Ca 2+) and magnesium (Mg 2+) ions
Cause water hardness
Do not pose health threat; but engage in reactions that leave insoluble mineral deposits. Iron (Fe 2+) and manganese (Mn 2+) ions
Can stain plumbing fixtures and clothes
Promotes bacterial growth, giving foul taste and odor to water. Adsorption
Process by which one substance is attached to the surface of another. Powdered Activated Carbon (PAC) – when added to water, organic compounds attach to surface of powder granules. PAC have large surface area
Carbon can be removed through filtration
Gravity filtration – water containing solid impurities (ex. precipitates) is passed through layers of porous material (ex. gravel & sand). Disinfection
Most health threats...
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