1. THE ATOM

2. History Lesson (400 B.C.) Democritus Greek philosopher
Belief that all matter is made up of tiny indivisible particles
NO EVIDENCE
Called this “atom”
Greek word – atomos = indivisible

3. Aristotle (350 B.C.) Believed all matter is made up of four elements
Fire
Water
Air
Earth

4. 1800’s
John Dalton
Developer of atomic theory

5. Dalton’s Atomic Theory
Elements are composed of tiny, indivisible particles called atoms
All atoms of an element are identical and have the same properties (proven wrong because elements have atoms with different masses aka isotopes
Atoms are indestructible (Law of conservation of mass)
Compounds contain atoms in small definite whole-number ratios (Law of definite proportions)
Atoms may combine in more than one ratio to form different compounds (Law of multiple proportions)

6. Law of Conservation of Mass
Mass is neither created not destroyed by any means including physical/chemical reactions
Can not make matter out of nothing
Can not make matter disappear/turn to nothing

8. Law of Definite Proportions
Every chemical compound contains fixed and constant proportions
Ex: Water will always be made up of 2 Hydrogen atoms to 1 Oxygen atom. Will always have a 2:1 proportion
Ex: NaCl: 39.34% by mass of Na and 60.66% by mass of Cl

9. Law of Multiple Proportions
Two or more different compounds can be composed of the same elements, but in different ratios
Ex: CO- Carbon Monoxide (1C:1O)
CO2 – Carbon Dioxide (1C:2O)
Different compounds = different properties

11. JJ Thomson Cathode Ray experiment
Proved the existence of negatively charged particles (electrons)
Cathode (negatively charged plate) and Anode (postively charged plate) are placed inside a vacuum sealed tube.
Electricity (contains electrons!) runs through the cathode
Electrons get repelled by the cathode/attracted to anode
Small hole in anode allows electrons to pass, creating a beam of electrons

12. Cathode Ray

13. Cathode Ray Experiment

14. Plum Pudding Model
Hypothesized that atoms were mostly empty space and electrons

15. Millikan (1909) Oil drop experiment
Observing tiny droplets of oil between two electrode plates
Oil drop would fall first without any voltage
Electric field strength would increase until oil drop was suspended in air
Able to determine charge of an electron

16. Oil Drop Experiment

17. Oil Drop Experiment

18. Rutherford (1910) Gold Foil Experiment
Alpha particles (positively charged) shot through a sheet of gold
Most went through, but some deflected
A few bounced back
This concluded that atoms have a dense positive charged nucleus that is surrounded by electrons

19. Gold Foil Experiment

20. Gold Foil Experiment

21. Bohr (1913) Provided the planetary model of the atom
Electrons circle the nucleus similar to planets around the sun
Explains spectral lines for quantized energy and colors/light

22. Bohr Model

23. Chadwick (1935)
Discrepancy between atomic number (# of protons) and atomic mass
Beryllium was bombarded with alpha particles
Radiation emitted, thought to be gamma rays
Radiation behaved like gamma rays, but different
Particle was capable of colliding and knocking away protons
Discovery of neutrons, n0
Isotopes
Atoms with the same # of protons (same element) but different number of neutrons