1. Pay Attention! You have a quiz Monday!
CH 04 & 05 History of the Atom
Pay Attention! You have a quiz Monday!

2. Democritus (400 B.C.) Proposed that matter was composed of atoms
Atoms were indivisible- cannot be divided
COINED THE TERM “ATOMS” Greek: atomos , means indivisible

3. John Dalton (1803)
1. Elements are composed of small indivisible particles called atoms.
2. Atoms of the same element are identical. EX: Same atomic weight
3. Atoms of different elements combine together in simple proportions to create specific compounds( EX: 2 to H20).
4. In a chemical reaction, atoms are simply rearranged to make new compounds.
Atoms are uniform, solid spheres

4. J. J. Thomson (1898) Cathode Ray Tube Experiment-
Discovered the Electron
negative particles within the atom
Plum-pudding Model- what Thomson thought the atom looked like!
Positive sphere (pudding) with negative electrons (plums) dispersed throughout

5. Ernest Rutherford- 1911 Gold Foil Experiment
Gold Foil Experiment
Shot positive alpha (+) particles through gold foil
MANY WENT STRAIGHT THROUGH- so heavy.. Like a cannonball going through tissue paper
Some particles hit something POSITIVE and bounced back or deflected to the side
CONCLUSION
Discovered the positive charge in the atom
Atoms have centrally located, dense positive nucleus with the electrons surrounding the nucleus
Atom is mostly empty space

6. Niels Bohr (1913)
Planetary Model of the Atom
Electrons circle the nucleus like planets circled the sun.
Electrons can only exist on specific energy levels
Cannot be found in between energy levels

7. James Chadwick (1932) Discovered the neutron
Particles in the nucleus of an atom with no charge
These ‘neutrons’ are approximately the same mass as the protons

8. Erwin Schrödinger (Schrö⋅ding⋅er) - 1926)
Electron cloud model
Electrons exist in orbitals not orbits like the planetary model
Orbitals are regions around the nucleus where electrons are likely to be found