WHY CAN'T WE SEE ATOMS?

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?10-7
  m (violet) and 7? 10-7 m (red)
• diameter of atoms 10-10 m
• 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
• CnH2n2 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

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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