THE ELECTRON
It is so electrifying
Is this physics or is this chemistry?
Hummmmmmm!!!!
A New Atomic Model
• Rutherford’s planet system model was
an improvement over earlier models,
but it was still not complete.
– Where are the electrons really?
• A new model evolved out of the
similarities discovered between the
behavior of light & electrons.
– This new connection led to
a revolution in science.
The Behavior of Light
• The behavior of an electron can be
modeled by the behavior of light.
– Light is a type of electromagnetic
radiation, that travels through space
as a wave.
– Other examples are (x-rays, radio
waves, gamma-rays, and cellular
waves)
Light as a Wave
• All waves, whether they are water
waves or electromagnetic waves, can
be described in terms of 4
characteristics
–
–
–
–
Amplitude
Frequency
Wavelength
Speed
speed
Light as a Wave
• Amplitude:
–Is the height of the wave measured
from the origin to its crest, or peak
–The brightness, or intensity of light
depends on the amplitude of the light
wave.
Light as a Wave
• Wavelength (l):
– the distance between successive
crests of the wave.
– the distance that the wave travels as
it completes one full cycle of up and
down motion
Light as a Wave
• Frequency ():
– How fast the wave oscillates.
– Measured by the # of times a light
wave completes a cycle of up and
down motion per sec.
– When a radio station
identifies itself it’s the
frequency used
Light as a Wave
• Speed (c):
–Regardless of its wavelength, moves
through space as a constant speed
• 3.00x108 m/s
–Because light moves at a constant
speed there is a relationship between
frequency and wavelength
Light as a Wave
• It is a mathematical relationship
between wavelength and the frequency of a wave.
–The shorter the wavelength the higher
the frequency
–The longer the wavelength the lower
the frequency
• Calculated using
the equation:
Light as a Wave
• When white light passes through a
prism or through raindrops you might
have noticed that the light can be
separated into a continuous array or
spectrum of colors
– All the mixture of
wavelengths that
make up white light
are spread apart
Light as a Wave
• The colors that combine to form white
light are red, orange, yellow, green,
blue, indigo, and violet (ROYGBIV)
• The different colors have different
wavelengths and frequencies
–Shortest l & highest  = violet
–Longest l & lowest  = red
Light as a Wave
• Visible light only constitutes a tiny
portion of the total light spectrum.
–The rest of the electromagnetic spectrum is invisible to the naked eye
• The next slide shows the relative
positions of the various types of EM
radiation in the EM spectrum
Light as a Wave
• Scientists soon discovered that
elements can also produce light or an
electromagnetic spectra.
– If you energize gaseous elements like
hydrogen and then diffract the
light produced through a prism
the result is an EM spectrum,
some of which is in the
visible range
Light as a Wave
• But instead of the spectrum being
continuous (one color bleeding into the
next) the spectrum splits into a pattern
of individual lines.
–It’s not a mixture of all
wavelengths, but a mixture
of specific, individual
wavelengths
Only specific
wavelengths of color
in the mixture
Quantum Theory
• Scientist’s had a hard time explaining
this line spectra
– Why were they specific lines of color
instead of all the colors?
• Finally along came a free thinking
scientist named Max Planck,
– He developed a new theory that is the
basis of modern physics.
• A.K.A. Quantum Theory.
Quantum Theory
• Planck hypothesized that energy,
instead of being given off in continuous waves of energy, is given off in
little packets of energy, or quanta.
– The word quantum means a fixed
amount, think of it as
flashes of energy
– Also called a photon
when describing a
quantum of light
Quantum Theory
• Planck’s idea was that one quantum
of energy (light) was related to its
frequency by the equation: E = h 
• The constant h (planck’s constant)
has a value of 6.6262 x 10-34 J-s, E is
the energy, and  is the frequency of
the radiation.
– The energy in wave form that is absorbed or emitted by atoms, is restricted to specific quantities (quantized)
Quantum Theory
• When we think of energy increasing,
or being absorbed, we usually think of
it increasing continuously.
If you accelerate in
a car, you are
accustomed to your
speed increasing
from 0 to 60 mph
continuously…
Quantum Theory
• When we think of energy increasing,
or being absorbed, we usually think of
it increasing continuously.
But Planck’s
theory means
that energy
increases in
discrete levels
(like steps)
Quantum Theory
• At the atomic level it might be like
being at rest (0 mph) and pressing the
accelerator
– If enough energy is absorbed then
the car leaps to 10 mph
– When enough energy has been
absorbed the car leaps to 20 mph
– When enough energy has been
absorbed the car leaps to 30 mph…
Quantum Theory
• Planck’s understanding works because
of the size of planck’s constant (h).
– Each quantum (leap) is 10-34, so it
feels like a continuous change of
energy at the macroscopic level
– Just like a drawn line with a computer
looks smooth unless you zoom in to
see it is actually blocks
Quantum Theory
• Planck’s theory of quantized energy
was a revolutionary idea, but most
scientists didn’t get it.
• Albert Einstein saw the potential of
quantized energy and proposed it to
be a new way of understanding light.
– He needed Planck’s work to
explain his Nobel Prize
winning research on the
photoelectric effect.
Photoelectric Effect
• Scientists noticed that when you
shined light onto some types of metal,
a voltage could be measured
– The light seems to transfer its energy
to the metal which causes an electric
current
• But, not every kind of light would
cause this to happen
– And it doesn’t help to initiate the
current by making the light brighter
Photoelectric Effect
• For each metal, a minimum frequency
of light is needed to release e– Red light cannot produce a current
– but violet can produce a current
Photoelectric Effect
• Einstein hypothesized that light must
exist as quantized energy
–Light must act as a collection of particles for it to have the ability to collide
with E-s at the surface of metal with
the power to drive the electrons out
• When a photon of light strikes a metal
electron, it acts much like a billiard ball
–The e- is then knocked out of the atom
which causes an electrical current
Photoelectric Effect
• Einstein reasoned that the energy (and
thus the frequency) of the photon determines whether or not it has sufficient
energy to knock an e- from the atom.
– There is a minimum frequency of light
required to establish a current
– Which explains why x-rays are damaging to organisms, while radio waves
have low frequencies and aren’t
hazardous.
Wave…I mean…Particle…I mean…
• The idea that light is a wave that travels at the speed of light, is now coupled with the fact that light can also act
as a particle
– Light can be thought of as a tiny ball
which can collide with an electron
• Light exhibits the properties of both
particles and waves.
Electrons and Quantum Theory
• Realizing that atoms can also gain or
lose energy in chunks or quanta, helps
us answer the question of how
electrons are arranged in the atom.
– Remember earlier we said that if you
split the light given off by H2 gas with
a prism you see set of colored lines
instead of a continuous spectrum.
• This would only happen if the energy
of an electron is quantized
Electrons and Quantum Theory
• Every element, when excited, emits or
absorbs light
– If emitted the light contains a unique
collection wavelengths
– If absorbed the light absorbs the same
pattern of wavelengths
• This gives each element a fingerprint
of spectral lines.
Electrons and Quantum Theory
• Scientists began to try to explain the
occurrence of the line spectra
• Neils Bohr put Rutherford’s atomic
model & Planck’s quantum theory
together to begin to explain the line
spectra
– Rutherford described the atom as a
planetary system with the nucleus
acting as the sun and the electrons
orbiting much like planets.
Electrons and Quantum Theory
• Bohr decided that the planetary model
couldn’t adequately explain the
occurrence of the spectral phenomena
– He reasoned that in order to get the
individual lines of energy released in
line spectra the energy of the e- must
be quantized.
– The electron is permitted to have only
certain orbits corresponding to different levels of energy.
Electrons and Quantum Theory
• Bohr labeled each energy level, or
orbit, by a number, n.
– an atom with its e-s occupying their
lowest energy levels the ground state
• If an e- at any level absorbs a particular amount of energy, it leaps to a
level of higher energy, an excited
state
– The excited e- will return to ground
state & release its absorbed energy
Electrons and Quantum Theory
• The energy released as the e- falls
back to ground state might be released
as a photon in the visible range (color).
– The more energy absorbed by the ethe higher the leap in energy
• The higher the leap - the farther the
electron falls
• Each fall; leads to specific frequencies;
therefore specific lines of color
Electrons and Quantum Theory
• Bohr used his theory to calculate the
frequencies & wavelengths emitted by
excited H atoms accurately
– which was
powerful evidence
in support of his
model.
– It only worked
successfully for
Hydrogen
Light = Wave & Particle
• Planck’s & Einstein’s theories lead us
to an understanding of the light as a
wave & as a discrete particle.
– When light travels through space it
has wavelike properties.
– When it interacts with matter its
behavior can be described as like a
stream of particles.
Matter = Wave & Particle
• If EM radiation has properties of
waves and particles, maybe matter
does too.
– This connection was made by Louis
de Broglie
• Louis de Broglie reasoned that even
particles of matter can behave like
waves and at times exhibit the characteristics of a wave, much like light.
Matter = Wave & Particle
• He developed a relationship between
the mass & velocity of a particle and the
wavelength it would exhibit
– l = h/mv.
• The eqn predicts that all objects in
motion have wavelike behavior
– it is only noticeable in objects with a
tiny mass.
– scientists routinely use this theory of
electrons having wavelike nature to
magnify objects