Post on 15-Dec-2015
2007, Prentice Hall
Chemistry: A Molecular Approach, 1st Ed.
Nivaldo Tro
Roy KennedyMassachusetts Bay Community College
Wellesley Hills, MA
The Behavior of the Very Small
electrons are incredibly smalla single speck of dust has more electrons than
the number of people who have ever lived on earth
electron behavior determines much of the behavior of atoms
directly observing electrons in the atom is impossible, the electron is so small that observing it changes its behavior
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A Theory that Explains Electron Behaviorthe quantum-mechanical model explains the
manner electrons exist and behave in atomshelps us understand and predict the properties
of atoms that are directly related to the behavior of the electronswhy some elements are metals while others are
nonmetalswhy some elements gain 1 electron when forming an
anion, while others gain 2why some elements are very reactive while others are
practically inertand other Periodic patterns we see in the properties of
the elements
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The Nature of Lightits Wave Naturelight is a form of electromagnetic radiation
composed of perpendicular oscillating waves, one for the electric field and one for the magnetic field an electric field is a region where an electrically charged
particle experiences a force a magnetic field is a region where an magnetized particle
experiences a force
all electromagnetic waves move through space at the same, constant speed3.00 x 108 m/s in a vacuum = the speed of light, c
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Characterizing Wavesthe amplitude is the height of the wave
the distance from node to crest or node to trough
the amplitude is a measure of how intense the light is – the larger the amplitude, the brighter the light
the wavelength, () is a measure of the distance covered by the wavethe distance from one crest to the next
or the distance from one trough to the next, or the distance between alternate nodes
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Characterizing Wavesthe frequency, () is the number of waves
that pass a point in a given period of timethe number of waves = number of cyclesunits are hertz, (Hz) or cycles/s = s-1
1 Hz = 1 s-1
the total energy is proportional to the amplitude and frequency of the wavesthe larger the wave amplitude, the more force it
hasthe more frequently the waves strike, the more
total force there is
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The Relationship Between Wavelength and Frequency
for waves traveling at the same speed, the shorter the wavelength, the more frequently they pass
this means that the wavelength and frequency of electromagnetic waves are inversely proportionalsince the speed of light is constant, if we know
wavelength we can find the frequency, and visa versa
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m
cs s
m1-
ExamplesCalculate the wavelength of red light with
a frequency of 4.62 x 1014 s-1
Calculate the wavelength of a radio signal with a frequency of 100.7 MHz
Colorthe color of light is determined by its wavelength
or frequencywhite light is a mixture of all the colors of visible
light a spectrumRedOrangeYellowGreenBlueViolet
when an object absorbs some of the wavelengths of white light while reflecting others, it appears coloredthe observed color is predominantly the colors
reflected
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12
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The Electromagnetic Spectrumvisible light comprises only a small fraction of
all the wavelengths of light – called the electromagnetic spectrum
short wavelength (high frequency) light has high energyradiowave light has the lowest energygamma ray light has the highest energy
high energy electromagnetic radiation can potentially damage biological moleculesionizing radiation
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Interferencethe interaction between waves is called
interferencewhen waves interact so that they add to make a
larger wave it is called constructive interferencewaves are in-phase
when waves interact so they cancel each other it is called destructive interferencewaves are out-of-phase
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Diffractionwhen traveling waves encounter an obstacle
or opening in a barrier that is about the same size as the wavelength, they bend around it – this is called diffractiontraveling particles do not diffract
the diffraction of light through two slits separated by a distance comparable to the wavelength results in an interference pattern of the diffracted waves
an interference pattern is a characteristic of all light waves
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The Photoelectric Effectit was observed that many metals emit
electrons when a light shines on their surfacethis is called the Photoelectric Effect
classic wave theory attributed this effect to the light energy being transferred to the electron
according to this theory, if the wavelength of light is made shorter, or the light waves intensity made brighter, more electrons should be ejected
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The Photoelectric EffectThe Problemin experiments with the photoelectric effect,
it was observed that there was a maximum wavelength for electrons to be emittedcalled the threshold frequencyregardless of the intensity
it was also observed that high frequency light with a dim source caused electron emission without any lag time
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Einstein’s ExplanationEinstein proposed that the light energy was
delivered to the atoms in packets, called quanta or photons
the energy of a photon of light was directly proportional to its frequencyinversely proportional to it wavelengththe proportionality constant is called Planck’s
Constant, (h) and has the value 6.626 x 10-34 J∙s
chhE
ExamplesCalculate the number of photons in a laser pulse
with wavelength 337 nm and total energy 3.83 mJ
What is the frequency of radiation required to supply 1.0 x 102 J of energy from 8.5 x 1027 photons?
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Ejected Electrons1 photon at the threshold frequency has just
enough energy for an electron to escape the atombinding energy,
for higher frequencies, the electron absorbs more energy than is necessary to escape
this excess energy becomes kinetic energy of the ejected electron
Kinetic Energy = Ephoton – Ebinding
KE = h -
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Spectrawhen atoms or molecules absorb energy, that
energy is often released as light energyfireworks, neon lights, etc.
when that light is passed through a prism, a pattern is seen that is unique to that type of atom or molecule – the pattern is called an emission spectrumnon-continuouscan be used to identify the material
Rydberg analyzed the spectrum of hydrogen and found that it could be described with an equation that involved an inverse square of integers
22
21
1-7
n
1
n
1m 10097.1
1
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Na K Li Ba
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Spectra of Mercury
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Bohr’s ModelNeils Bohr proposed that the electrons could
only have very specific amounts of energyfixed amounts = quantized
the electrons traveled in orbits that were a fixed distance from the nucleusstationary statestherefore the energy of the electron was
proportional the distance the orbital was from the nucleus
electrons emitted radiation when they “jumped” from an orbit with higher energy down to an orbit with lower energythe distance between the orbits determined the
energy of the photon of light produced
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Wave Behavior of Electronsde Broglie proposed that particles could have
wave-like characterbecause it is so small, the wave character of
electrons is significantelectron beams shot at slits show an
interference patternthe electron interferes with its own wave
de Broglie predicted that the wavelength of a particle was inversely proportional to its momentum
)s(m(kg)
s
mkg
m1-
2
2
velocitymass
h
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however, electrons actually present an interference pattern, demonstrating the behave like waves
if electrons behave like particles, there should only be two bright spots on the target
examplesCalculate the wavelength of an electron traveling
at 2.65 x 106 m/s
Determine the wavelength of a neutron traveling at 1.00 x 102 m/s(Massneutron = 1.675 x 10-24 g)
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Uncertainty PrincipleHeisenberg stated that the product of the
uncertainties in both the position and speed of a particle was inversely proportional to its massx = position, x = uncertainty in positionv = velocity, v = uncertainty in velocitym = mass
the means that the more accurately you know the position of a small particle, like an electron, the less you know about its speedand visa-versa
m
1
4
hv
x
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any experiment designed to observe the electron results in detection of a single electron particle and no interference pattern
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Determinacy vs. Indeterminacyaccording to classical physics, particles move
in a path determined by the particle’s velocity, position, and forces acting on itdeterminacy = definite, predictable future
because we cannot know both the position and velocity of an electron, we cannot predict the path it will followindeterminacy = indefinite future, can only predict
probabilitythe best we can do is to describe the
probability an electron will be found in a particular region using statistical functions