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 & again…At what point would you find that you couldn’tsubdivide 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/particleadventure/frameless/sitemap.html
What is matter ? • We are ! And lots of other thingsaround us…Almost everything around you ismatter…But, what we’re really interestedin is:What is matter at its most fundamental level ?
0.0002” What are we made of ? • We’re 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 moleculeswhich contains your genetic code! • But, what are molecules made of ?
The Elements • Molecules are complex structures of the elements
Early“Plum-Pudding”Model Corpuscles (Electrons) Positive Charge (uniformly distributed) The positive charge is spread out like a “plum-pudding” 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 ? ”
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). Radiation is going through you atthis very moment! http://www.atral.com/U238.html
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 238U.
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)
Where do these ‘particles’ come from ? • These particles generally come from the nuclei of atomic isotopeswhich are not stable. • The decay chain of Uranium produces all three of these formsof radiation. • Let’s look at them in more detail…
Note: This is theatomic weight, whichis the number ofprotons plus neutrons Alpha Particles (a) 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) It’s the same as the element Helium, but without the electrons !
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 nitrogennucleus. In symbolic notation, the following process occurred:
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.
Gamma Rays Neon Ne20 Neon Ne20 + The gamma from nuclear decayis in the X-ray/ Gamma ray part of the EM spectrum(very energetic!)
How do these particles differ ? mp = proton mass mn = neutron mass Back to “Structure of Matter”
a Scattering Experiments If the plum-pudding model was right, then matteris “soft”. There’s 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…
a a 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 came…The theoretical expectationwas 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.”
a a a a a The (only) interpretation The atom must have a solid core capable of imparting largeelectric forces onto an incoming (charged) particle.
Atom: the smallest particle of an element that can exist eitheralone or in combination The Modern Atom Electrons Nucleus 2x10-13 cm 0.0000000002 cm (2 x 10-10 cm)
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.
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 !!!
Next time…What’s inside the nucleus ?What’s filling all the space ?