Discovering Atomic Discovering Atomic StructureStructure

Chapter 3-1Chapter 3-1

Early Models of the AtomEarly 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?

• 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.

• 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.

Scanning tunneling microscopeScanning tunneling microscope

• 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

• So, a - and a - do what?

• REPEL

• So, a + and a - do what?

• ATTRACT

• But what about a – and a + ??????

• ATTRACT

Cathode Ray Tube – gave us Cathode Ray Tube – gave us an understanding of electronsan understanding of electrons

Cathode Ray Tubes (contd.)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

• Thompson was able to use magnets to deflect the cathode rays. He called the particles electrons.

• He could calculate the ratio of Electron ChargeElectron 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

• 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

• 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.

Rutherford gold foil experimentRutherford gold foil experiment• http://www.mhhe.com/physsci/chemistry/essent

ialchemistry/flash/ruther14.swf

• 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.