WHY CAN'T WE SEE ATOMS?
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“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:
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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
WHAT IS INSIDE AN ATOM?
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GREEK MODEL OF ATOM
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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
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(“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)
RUTHERFORD MODEL OF ATOM
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RUTHERFORD MODEL OF ATOM
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(“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
CHEMICAL SYMBOLS ANDFORMULAE
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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)
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Fe = iron (Ferrum)
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K = potassium (Kalium)
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Na = sodium (Natrium)
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H = hydrogen (Hydrogenium = water maker)
chemical compounds, and their molecules, are
denoted by their chemical formula
chemical formula gives composition, e.g.
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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
chemical reactions
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atoms can bond together to form compounds;
compounds can react with each other to make new
compounds;
some chemical reactions:
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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