Atomic History
Part 2
Bohr Model
• Each element gave off different colors of
light when they absorbed energy from some
source and then released it.
• These colors of light are known as an
element’s line emission spectrum.
Bohr Model Cont’d
• Since only certain colors of light are emitted,
only certain types of photons are produced.
• Each type of photon has a specific amount of
energy with it.
• The excited electrons only gave off specific
amounts of energy so we know that only
certain energy changes are occurring.
• Therefore, the electrons have specific energy
levels that they can move between.
Bohr Model Cont’d
• Bohr constructed a model of the atom with
electrons moving in circular orbits or
energy levels.
• These electrons could jump to different
orbits by absorbing or releasing a photon of
light with exactly the correct energy
content.
• Bohr’s model only fit the line emission
spectra for hydrogen. So more research had
to be done.
Erwin Schrödinger
• 1926
• Thought that since light
could act as wave and a
particle that maybe an
electron could too.
• Used math to predict the
location of an electron as a
probable region in space
• His model is known as the
wave-mechanical model.
Electrons don’t travel in orbits…
• It became clear to Schrödinger that
electrons didn’t travel in circular orbits like
Bohr thought.
• In fact, we cannot pin point where an
electron is at any given time.
• We can only predict where is most likely to
be.
• A region where an electron is likely to be
found is called an orbital.
Schrödinger’s Electron Cloud
• Orbital – A region in space that describes
the PROBABLE location of an electron.
• Quantum Mechanics – study of orbitals
James Chadwick
• Worked under Rutherford for two
years.
• Accepted a scholarship to work with
Geiger in Berlin
• WWI broke out and he was
imprisoned in Germany for the
duration of the war.
• At the end of the war he returned to
work with Rutherford.
• Later in life he moved to the US to
help with the Manhattan Project.
James Chadwick
• Scientists had already noticed that the mass
of the atom didn’t match the number of
positive particles.
• Ex: Helium has a mass of 4, but an atomic
number of 2.
• When he went back to work with
Rutherford for the second time, he
discovered the neutron.
Chadwick’s Experiment
• Other scientists discovered that if a light
element like Beryllium was hit with alpha
particles it released a type of radiation.
• Many thought this radiation was just
gamma rays.
• Chadwick aimed this radiation a block of
paraffin wax.
• This caused the paraffin wax to give off
protons.
Chadwick’s Experiment
• Since the “radiation” produced by the Beryllium
was able to knock protons in the paraffin loose, it
must be made of particles similar in size to a
proton. (Marble/ Pool ball scenario)
• Since these particles were not attracted to
positively or negatively charged objects they must
be neutral.