What is Matter

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What is Matter
If you took a piece of paper, and ripped itin
half.Take one of the halves, and rip it in
half.Repeat this again again againAt
what point would you find that you
couldntsubdivide the material anymore?What
would you have in your hand at this point?
A very good reference for the things we are going
to cover is at http//particleadventure.org/par
ticleadventure/frameless/sitemap.html
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What is matter ?

• We are ! And lots of other thingsaround
  usAlmost everything around you
  ismatterBut, what were really interestedin
  is What is matter at its most
  fundamental level ?

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What are we made of ?

• Were made of cells whichcontain DNA. -
  Different cells serve different functions in
  your body.
• The cells contain a nucleus, which contains
  your DNA !
• And the DNA is a wonderful, complex chain of
  molecules which contains your genetic code!
• But, what are molecules made of ?

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The Elements

• Molecules are complex structures of the elements

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But whats inside an element
For each element, we can associate an
atom. Prior to 1905, nobody really knew
What does the inside of an atom look like ?
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A digression on radiation
Radiation The process of emitting energy in the
form of waves or particles. Where does
radiation come from? Radiation is generally
produced when particles interact or decay.A
large contribution of the radiationon earth is
from the sun (solar) or from radioactive
isotopes of the elements (terrestrial). Radiati
on is going through you atthis very moment!
http//www.atral.com/U238.html
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Isotopes
Whats an isotope? Two or more varieties of an
element having the same number of protons but
different number of neutrons. Certain isotopes
are unstable and decay to lighter isotopes or
elements.Deuterium and tritium are isotopes of
hydrogen. In addition to the 1 proton, they have
1 and 2 additional neutrons in the nucleus
respectively. Another prime example is
Uranium-238, or just 238U.
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Radioactivity

• By 1900, it was known that certain isotopes emit
  penetrating rays. Three types of radiation were
  known
• Alpha particles (a)
• Beta particles (b)
• Gamma-rays (g)

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Where do these particles come from ?

• These particles generally come from the nuclei
  of atomic isotopes which are not stable.
• The decay chain of Uranium produces all three
  of these formsof radiation.
• Lets look at them in more detail

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Alpha Particles (a)
Note This is theatomic weight, whichis the
number ofprotons plus neutrons
Radium R226
Radon Rn222
p

n
n
p
a (4He)
88 protons 138 neutrons
86 protons 136 neutrons
2 protons 2 neutrons
The alpha-particle (a) is a Helium nucleus
(charge 2) Its the same as the element
Helium, but without the electrons !
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Beta Particles (b)
Carbon C14
Nitrogen N14
n


e-
electron (beta-particle)
6 protons 8 neutrons
7 protons 7 neutrons
More on thisbugger later
n ? p e- n
The electron emerges with relatively high energy
in this disintegration (decay) process.
We see that one of the neutrons from the C14
nucleus converted into a proton, and an
electron was ejected. The remaining nucleus
contains 7p and 7n, which is a nitrogen nucleus.
In symbolic notation, the following process
occurred
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Gamma particles (g)
In much the same way that electrons in atoms can
be in an excited state, so can a nucleus.
Neon Ne20
Neon Ne20

10 protons 10 neutrons(in excited state)
10 protons 10 neutrons(lowest energy state)
gamma
A gamma is a high energy light particle (short
for gamma ray). It is NOT visible to your naked
eye because it is not in the visible part of the
EM spectrum.
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Gamma Rays
Neon Ne20
Neon Ne20

The gamma from nuclear decayis in the X-ray/
Gamma ray part of the EM spectrum(very
energetic!)
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How do these particles differ ?
mp proton mass mn neutron mass
Back to Structure of Matter
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Scattering Experiments
If the plum-pudding model was right, then
matteris soft. Theres no central, hard core
Ernest Rutherford1871-1937
Alphaparticlesource
Awarded the Nobel Prize in 1908
Calculations, based on the known laws of
electricity and magnetismshowed that the heavy
alpha particles should be only slightly
deflectedby this plum-pudding atom
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Au Contraire
Contrary to expectations, Rutherford found that a
significantlylarge fraction (1/8000) of the
alpha particles bounced back in the same
direction in which they cameThe theoretical
expectationwas that fewer than 1/10,000,000,000
should do this ???
Gold foil
Huh ???
In Rutherfords wordsIt was quite the most
incredible event that ever happened to me in my
life. It was as if you fired a 15-inch naval
shell at a piece of tissue paper and the shell
came right back and hit you.
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The (only) interpretation
The atom must have a solid core capable of
imparting largeelectric forces onto an incoming
(charged) particle.
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The Modern Atom
Atom the smallest particle of an element that
can exist eitheralone or in combination
Electrons
Nucleus
2x10-13 cm
0.0000000002 cm (2 x 10-10 cm)
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Atoms and Space
Approximately what fraction of the volume of an
atom does thenucleus consume? Assume that an
atom can be approximated by a sphere with a
radius given by the electrons orbit radius?
Use the following data.

• The radius of the nucleus is 2x10-13 cm.
• The electrons orbits at a radius of 2x10-10 cm
• Ignore the electrons size, as it is unimportant.
• The volume of a sphere is (4/3)pR3.

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Answer
a) First find the volume of the entire
atom Volume (4/3)p(2x10-10)3 3.4 x 10-29 cm3
b) Now find the volume which contains the
nucleus. Volume (4/3)p(2x10-13)3 3.4 x
10-38 cm3
c) Now compute the fraction Fraction (3.4 x
10-38 / 3.4 x 10-29 ) 0.000000001
In other words, 99.99999999 of an atom is empty
space !!!
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Next timeWhats inside the nucleus ?Whats
filling all the space ?