Development of Atomic Theory
Atomic Theory Poem/Song
Write a song or poem:
• With a stanza for each theorist – Dalton,
Thomson, Rutherford & Bohr
• Mentioning their contribution to our
understanding of the atom.
• Mentioning key evidence to support each of
their findings.
• Include a noticeable rhythm, rhyme scheme or
beat.
• Be prepared to read or sing in front of class.
John Dalton
• British
• A small town
school teacher at
the age of 12.
• Introduced his
atomic theory in
1803.
Previous Findings
1. Law of Conservation of Mass
Matter is neither created or destroyed in a chemical
reaction. (Antoine Lavoisier)
2. Law of Definite Proportions
The percentage by mass of elements in a compound
is constant for any sample. Ex: H2O
3. Law of Multiple Proportions
Compounds composed of the same two elements
differ in one element by simple ratios.
Ex: CO vs CO2; H2O vs H2O2
Daltons Atomic Theory
1. All matter is composed of tiny indivisible particles
called atoms
2. All atoms of the same element are identical
3. Different elements have different types of atoms
4. Compounds are formed from simple combinations
of atoms of different elements.
5. In a chemical reaction atoms are simply
rearranged.
*Activity: Ball & Stick Reactions
Picture Dalton’s Atomic Theory
Updates to Dalton’s Theory
1a. Atoms are divisible into protons, neutrons &
electrons (& even smaller!).
1b. In nuclear decay they actually fall apart!
2. All atoms of a single element have the same
number of protons, but not neutrons.
(isotopes)
4. Compounds may be very complex!
Cathode Rays
• Tape Lab – Static electricity
attractions & repulsions.
Where do the charges
originate?
• An evacuated glass tube
when placed in an electric
field
• Crooke’s observed a glowing
inside.
• Thomson repeated Crooke’s
experiment and did
additional experiments.
(-)
(+)
Thomson’s Experiment #1
• Setup: A cross was
placed in the path of the Cathode (-)
glowing beam. (D?)
• Observation: A shadow
appeared on the anode Anode (+)
(+) side. (D?)
• Interpretation: The rays
come from the cathode
(-) side.
Thomson’s Experiment #2
• Setup: The cathode ray
tube was placed in an
electric field: (-)
electrode on top, (+)
electrode on bottom.
(DPath?)
• Observation: The
cathode rays were
attracted towards the (+)
electrode. (D?)
• Interpretation: Cathode
rays must be negative (-).
Thomson’s Experiment #3
• Setup: Cathode rays
were placed in a
magnetic field.
• Observation: Cathode
rays are bent
perpendicular to the
magnetic field.
• Interpretation: Cathode
rays are not a form of
light.
Thomson’s Experiment #4
• Setup: A glass wheel was placed on a level
track inside the cathode ray tube.
• Observation: Cathode rays can rotate the
glass wheel.
• Interpretation: Cathode rays are particles
with mass.
Thomson Experiment #5
• Setup: Thomson made cathode ray tubes with
a variety of different gases & metal electrodes
in the tube.
• Observation: Every tube produced the same
cathode rays.
• Interpretation: Cathode rays are fundamental
to matter. He called cathode rays “electrons!”
Discovered in 1897.
Thomson’s Plum Pudding Model
• Thomson concluded
that all atoms must
have negative charges
and positive charges to
balance them.
• Thomson assumed that
(+) & (-) charges would
be evenly distributed.
Uses of cathode rays
• 1. A cathode ray tube (CRT) is widely used in research laboratories to
convert any signal (electrical, sound, etc) into visual signals. These are
called CRT or oscilloscopes.
• 2. CRT is the basic component in all television and computer screens. The
signals are sent to the vertical and horizontal deflecting plates, which
produce a pattern on the fluorescent screen.
• High energy cathode rays when stopped suddenly produce X-rays. The Xrays have many medical and research applications.
Millikan’s Oil Drop Experiment
• http://www.youtube.com/watch?v=XMfYHag7
Liw&feature=related
Ernest Rutherford
• New Zealander
• Discoverer of alpha,
beta & gamma
radiation.
• Discovered nucleus of
atom in 1912.
Discovery of Alpha, Beta & Gamma
Radiation
Rutherford’s Experimental Design
•
•
•
•
Uranium alpha emitter.
Slits to focus radiation
Gold foil target.
Scintillation screen of
zinc sulfide to flash
when hit.
Rutherford’s Prediction
Positive alpha particles
would go straight
through or have minor
deflections due to the
electrons embedded in
a sea of positively
charged matter.
Rutherford’s Observations (1 of 2)
1. http://www.mhhe.com/physsci/chemistry/e
ssentialchemistry/flash/ruther14.swf
2. Vary the slit width and see what happens:
3. http://micro.magnet.fsu.edu/electromag/jav
a/rutherford/
4. http://chemteacher.chemeddl.org/services/c
hemteacher/index.php?option=com_conten
t&view=article&id=74
Rutherford’s Observations (2 of 2)
Interpreting the Results
• Most positive alpha particles went straight through or were slightly
deflected.
• Therefore the atom is mostly empty space.
• A few positive alpha particles bounced back radically!
• Thus the atom must have a large concentration of positive charge!
Rutherford’s Atomic Model
Development of the Bohr Model
• In 1913 Danish physicist
Neils Bohr proposed a
new model of the atom.
• Bohr’s Model explained
the emission and
absorption patterns of
light discovered by
Bunsen in flames &
lamps.
Emission Lamps
Emission Spectra
• Each element emits a unique set of bright line wavelengths.
Emission Spectra of All the Elements
• http://chemistry.beloit.edu/bluelight/moviepa
ges/em_el.htm
• http://jersey.uoregon.edu/vlab/elements/Ele
ments.html
4 Principles of the Bohr Model
1)Electrons assume only certain orbits around the
nucleus. These orbits are stable and called
"stationary" orbits.
2)Each orbit has an energy associated with it. The
lowest energy levels are close to the nucleus. The
farther from the nucleus corresponds to higher
energy levels. Electrons tend to occupy the lowest
energy levels available.
3)Light is emitted when an electron jumps from a
higher orbit to a lower orbit. Light is absorbed when
it jumps from a lower to higher orbit.
4)The quantity of energy and wavelength of light
emitted or absorbed is given by the difference
between the two orbit energies. (Quantum Leaps!)
•
•
•
With these conditions
Bohr was able to explain
the stability of atoms as
well as the emission
spectrum of hydrogen.
Line spectra correspond
to quantum leaps
between levels of specific
energies.
Violet light corresponds
to high energy quantum
leaps while red light
corresponds to low
energy. ROYGBIV
Excited State
Ground State
Green light
emitted
Red light
emitted
Excited State
Semi-Excited State
Excited vs Ground States
• Light is absorbed when electrons jump up to higher
“excited” energy levels.
• Light is emitted when electrons jump back down to
their lowest energy “ground” state energy levels.
• Animated Absorption & Emission
• Fluorescent lights are constantly exciting gas atoms
to emit light by passing a stream of electrons through
the interior gas.
The Sun’s Spectra
• Many elements
can be identified
by their unique
lines.
• Helium was 1st
discovered in the
Sun’s (Helios)
spectrum
Emission vs Absorption