2Early Models of the Atom Cut a piece of Al foil in ½ again and again…….450BC Democritus , AtomosThe ATOM: Smallest piece of an element that retains identity of that element.But what holds them together?
3Lavoisier – 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.
4Dalton’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 itRecent advances (the scanning-tunneling microscope) have almost allowed us to “see” atoms. Daltons postulates have been proven as fact.
6John 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
7So, a - and a - do what?REPELSo, a + and a - do what?ATTRACTBut what about a – and a + ??????
8Cathode Ray Tube – gave us an understanding of electrons
9Cathode Ray Tubes (contd.) The tubes would glow when the end screen was coated with some substancesThe ‘cathode rays’ could spin a paddle wheel so they must have had mass (particles)A magnet deflects the particles just like negatively charged objects
10Thompson 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 ofElectron ChargeElectron MassMillikan 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
11RADIOACTIVITY: (led to understanding of nucleus) Bequerel had some mysteriously exposed photographic plates. Uranium- radioactiveCuries isolated Radium and PoloniumRutherford – radioactive substances show changes in their properties with time.His experiment showed + alpha particles,- beta particles and uncharged Gamma radiation
12Thompson showed that most atoms are neither net (+) or (–) But (–) electrons must imply a positive part of the atom , the nucleusThe further gold foil experiment proved the true nature of atoms.
13Rutherford gold foil experiment A very small, dense core (nucleus) containing + charged “protons” and uncharged neutronsA lot of space in which electrons traveled – the electron cloud, surrounds the nucleus.