(1) It took over a century of work by many scientists to discover the structure of the atom due to its incredibly small size. (2) Early models including Dalton's theory of atoms and Thomson's "plum pudding" model were later shown to be inaccurate. (3) Rutherford's gold foil experiment demonstrated that atoms have a small, dense nucleus, with most of their volume being empty space. (4) Later discoveries of subatomic particles like protons, neutrons, and electrons led to Bohr and quantum mechanical models that better explained atoms' properties.

1. LESSON 1: INVISIBLE ATOMS
CP Chemistry
UNIT 1: STRUCTURE & PROPERTIES OF MATTER
PowerPoint adapted from Los Banos High School’s History of an Atom: http://www.losbanosusd.k12.ca.us/lbhs/Class/462-Environmental-Chemistry/3130-PowerPoint-Lectures.html

2. REMINDERS FOR GOOD NOTE-TAKING:
• Only write the important stuff. Focus on the objectives and ask, does this information lead
to the goal? If so, write it down!
• Use key words and key definitions.
• Be accurate.
• Use abbreviations but make sure you know what they stand for!
• Review your notes after the lesson. Make sure everything makes sense. If not, ask.

3. WHAT DO WE KNOW ABOUT ATOMS?
• Brainstorm with a partner and make a
list of the things we know.
• Share your ideas with the class!

4. FINDING A MODEL FOR THE ATOM
• It took us over a century and the collaboration of many scientists to discover the structure
of an atom
• Think-Pair-Share: Why do you think that it took us so long?
• The atom is TINY! How tiny? Let’s check out the scale of our universe:
• Scale of the Universe

5. DALTON’S THEORY OF ATOMS (1808)
John Dalton used observations and measurements of
chemical reactions and substances to produce the atomic
theory:
• All matter consists of tiny, spherical particles called
“atoms”
• Atoms of one element can neither be subdivided nor
changed into atoms of any other element.
• Atoms can neither be created nor destroyed.
• All atoms of the same element are identical in mass,
size, and other properties.
• Atoms of one element differ in mass and other
properties from atoms of other elements.
• In compounds, atoms of different elements combined
in simple, whole number ratios.

6. J.J. THOMSON’S CATHODE RAY TUBE (1897)
Think-Pair-Share:
1. What can you infer about these observations?
2. What are some possible conclusions?

7. THE ELECTRON
• Cathode rays are a stream of negatively charged
particles
• Thomson called these particles “electrons”
• He extended this experiment to prove all atoms
can be made to emit these electrons
• Thomson’s findings were:
• All atoms must have electrons
• Because atoms don’t have a negative
charge, there must be something that
balances the charge of the electron
• Electrons are very light: 9.109 x 10-31 kg
• Additional findings:
• Electrons are responsible for a substance’s
reactivity
“Plum and Pudding Model”

8. RUTHERFORD’S GOLD-FOIL EXPERIMENT (1909)
• Think-Pair-Share: If Thompson’s plum and pudding model is an accurate model for the
atom, describe what you would expect to see if you were to pass a beam of positively
charged particles (alpha particles) through a thin layer of atoms.

9. GOLD-FOIL EXPERIMENT: RESULTS
Think: What do you see happened to the alpha particles when it hit the gold foil?

10. GOLD-FOIL EXPERIMENT: RESULTS
1. Most of the alpha particles passed through the gold foil at different angles.
2. A very small amount of alpha particles were heavily deflected backwards
Think-Pair-Share:
1. Do these observations support Thomson’s model? Why or why not?
2. If most of the particles passed through the gold foil, what can that tell you about the
atom?
3. If alpha particles (positively charged particles) were heavily deflected, what do you think it
collided with?

11. GOLD-FOIL EXPERIMENT: CONCLUSIONS
1. Atom must have a very small, dense, positively charged center to cause only a few alpha
particles to deflect so drastically.
• Rutherford called this the nucleus
2. Most of the atom is empty space, surrounding the nucleus, allowing most of the alpha particles
to pass through
• This was called the electron cloud.
Nuclear atom
Nucleus
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Alpha particles
Plum-pudding atom

12. THE NUCLEUS
Think: How does the nucleus stick together if it is made up of only protons?
The work of Rutherford led to the discovery of:
• The neutron
• A particle with no charge (0)
• Heavy mass compared to electron: 1.673 x 10-27 kg
• The proton
• A positively charged particle (+1)
• Heavy mass compared to electron: 1.673 x 10-27 kg
• Determines the identity of an atom
• For example, Carbon will always have 6 protons, though the number of neutrons
and electrons may vary.

13. NIELS BOHR’S MODEL (1913)
Key Findings
• Electrons can occupy only certain regions of space, called orbits
• Orbits closer to the nucleus are on lower energy levels and are more stable
• Electrons can move from one orbit to another by absorbing or emitting energy
• Produces spectra lines
Limitations
• Model could not explain spectra of atoms heavier than hydrogen
• Not entirely accurate…

14. NIELS BOHR’S MODEL OF THE ATOM

15. NIELS BOHR’S MODEL OF THE ATOM

16. QUANTUM MECHANICAL MODEL (1926)
• Discovered through the work of Erwin Schrodinger
• Describes electronic structure of the atom as the probability of finding electrons within
certain regions of space (orbitals)
Think-Pair-Share:
1. How is this different from all of the other models we’ve seen?
2. What do you think the “probability of finding electrons” implies?

17. QUANTUM MECHANICAL MODEL
• Atom is mostly empty space (Rutherford’s
experiment)
• Two regions of the atom:
• Nucleus: contains protons and neutrons
• Electron cloud: region where you might find an
electron
• Orbital is the region in space where there is
a 90% probability of finding an electron

18. WHY IS THIS SO IMPORTANT?
• Because we’ve discovered the structure of an atom, we now understand:
• The part of the atom gives it its identity
• The part of the atom that causes it to react a particular way
• How the subatomic particles fit together
• Chemistry has been built off of these findings
• Think-Pair-Share: Think of something you do as a daily routine. Do you notice any
chemistry involved?
• Microwaves, DVD players, and cars are an application of the understanding of how
electrons move within an atom.
• The toothpaste you use is based on the understanding of how sodium and fluoride’s
electrons bond
• The solution to our energy crisis is nuclear reactors. Nuclear reactors are an
application of this work on the structure of atoms.

19. SUMMARY: WHAT’S INSIDE AN ATOM?
Proton Neutron Electron
Location
Mass
Charge
Importance
In the nucleus In the nucleus Outside the
nucleus
Heavy Heavy Very light
+1 0 -1
Determines the
atom’s identity
Keeps the
nucleus
together
Responsible for
reactivity

20. SUMMARY OF ATOMIC MODELS
1800 1805 .... 1895 1900 1905 1910 1915 1920 1925 1930 1935 1940 1945
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1803 John Dalton
pictures atoms as
tiny, indestructible
particles, with no
internal structure.
1897 J.J. Thomson
discovers the electron.
He pictures electrons
embedded in a sphere
of positive electric
charge.
1911 Ernest Rutherford
states that an atom
has a dense, positively
charged nucleus.
Electrons move
randomly in the space
around the nucleus.
1913 In Niels Bohr's
model, the electrons
move in spherical
orbits at fixed
distances from the
nucleus.
1926 Schrödinger
develops mathematical
equations to describe
the motion of electrons
in atoms. His work
leads to the electron
cloud model.

21. JUST HOW SMALL IS AN ATOM?
Video found: https://www.youtube.com/watch?v=yQP4UJhNn0I