Unanswered Questions
• Rutherford’s work on the atom was
important but left big questions:
– How are electrons arranged around the
atom?
– Why aren’t electrons pulled into the
positive nucleus?
– What accounts for the difference in
chemical behavior among atoms?
Unraveling Chemical Behavior
• In the early 1900’s chemists found that
certain elements emitted visible light when
heated in a flame (now called atomic
emission spectra).
• They found that this and all of an
element’s chemical behavior is related to
the arrangement of the electrons in its
atoms.
Electromagnetic Radiation
• A form of energy that exhibits wavelike
behavior as it travels through space.
• Includes visible light, ultraviolet, infrared,
TV, radio, X-ray, microwaves & gamma
• All e-m waves travel at the speed of light
(c) which is 3.00 X108 m/s.
• All e-m waves have a wavelength,
frequency and amplitude.
• This is the wave model of light.
Electromagnetic Spectrum
•
•
•
•
C=λxv
C = speed of light in m/s and λ = wavelength
Wavelength is given in meters and frequency
is given in hertz or s-1.
Wavelength and frequency have an inverse
relationship (one goes up the other goes
down).
Greater frequency = greater energy
Try It 1
A helium-neon laser emits light with
a wavelength of 633 nm. What is
the frequency of this light?
Try It 2
What is the wavelength of X rays
having a frequency of 4.80 x 1017
Hz?
Need for a new model
• The wave model of light cannot explain
why heated objects emit only certain
frequencies (colors) of light at a given
temperature
• Or why some metals emit electrons when
colored light of a specific frequency shines
on them.
Planck’s Work
• Found that heated metals emit specific
colors (frequencies) of light (flame tests).
• As objects get hotter and possess more
energy, the metal emits different colors
(frequencies) of light.
• Found matter can only gain or lose energy
in small, specific amounts (a quantum)
Quantum
• The minimum amount of energy that can
be gained or lost by an atom at a time.
• Equantum = hv
• Since C = λ x v,
then E = hc / λ
• h = Planck’s constant:
6.626 x 10-34 Joules.seconds or J.s
More quantum ideas
• According to Planck’s theory, matter can
have only certain amounts of energy—
quantities of energy between these values
do not exist.
• So this was a big deal at the time!
Try It 3
Calculate the energy of a gamma
ray photon whose frequency is
5.02 x 1020 Hz.
= (6.626 x 10-34 J.s) (5.02 x 1020 Hz)
= 3.33 x10-13 J
Photoelectric Effect
Increase light’s intensity and more electrons are
ejected.
Increase light’s frequency and ejected electrons
move faster.
Photoelectric Effect
• Each metal has a minimum frequency
necessary for this to happen.
• Varying intensity and frequency changes
the number of electrons ejected & their
speed.
• Led to development of photoelectric cells
used in street lights, solar calculators, etc.
Einstein’s Proposal
• The wave model couldn’t explain the
photoelectric effect.
• So in 1905 Einstein proposed the “particle
nature” of light.
• Says that light is both a wave and particle
• Light is really a stream of tiny particles or
bundles of energy called photons.
Photons
• Bundles of energy that make up all EM
waves like light.
• Have no mass but each photon carries a
quantum of energy so
Equantum = Ephoton = hv
Atomic Emission Spectra
• If a gas is heated, it absorbs energy which can
cause some electrons to be “excited” or bumped
up to the next energy level.
• These excited electrons are unstable, so emit or
lose the energy and drop back to their original
energy level.
• The emitted energy is seen as different colors of
light.
• The process continues over & over as long as
energy is put into the gas.
• Each element has a unique emission spectra
that consists of certain frequencies or colors.
• Elements can be identified by their spectra.
Try It 4
• An FM radio station broadcasts at
a frequency of 98.5 MHz. What is
the wavelength of the station’s
broadcast signal?
98.5 MHz x 106 Hz / 1 MHz = 98.5 x 106 Hz
= (3.00 x 108 m/s) / 98.5 x 106 Hz
=3.05 meters
Try It 5
Calculate the energy of a photon of
ultraviolet light that has a
wavelength of 49.0 nm.
E = (6.626 x 10-34 J.s) (3.00 x 108 m/s) / (49.0 x10-9 m)
E = 4.06 X 10-18 J