1. Atomic Models Through Time

2. Democritus
@400 B.C.
Theorized that if you were to cut something in half, then cut it in half again and again…
Eventually you would be left with something too small to be cut.
Therefore, Democritus gave us “atomos,” or “uncuttable,” to be the word for the smallest individual particle of a substance.

3. John Dalton Dalton’s Atomic Theory ~ 1808
All matter is composed of atoms.
Atoms of each element are the same size, mass, etc., but are different from atoms of other elements.
Atoms can’t be subdivided (cut) or destroyed.
Envisioned atoms as “tiny, solid balls”

4. Cathode Ray Tube If you run electricity through a gas filled tube,
you see streams of particles
How do we know they are particles?
A cross left a “shadow”, so something must be traveling in a straight line.
How do we know it isn’t light?
J.J. Thomson put a magnet next to the tube and it bent the beam.
Light is not magnetic!
Beam had a negative charge

5. J.J. Thomson Thomson’s Atomic Theory ~ 1897 “Muffin” Model
Discovered negatively charged electrons.
Envisioned atoms as a “positively charged sphere with electrons embedded in it.”
“Muffin” Model
Positively charged bread
Negatively charged “blueberries” floating around (electrons)

6. Hantaro Nagaoka Nagaoka’s Atomic Theory~ 1904 “Planetary” Model
Proposed a nucleus
Envisioned electrons orbiting a positively charged sphere in the center.
“Planetary” Model
Electrons = planets
Positive Center Sphere = Sun

7. Alpha Particle Cannon
Ernest Rutherford shot fast moving positively charged alpha particles at thin gold foil…
If Thomson’s model was correct, the alpha particles would pass right through the foil.
Instead, some deflected back at large angles!

8. Alpha Particle Cannon Conclusions: Thomson’s model was inaccurate
A new model must be created
1. The atom is mostly empty space
2. There is a dense, positively charged nucleus in the center

9. Ernest Rutherford Rutherford’s Atomic Theory ~ 1911 “Peach” Model
The atom is composed of mostly empty space.
Small, dense, positively charged nucleus containing almost all of the atom’s mass.
“Peach” Model
Peach Pit = Nucleus
Meat = Empty space with electrons randomly orbiting nucleus.

10. Refining Atomic Theory in the Early 1900’s
Masses of protons are measured.
Protons are 2000 times more massive than electrons.
Charge of protons is determined to be positive.
Protons provide equal & opposite charge to electrons.

11. Niels Bohr Bohr’s Atomic Theory ~ 1913 Bohr’s Model
Electrons do not “randomly” orbit the nucleus.
Electrons move in specific “layers” or “energy levels” / “shells” (3-dimensional).
Atoms absorb or give off energy when electrons move from one “level” to another.
Bohr’s Model
“onion” model
Electrons move on layers of an onion.

12. Valence Electrons
Valence electrons are the electrons located furthest from the nucleus
They are a determining factor in chemical bonding.
If you visualize the atom like Bohr (an onion), the valence electrons are the electrons located on the outer layer.

13. New Problem
As scientists find the masses of atoms, they always have about twice the mass predicted by the number of protons
Remember, the mass of electrons is so small it is insignificant
So, there must be something else in the atom…

14. James Chadwick Contribution to Modern Model ~ 1932
Discovered the neutron
Same mass as proton
No electrical charge
This is why it was not “discovered” earlier
Existence of neutron explained why atoms were heavier than the total mass of the protons and electrons.

15. Modern Model Electron Cloud Model ~ 1920’s to present
Electrons form a negatively charged cloud around the nucleus.
It is impossible to determine exactly where an electron is at any given time. (probability instead)
Higher probability of electrons near nucleus.