What is Matter If you took a piece of paper, and ripped it in half. Take one of the halves, and rip it in half. Repeat this again & again & again… At what point would you find that you couldn’t subdivide 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:

What is matter ?  We are ! And lots of other things around us… Almost everything around you is matter… But, what we’re really interested in is: What is matter at its most fundamental level ?

What are we made of ?  We’re made of cells which contain 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 ? ”

The Elements  Molecules are complex structures of the elements

But what’s 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 ? ” Positive Charge (uniformly distributed) Corpuscles (Electrons ) The positive charge is spread out like a “plum-pudding” Early “Plum-Pudding” Model

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 radiation on earth is from the sun (solar) or from radioactive isotopes of the elements (terrestrial). Radiation is going through you at this very moment!

Isotopes What’s 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 238 U.

Radioactivity By ~1900, it was known that certain isotopes emit penetrating rays. Three types of radiation were known: 1)Alpha particles (  ) 2)Beta particles (  ) 3)Gamma-rays (  ) By ~1900, it was known that certain isotopes emit penetrating rays. Three types of radiation were known: 1)Alpha particles (  ) 2)Beta particles (  ) 3)Gamma-rays (  )

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 forms of radiation.  Let’s look at them in more detail…

Alpha Particles (  ) Radium R protons 138 neutrons Radon Rn 222 Note: This is the atomic weight, which is the number of protons plus neutrons 86 protons 136 neutrons + n n p p   He) 2 protons 2 neutrons The alpha-particle  is a Helium nucleus (charge = +2) It’s the same as the element Helium, but without the electrons !

Beta Particles (  ) Carbon C 14 6 protons 8 neutrons Nitrogen N 14 7 protons 7 neutrons + e-e- electron (beta-particle) We see that one of the neutrons from the C 14 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: + More on this bugger later… The electron emerges with relatively high energy in this “disintegration” (decay) process. n  p + e - +

Gamma particles (  ) In much the same way that electrons in atoms can be in an excited state, so can a nucleus. Neon Ne protons 10 neutrons (in excited state) 10 protons 10 neutrons (lowest energy state) + gamma Neon Ne 20 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. 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.

Gamma Rays Neon Ne 20 + The gamma from nuclear decay is in the X-ray/ Gamma ray part of the EM spectrum (very energetic!) Neon Ne 20

How do these particles differ ? ParticleMassCharge Gamma (  ) 00 Beta (  ) (Electron) Electron mass is ~1/2000 th of a proton’s mass Alpha (  ) ~4 times a proton’s mass (since m p  m n). +2 Back to “Structure of Matter” m p = proton mass m n = neutron mass

Scattering Experiments Ernest Rutherford If the plum-pudding model was right, then matter is “soft”. There’s no “central, hard core”… Calculations, based on the known laws of electricity and magnetism showed that the heavy alpha particles should be only slightly deflected by this “plum-pudding” atom… Awarded the Nobel Prize in 1908  Alpha particle source

Au Contraire Contrary to expectations, Rutherford found that a significantly large fraction (~1/8000) of the alpha particles “bounced back” in the same direction in which they came…The theoretical expectation was that fewer than 1/10,000,000,000 should do this ??? Gold foil  Huh ??? In Rutherford’s words… “It 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.”

The (only) interpretation The atom must have a solid core capable of imparting large electric forces onto an incoming (charged) particle. 

The Modern Atom cm (2 x cm) 2x cm Electrons Nucleus Atom: the smallest particle of an element that can exist either alone or in combination

Atoms and Space Approximately what fraction of the volume of an atom does the nucleus 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 ~ 2x cm. The electrons orbits at a radius of ~ 2x cm Ignore the electrons size, as it is unimportant. The volume of a sphere is (4/3)  R 3.

Answer… a) First find the volume of the entire atom Volume = (4/3)*  2x ) 3 = 3.4 x cm 3 b) Now find the volume which contains the nucleus. Volume = (4/3)*  2x ) 3 = 3.4 x cm 3 c) Now compute the fraction: Fraction = (3.4 x / 3.4 x ) = In other words, % of an atom is empty space !!!

Next time… What’s inside the nucleus ? What’s filling all the space ?