2. Atomos small hard particles.
Made of a single material that’s formed into different shapes and sizes.
always moving
form different materials by joining together.

3. Democritus’ theory was ignored and forgotten for more than 2000 years!

4. Why?
The eminent philosophers of the time, Aristotle and Plato, had a more respected, (and ultimately wrong) theory.
Aristotle and Plato favored the earth, fire, air and water approach to the nature of matter. Their ideas held sway because of their eminence as philosophers. The atomos idea was buried for approximately 2000 years.

6. 1808: John Dalton
Dalton’s atomic model is called the “Billiard Ball Model”
All substances made of atoms that cannot be created, divided, or destroyed.
Atoms of the same element are exactly alike and atoms of different elements are different.
Atoms join with other atoms to form different substances

7. Thomson’s Plum Pudding Model
1897
provided the first hint that an atom is made of even smaller particles.

8. CATHODE RAY EXPERIMENT: Thomson concluded that a cathode ray consists of a beam of negatively charged particles (electrons) and that electrons are constituents of all matter.

9. Thomson’s Atomic Model
J. J. Thomson
proposed the “Plum Pudding” model.
Atoms made from a positively charged substance with negatively charged electrons scattered about, like raisins in a pudding.

10. CONCLUSIONS:
the negative charges came from within the atom.
A particle smaller than an atom had to exist.
The atom was divisible!
called the negatively charged “corpuscles,” today known as electrons.
Since the gas was known to be neutral (no charge), he reasoned that there must be positively charged particles in the atom.

11. ~
Dmitri Mendeleyev

12. Subatomic Particles Particle Charge Mass (g) Location Electron (e-) -1
9.11 x 10-28
Electron cloud
Proton (p+) +1
1.67 x 10-24
Nucleus
Neutron
(no)

13. Ernest Rutherford’s Gold Foil Experiment - 1911
Alpha particles are helium nuclei - The alpha particles were fired at a thin sheet of gold foil
Particles that hit on the detecting screen (film) are recorded

14. Rutherford’s Findings
Observations/Evidence:
Most of the particles passed right through
A few particles were deflected
VERY FEW were greatly deflected
“Like howitzer shells bouncing off of tissue paper!”
Inference/Conclusions:
The nucleus is small
The nucleus is dense
The nucleus is positively charged

16. The Bohr Model of the Atom (1913) What did Bohr learn about electron movement?
Bohr proposed that electrons move in paths at certain distances around the nucleus.
Electrons can jump from a path on one level to a path on another level.
Click here

18. How can bookshelves help you understand the movement of electrons?
Each shelf represents an energy level
Each book represents an electron
You can move a book to a higher or lower shelf with the correct amount of energy.
A book cannot be between shelves
(An electron can move by gaining or losing energy but can never be between energy levels)

19. James Chadwick
1932
discovered neutron in the nucleus.

20. You can compare the head of a pin to the diameter of a stadium to think about atom size….
The diameter of a pinhead is 100,000 times smaller than the diameter of a stadium.
Likewise the diameter of the nucleus of an atom is 100,000 times smaller than the diameter of an atom

21. The Modern Theory of the Atom
Electrons travel in regions called “electron clouds”
You cannot predict exactly where an electron will be found

22. How small are atoms?
THERE ARE 2 X 1022 ATOMS IN A PENNY. If all the atoms in a penny were blown up to the size of a grain of sand they would cover the entire state of California

23. What can a scanning tunneling electron microscope show us?
These images do not show an actual picture of an atom. They show a color-enhanced image of the surface of a material at the atomic level.

24. What is the structure of an atom?
Bohr Model
“Planetary Model”
Schrödinger Model
“Electron Cloud Model”

25. Quick atomic model review and the scientists credited with the model
A review: