Download
why can t we see atoms n.
Skip this Video
Loading SlideShow in 5 Seconds..
WHY CAN'T WE SEE ATOMS? PowerPoint Presentation
Download Presentation
WHY CAN'T WE SEE ATOMS?

WHY CAN'T WE SEE ATOMS?

180 Vues Download Presentation
Télécharger la présentation

WHY CAN'T WE SEE ATOMS?

- - - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript

  1. WHY CAN'T WE SEE ATOMS? • “seeing an object” • = detecting light that has been reflected off the object's surface • light = electromagnetic wave; • “visible light”= those electromagnetic waves that our eyes can detect • “wavelength” of e.m. wave (distance between two successive crests) determines “color” of light • wave hardly influenced by object if size of object is much smaller than wavelength • wavelength of visible light: between 410-7m (violet) and 7 10-7 m (red); • diameter of atoms: 10-10m • generalize meaning of seeing: • seeing is to detect effect due to the presence of an object • quantum theory  “particle waves”, with wavelength 1/(m v) • use accelerated (charged) particles as probe, can “tune” wavelength by choosing mass m and changing velocity v • this method is used in electron microscope, as well as in “scattering experiments” in nuclear and particle physics

  2. WHAT IS INSIDE AN ATOM? • GREEK MODEL OF ATOM • atom = small, unchangeable particles; come in many varieties (corresponding to different “elements”) • move and combine in various ways. • this picture sufficient to describe all observations of early chemistry and physics (up to about middle of 19th century); • late 19th century: indications that there may be more: • observation of ions (``charged atoms''); • regularity seen in periodic table of elements due to atoms made up of smaller constituents? • where does electric charge come from? • THOMSON'S MODEL OF ATOM • (“RAISIN CAKE MODEL”): • J.J. Thomson (1897): discovery of electron; electron = charged particle, much less massive (2000 times) than lightest known ion; • appeared to come out of matter - part of atoms? • Thomson's atom model: • atom = sphere of positive charge (diameter 10-10 m), • with electrons embedded in it, evenly distributed (like raisins in cake)

  3. RUTHERFORD MODEL OF ATOM • RUTHERFORD MODEL OF ATOM • (“PLANETARY MODEL”): • observations in “scattering experiments” (Geiger, Marsden, Rutherford, 1906 - 1911): measured angular distribution of scattered particles did not agree with expectations from Thomson model (only small angles expected), • but did agree with that expected from scattering on small, dense positively charged nucleus with diameter 10-14 m , surrounded by electrons at distance  10-10 m; • “planetary model”: • positive charge concentrated in nucleus (10-14 m); • negative electrons in orbit around nucleus at distance 10-10 m ; • electrons bound to nucleus by “electric” force; • problem with planetary atom model: electron orbiting in circular or elliptic orbit would lose energy by radiation orbit decays, and atoms would be unstable (lifetime  10-10 sec) • we would not exist to think about this!! • new theory needed to explain contradictions - Quantum Theory

  4. CHEMICAL SYMBOLS ANDFORMULAE • chemical elements, and their atoms, are denoted by chemical symbols of one or two letters, (from their Latin or Greek name), e.g.: • O = oxygen (Oxygenium = acid maker) • Fe = iron (Ferrum) • K = potassium (Kalium) • Na = sodium (Natrium) • H = hydrogen (Hydrogenium = water maker) • chemical compounds, and their molecules, are denoted by their chemical formula chemical formula gives composition, e.g. • H2 O= water molecule, contains 2 hydrogen and one oxygen atoms • C O2 = carbon dioxide, contains one carbon and 2 oxygen atoms • O3 = ozone, contains 3 oxygen atoms • NaCl = sodium chloride = table salt • CnH2n+2= “alkane”, the simplest group of “hydrocarbon” compounds, such as: • C H4 = methane, • C2 H6 = ethane, • C3 H48 = propane, • C4 H10 = butane, • C8 H18 = octane, etc. • C2 H5 OH= ethanol, ethyl alcohol

  5. chemical reactions • atoms can bond together to form compounds; • compounds can react with each other to make new compounds; • some chemical reactions: • oxidation = formation of new compound by bonding with oxygen, e.g. • burning of carbon: C + O2 CO2 + energy • burning of methane: CH4 + 2O2 CO2 + 6H2 O + energy • respiration: C2 H12 O6 + 6O2 6CO2 + 6H2O + energy respiration is the process by which animals retrieve energy stored in glucose, breathing in oxygen produced by plants, • Photosynthesis = mechanism by which plants convert energy of sunlight into energy stored in carbohydrates, generating oxygen as a “by-product” reaction: solar energy + CO2 + H2O  carbohydrates + O2