JOURNAL #37

What is the Bohr model? Refer to your textbook
as needed.
INTRODUCTION
TO ATOMIC
STRUCTURE
TODAY’S LEARNING GOAL:

We will describe the Bohr model.
LIGHT AND SOUND
 In
1905 Einstein derived an equation
relating mass and energy. You should be
familiar with this equation:
 E = mc2
 This equation has been changed a bit
since, but a relationship has now, for the
first time in history, been established
between matter and energy and between
physics and chemistry
LIGHT AND SOUND
 Because
Einstein was able to prove a
relationship between matter and energy,
we today can understand more about
matter by learning all about energy.
 We
can see this relationship between
energy and matter specifically when we
look at some of the unusual properties of
the wave nature of energy
THE NATURE OF LIGHT:
WAVE OR PARTICLE?
 The
nature of light has been debated for
thousands of years.
 In
the 1600’s Newton argued that light
was a stream of particles. Huygens
countered that it was a wave. Both had
good arguments, but neither could prove it
YOUNG’S DOUBLE SLIT
EXPERIMENT
 In
1801, Thomas Young settled the
argument with his Double Slit
Experiment.
 We will take a closer look at the results of
this experiment, but first we need to
understand waves…
http://video.mit.edu/watch/thomas-youngs-double-slit-experiment-8432/
YOUNG’S DOUBLE SLIT
EXPERIMENT
 Young
tested to see if light was a wave by
seeing if it created an interference pattern
when it went through the 2 slits, like a wave
would.
 The double slit experiment relies on 2
properties of waves:


Diffraction
Interference
 Each
slit generates a new wave due to
diffraction. Those waves then either
constructively or destructively interfere on a
far away screen.
LET’S REVIEW WHAT WE’VE LEARNED:
What
principle is responsible for
light spreading as it passes
through a narrow slit?
A. Diffraction
B. Polarization
C. Dispersion
D. interference
Answer: A
DOUBLE SLIT MAXIMA
AND MINIMA
 Interference
occurs because each point on
the screen is not the same distance from
both slits. Depending on the path length
distance, the wave can interfere
constructively or destructively
 Bright lines- Maxima
 Dark lines -Minima
LET’S REVIEW WHAT WE’VE LEARNED:
What
principle is responsible for
alternating light and dark bands
when light passes through 2 or
more narrow slits?
A. Diffraction
B. Polarization
C. Dispersion
D. interference
Answer: D
IF LIGHT IS A
WAVE…WHAT IS WAVING?
 In
sound waves, we know it’s the pressure
in the air.
 In any simple harmonic motion there has
to be 2 forms of energy and a means to
move between them.
 But what does that mean for light?
ACCELERATING CHARGES
CREATE E-M WAVES
A
great way to start this up is to make a
charge (like an electron) accelerate.
 That creates a changing electric field
which creates a changing magnetic field
 Which creates a changing electric field…
which creates a changing magnetic field
 Which creates a changing electric field…
which creates a changing magnetic field
 Which creates a changing electric field…
which creates a changing magnetic field
CREATING ELECTROMAGNETIC
WAVES
 In
physics we learned that changing
magnetic field produces an electric field.
 Changing an electric field produces a
magnetic field as well
 Once these changing fields are first
started up, they keep creating each
other…and travel on their own.
 These traveling fields are called
electromagnetic waves.
LET’S REVIEW WHAT WE’VE LEARNED:
An
electric field is produced by a
A. Constant magnetic field
B. Changing magnetic field
C. Either a constant or a
changing magnetic field
D. Gravitation
Answer: B
LET’S REVIEW WHAT WE’VE LEARNED:
A
changing electric field will
produce a
A. Current
B. Gravitation field
C. Magnetic field
Answer: C
LIGHT IS AN
ELECTROMAGNETIC WAVE
 Young
showed that light is a wave.
 Electromagnetic waves exist and travel at the
speed of light
 Light was shown to be an electromagnetic wave
 The frequency of an electromagnetic wave is
related to its wavelength. For electromagnetic
waves, in a vacuum
C=λV
C = speed of light, λ = wavelength (m) V = frequency

ELECTROMAGNETIC SPECTRUM
All electromagnetic radiation travels at the same
velocity: the speed of light ©
 C= 3.00 x 108 m/s

LET’S REVIEW WHAT WE’VE LEARNED:
 All
electromagnetic waves travel
through a vacuum at
A. Same speed
B. Speeds that are proportional to
their frequency
C. Speeds that are inversely
proportional to their frequency
D. Speeds too slow to measure
Answer: A
WHY DOES THIS ALL MATTER?
 Light
behaves like a wave and so does matter!
 Electrons fired on at a time towards two slits
show the same interference pattern when
they land on a distant screen.
 Since all matter and energy are now
understood they share certain properties
(wavelength for example) the interaction of
matter with light has allowed us to probe the
nature of matter itself, from the structure of
the atom to the unique behavior of molecules.
The structure and behavior of matter is the
domain of the chemist!!
QUANTUM
A
quantum of energy is the minimum
quantity of energy that can be lost or gained
by an atom
 The relationship between a quantum of
energy and the frequency of radiation is
E=hv
 E= energy (joules), V= frequency and h= is a
funamental physical constant (planck’s constant)

6.626 x 10-34J.s
THE HYDROGEN ATOM LINE
EMISSION SPECTRUM
 When
current is passed through a gas at low
pressure, the potential energy of some of the
gas atoms increases.
 The lowest energy state of an atom is its
ground state.
 A state in which an atom has a higher
potential energy than it has in its ground stat
is an excited state.
 When an excited atom returns to its ground
state, it gives off the energy it gained in the
form of electromagnetic radiation.

(ex: neon lights)
THE HYDROGEN ATOM LINE
EMISSION SPECTRUM
 When
a narrow beam of emitted light was
shined through a prism, it was separated into
four specific colors of the visible spectrum.
 The four bands of light were part of what is
known as Hydrogen’s line-emission spectrum.
 Attempts to explain why hydrogen atoms
gave off only specific frequencies of light is
called quantum theory.
THE HYDROGEN ATOM LINE
EMISSION SPECTRUM
 When
an excited Hydrogen atom falls to its
ground state or to a lower energy excited
state, it emits a photon of radiation.
 The energy of this photon (E=hv) is equal to
the difference in energy between the atoms
initial state and its final state.
 The fact that hydrogen atoms emit only
specific frequencies of light indicated that the
energy differences between the atom’s energy
states were fixed. (hydrogen atoms exists only
in very specific energy states)
 The
BOHR MODEL
puzzle of the hydrogen atom spectrum was
solved by Niels Bohr.
 The electron can circle the nucleus only in
allowed paths, or orbits.
 When the electron is in one of these orbits, the
atom has a definite, fixed energy.
 The electron is in its lowest energy state when
it is in the orbit closest to the nucleus.
 This orbit is separated from the nucleus by a
large empty space where the electron cannot
exist.
 The energy is higher when the electron is in
orbits that are farther from the nucleus.
BOHR MODEL
 Example:
When standing on a ladder, the higher up you go,
the more potential energy you have.
 Your energy cannot correspond to standing
between 2 steps because you cannot stand in
midair.
 The same way for electrons. They can be in one
orbit or another, but not in between.

BOHR MODEL
 How
does this explain the observed spectral
lines?
 While in a given orbit, the electron is neither
gaining nor losing energy
 However, it can move to a higher energy orbit
by gaining an amount of energy equal to the
difference in energy between the higher energy
orbit and initial lower energy orbit.
BOHR MODEL
 When
a H atom is in excited state, its electron
is in one of the higher energy level orbits.
 When the electron falls to a lower energy level,
a photon is emitted – called emission.
 Absorption is the process in which energy
must be added to an atom in order to move an
electron from a lower energy level to a higher
energy.
YOUR ASSIGNMENT
 Chapter
4 section 1 review (pg 103)
 Do problems 1-5.