1. Atoms

2. 400 b.c. Greeks
Greeks philosophers ponder the nature of matter: what is it made of?
Democritus: basic particle of matter = “atom” which means “indivisble”.
Envisions these to be “hard spheres”
Aristotle: does not believe in atoms

3. Dalton’s Atomic Theory 1803
1) All matter is composed of extremely small particles called atoms

4. Dalton’s Atomic Theory 1803
Atoms of one element are all alike
(size, mass)

5. Dalton’s Atomic Theory 1803
3) Atoms can’t be created, destroyed or subdivided

6. Dalton’s Atomic Theory 1803
Atoms of different elements combine in
simple whole number ratios to form
compounds

7. Dalton’s Atomic Theory 1803
5) In a chemical reaction, atoms are combined, separated, or rearranged

8. Conservation of mass in chemical reactions

9. Conservation of mass in chemical reactions

10. 1897 J.J. Thompson Cathode Ray Experiments
The electron is discovered….
“PLUM PUDDING” model

11. Rutherford’s Gold Foil Experiment 1909
Ernst Rutherford bombards a piece of gold foil with positively charged alpha particles (the nucleus of a helium atom)

12. Some alpha particles
bounce back

13. Rutherford’s Gold Foil Experiment
Observations:
Most of the alpha particles passed straight through the gold foil as if it wasn’t even there!
The nucleus of the atom is small, dense, and positively charged

14. Rutherford’s Gold Foil Experiment
Interpretation (conclusions):
Atoms are mostly empty space and therefore MATTER is mostly empty space
Atoms have a small, dense, positively charged nucleus

15. + + +
This alpha particle
passes through the atom’s
empty space + + +
Alpha particles have + charge
two + charges will repel each other

16. Subatomic Particles (Table O)
Proton
Electron
Neutron

17. Protons Located in the atom’s nucleus Positive charge +1
Mass = 1 amu (Atomic Mass Unit)

18. Neutrons Located in the atom’s nucleus zero charge (neutral)
Mass = 1 amu (Atomic Mass Unit)

19. Electrons Located in the electron cloud outside the nucleus
Negative charge -1
Mass = 1/1836 amu (negligible)

20. What are the subatomic particles in lithium and carbon?

21. Models of the atom

22. A model of the atom to scale
If YOU are the nucleus of an atom, your closest electron would be….
……..in downtown Rhinebeck!

23. Actual image of 2 atoms