Name ___________________
Period _____
Model 1
Dalton’s Experiment
0.6 L of hydrogen gas
+
1.2 L of hydrogen gas
+
0.6 L of hydrogen gas
+
1.2 L hydrogen gas +
0.6 L chlorine gas
0.6 L chlorine gas
1.2 L chlorine gas
yields
yields
1.2 L chlorine gas
yields
1.2 L muriatic acid gas
1.2 L muriatic acid gas
1.2 L muriatic acid gas
yields
+
+
0.6 L hydrogen gas
0.6 L chlorine gas
2.4 L Muriatic Acid gas
Dalton’s 5 Part Atomic Theory (1803)

All elements are made of atoms.

Atoms of an element are identical.

Atoms of different elements are different.

Atoms combine in simple whole number ratios to make compounds.

Atoms may rearrange in chemical reactions, but are not changed in the reaction.
Dalton’s Model of the Atom: Dalton described the atom (his model) as
a small hard sphere. All elements were made of many of these spheres.
Since atoms of each element are different from each other, the sphere
for each element would be slightly different.
Critical Thinking Questions
1.
Evaluate the information provided for Dalton’s experiment. Which one of the five
parts of his theory is best supported by the experimental data? Use specific data
from the model to explain how the data supports his theory.
2. What part of Dalton’s model of the Atom do we still agree with today? What part
of the model has been discarded?
Model 2
Thomson’s Experiments (1897)
Part 1: Thomson used a hollow glass tube
with a metal plate on either end. Thomson
removed the air from the tube, and
connected the tube to an electricity source.
He found that “rays” travelled across the
tube from the cathode to the anode.
Part 2: Thomson exposed the ray to a
magnet and found that the ray bent toward
the positive end of a magnet, and away from
the negative end of a magnet.
Part 3: Thomson used an electric field to measure the mass to charge ratio of the ray.
He found this ratio to be almost 2000 times larger than the mass to charge ratio for a
charged hydrogen atom. So... either the ray particles had a HUGE charge, or a very small
mass.
Critical Thinking Questions
3. What technological advancement was required for Thomson to conduct his
experiments?
4. What part of the atom did Thomson discover? Give specific evidence from his
experiments to support your claim.
Thomson’s Model of the Aton: Thomson held on to
Dalton’s idea that atoms were solid objects. He modified
Dalton’s model to add a positively charged “base” material
that had small negatively charged particles imbedded. He
called this the Plum Pudding model.
5. What part of Thomson’s model of the Atom do we still agree with today? What
part of the model has been discarded?
Model 4
Rutherford’s Experiment (1911)
Rutherford set out to test Thomson’s model of the atom using alpha particles. Alpha
particles are essentially a Helium nucleus that is emitted from a larger atom during
radioactive decay. These small particles are able to pass through paper, and thin layers of
other materials.
The experiment involved using a sheet of gold that was only 2-3 atoms thick. The alpha
particles were released from a lead box through a slit. 99.999% of the particles passed
directly through the gold foil as if it wasn’t there. A very small number of particles were
deflected, some a little, some were deflected almost straight back.
Critical Thinking Questions
6. What technological advancement was required for Rutherford to conduct his
experiment?
7. What part of the atom did Rutherford discover? Give specific evidence from his
experiment to support your claim.
Rutherford’s Model of the Atom: Rutherford held on
to Thomson’s idea that atoms were made up of positive
and negative parts. He modified Thomson’s model to
specify that the positive part of the atom was contained
in a small, dense nucleus, and the electrons orbited the
nucleus like planets orbit the sun.
8. What part of Rutherford’s model do we still agree with today? What part of the
model has been discarded?
Model 5
Bohr’s Experiment (1922)
Bohr observed that when a sample of hydrogen gas was exposed to electric current it
released energy as light. When this light was viewed with a spectroscope, specific
wavelengths of light were seen. Different wavelengths of light correspond to different
levels of energy. Bohr used this same method to test other elements and found that each
emitted its own distinct color of light, each made up of its own wavelength combinations.
high energy
low energy
Bohr realized that this phenomenon was not possible with Rutherford’s model of the atom,
since electrons travelling in an elliptical orbit would produce a continuous rainbow of colors
instead of the distinct lines seen.
Bohr’s Model of the Atom: Bohr held on to
Rutherford’s idea that atoms were made up of a positive
nucleus with electrons orbiting around the nucleus. He
modified Rutherford’s model to specify that the
electrons travel in an orbit that is a set distance from
the nucleus. This results in each electron orbit
containing a specific amount of energy.
Critical Thinking Questions
9. What property of atom behavior did Bohr discover? Give specific evidence from
his experiment to support your claim.
10. What part of Bohr’s model do we still agree with today? What part of the model
has been discarded?
11. Our understanding of the atom has changed from Dalton to today. When changes
were made to the model of the atom were all previous models thrown out, or were
they modified? Why do you think this is the case?
Model 6: The Quantum Mechanical Model of the Atom (Our
current model!)
Schrodinger and Heisenberg
Erwin Schrodinger did some fancy math and figured out what he called quantum
numbers.
 The principal quantum number (n) describes the energy of an electron. In
other words, it’s the same thing as Bohr’s energy levels. Thus, we’ll
interchangeably refer to principal quantum numbers and energy levels (so
energy level 2 = principal quantum number 2 or n+2).

We’re not going to worry too much about the other quantum numbers,
instead we’ll refer to orbitals.
Werner Heisenberg did some experiments and came up with the Heisenberg
Uncertainty Principle that basically says that once you locate a particle, it’s gone.
 So, because of the uncertainty principle we only know areas where we can
probably find electrons: We call these areas of probability orbitals.
 Orbitals kind of look like this:
12. If Bohr told you an electron were found on the fifth energy level, how would you
refer to the electron using the Quantum Mechanical Model?
13. How did Heisenberg change Rutherford’s idea of the atom?
14. Our understanding of the atom has changed from Dalton to today. When changes
were made to the model of the atom were all previous models thrown out, or were
they modified? Why do you think this is the case?