Download Light

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Many scientists found Rutherford’s Model to
be incomplete
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He did not explain how the electrons are arranged
He did not explain how the electrons were not
pulled in by the positive nucleus
He did not explain the differences in chemical
behavior
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In the early 1900’s, scientists began to
analyze the visible light that elements emitted
when heated
An element’s chemical behavior is related to
the arrangement of its electrons
To better understand chemical behavior, we
must first better understand light
Light
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By 1900, evidence supported that light
consists of waves.
Wave Properties
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Amplitude- wave’s height from zero to crest.
Wavelength- (λ) length from one point of one
wave to same point on next wave.
Frequency- (f) number of wave cycles to pass a
given point per unit of time.
Hertz- units of frequency = cycles/sec
Wavelength and frequency are inversely
proportional to one another.
Speed = wavelength x frequency (V = λf)
Wave Nature of Light
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Consists of radiation (energy) over several
wavelengths. Visible light portion is very small
Light consists of electromagnetic waves.
Electromagnetic Radiation- includes radio
waves, microwaves, infrared waves, visible
light, ultraviolet waves, X-rays, and gamma
rays.
All waves travel at the speed of light (c), which
is 3.0 x 108 m/s.
Practice
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If the wavelength of yellow light is 580 nm,
determine the frequency.
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The wave model is great, but it does NOT
explain some important aspects of light’s
interactions with matter
Particle Nature of Light
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Max Planck studied the light emitted from
heated objects
He concluded that matter can gain or lose
energy in small specific amounts (quanta)
Quantum – minimum amount of energy that
can be gained or lost by an atom
Photoelectric Effect
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Electrons are emitted from a metal’s surface
when certain frequency light shines on it
The frequency of light that required to do this
varies by metal
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Albert Einstein- proposed that light could be
described as quanta of energy.
Light quanta are called photons.
Wave-Particle behavior of light.
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Energy of photon = hv (E=hf )
h- Planck’s constant = 6.626 x 10-34 Js
v/f – frequency
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Also, remember c = v λ
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So… E = hc / λ
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Practice
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How much energy is emitted when light with
a wavelength of 350 nm strikes an atom?
Atomic Spectra
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Electrons are normally at ground state
By absorbing energy, the energy level rises to
2, 3, 4,or more (excited state)
These electrons lose energy by emitting light
when they return to lower energy levels.
Atomic emission spectrum- frequencies of
light emitted by an element separate into
specific lines.
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Each line in an emission spectrum matches
to one exact frequency of light emitted by the
atom.
No two elements have the same emission
spectra. They are like the fingerprints of
elements!