Discovering Atomic Structure Chapter 3-1
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?
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.
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 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 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
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 experiment • http://www.mhhe.com/physsci/chemistry/essentialchemistry/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.