1. Discovering Atomic Structure
Chapter 3-1

2. Early Models of the Atom
Cut a piece of Al foil in ½ again and again…….
450BC Democritus , Atomos
The ATOM: Smallest piece of an element that retains identity of that element.
But what holds them together?

3. Lavoisier – Conservation of matter.
Proust – Law of constant composition (compounds always have the same proportions of the elements comprising it).
Ex. Water 88.9% O and 11.1% H.

4. Dalton’s Atomic Theory:
Extremely small particles (atoms).
Atoms of a given element are identical but different from all other elements.
Atoms never created nor destroyed.
compounds always have the same proportions of the elements comprising it
Recent advances (the scanning-tunneling microscope) have almost allowed us to “see” atoms. Daltons postulates have been proven as fact.

5. Scanning tunneling microscope

6. John Dalton had believed atoms to be hard spheres like marbles.
1839 – Faraday suggested electrical phenomena related to atomic structure.
Elektron (amber), static (stationary) electricity.
Ben Franklin. 2 types of charge, + or - .
Opposites attract, like charges repel.
So, a + and a + do what?
REPEL

7. So, a - and a - do what?
REPEL
So, a + and a - do what?
ATTRACT
But what about a – and a + ??????

8. Cathode Ray Tube – gave us an understanding of electrons

9. Cathode Ray Tubes (contd.)
The tubes would glow when the end screen was coated with some substances
The ‘cathode rays’ could spin a paddle wheel so they must have had mass (particles)
A magnet deflects the particles just like negatively charged objects

10. Thompson was able to use magnets to deflect the cathode rays
Thompson was able to use magnets to deflect the cathode rays. He called the particles electrons.
He could calculate the ratio of
Electron Charge
Electron Mass
Millikan measured the Electron charge.
Solving, the electron mass was found to be 1/1800 mass of a hydrogen atom.
At this time most believed in the “plum pudding” model of the atom

11. RADIOACTIVITY: (led to understanding of nucleus)
Bequerel had some mysteriously exposed photographic plates. Uranium- radioactive
Curies isolated Radium and Polonium
Rutherford – radioactive substances show changes in their properties with time.
His experiment showed + alpha particles,
- beta particles and uncharged Gamma radiation

12. Thompson showed that most atoms are neither net (+) or (–)
But (–) electrons must imply a positive part of the atom , the nucleus
The further gold foil experiment proved the true nature of atoms.

13. Rutherford gold foil experiment
A very small, dense core (nucleus) containing + charged “protons” and uncharged neutrons
A lot of space in which electrons traveled – the electron cloud, surrounds the nucleus.