Lecture 32 April 08. 2016. Hydrogen Discharge Tube and Emission of Discrete Wavelengths Description of the discrete Hydrogen Emission Spectrum by the Balmer (1884) Rydberg Ritz formula (1908) Cathode Ray Tube and Thomson's discovery of electrons, particles and photons (1897) Cathode Ray Tube and Röntgen s discovery of X-ray photons Black Body radiation and Max Planck s quantum hypothesis: E h Photo Electric Effect and its interpretation by Einstein 4/7/2016 Physics 214 Spring 2016 1
Bohr s hypothesis of the Hydrogen Atom a.) Electron orbits around proton in its Coulomb field like the solar system b.) Only certain stable orbits are permitted c.) Photons are emitted or absorbed in the transitions between these stable orbits d.) Electrons do not radiate when move in stable orbits Periodic Table and Rutherford s Experiment Nuclear structure and radioactivity 4/7/2016 Physics 214 Spring 2016 2
Improvements over Guericke's (Mayor of Magdeburg 1654) vacuum pump and Volta-Galvani s battery, resulted in Hydrogen gas discharge tube and in high vacuum and high voltage cathode ray tube. The Hydrogen discharge tube produced a remarkable four discrete line spectrum: a red, a blue and two violet lines. See Fig. 18.17 on page 399. Balmer (1884) Rydberg Ritz (1908) described the above observation by the equation: 1 1 1 R( ) n 2 m 2 Where R is called the Rydberg constant and n, m are integers. This formula was the forerunner of quantum rather then continuum physics. 4/7/2016 Physics 214 Spring 2016 3
4/7/2016 Physics 214 Spring 2016 4
Upon heating the cathode of the cathode ray tube (See Fig. 18.6 on page 391) three kind of beams (1897) emerged: particles particles particles Which we identify: particle with Helium nucleus particle with the electron of Benjamin Franklin by Thomson in 1897. See Fig. 18.7 and 18.8 on page 393. particle with high energy photon by Röntgen. See Fig. 18.10 and 18.11 on page 394. 4/7/2016 Physics 214 Spring 2016 5
4/7/2016 Physics 214 Spring 2016 6
4/7/2016 Physics 214 Spring 2016 7
4/7/2016 Physics 214 Spring 2016 8
Max Planck painted black the inside of a closed cube and drilled a small hole on one of its side and measured the distribution of of the emitted (black body) radiation. (See Fig. 18.18 on page 400) Max Planck could explain the observed spectral distribution of the black body radiation only if he assumed that the relation between the energy and frequency of the radiation is Using this relation Einstein could explain the Photo Electric Effect, the emission of bound electron from metals and obtain the Nobel prize. E Ee h h 4/7/2016 Physics 214 Spring 2016 9
4/7/2016 Physics 214 Spring 2016 10
Planck s relation and its interpretation by Einstein suggests that the discrete spectral lines of H 2 gas ( See Fig. 18.17 on page 399) with fixed can be interpreted as an electron transition from one stable orbit with energy level E1( r1) with radius r1 to another stable orbit with energy level E2( r2) with radius r2. To make Bohr s hypothesis possible he added the corollary that electrons do not radiate in stable orbits, only when they are in transition from r. 1 r2 See Fig. 18.19 on page 401. But how is the + charge, which attracts the, is distributed In 1909-1911 Rutherford used particles emerging from decaying nuclei and bombarded a gold foil. He found amazing events where particles scatter backward. This indicates that a positive charge is concentrated in a small sphere thus the particle s kinetic energy is converted into potential energy, stops and recoils. 2 mv q k nucleus 2 r See graph 3 on page 406. e 4/7/2016 Physics 214 Spring 2016 11
4/7/2016 Physics 214 Spring 2016 12
4/7/2016 Physics 214 Spring 2016 13
Mendeleev s periodic table can be interpreted as z N Atom = Atom contains Z proton and N nucleons (protons + neutrons) it is located at the th z position in the periodic table and Z electrons orbit around the protons. Atoms in one columns have equal number of electrons on the outer orbits. See Fig. 18.5 on page 390. 4/7/2016 Physics 214 Spring 2016 14
4/7/2016 Physics 214 Spring 2016 15
The basic building blocks The basic building blocks that make up atoms are charge mass Proton 1.6 x 10-19 1.672 x 10-27 kg=938.27mev Neutron 0 1.675 x 10-27 kg=939.56mev Electron -1.6 x 10-19 9.1 x 10-31 kg=0.51mev Photon 0 E = hf p = h/λ h = Planck s constant = 6.626 x 10-34 J.s 4/7/2016 Physics 214 Spring 2016 16
The nucleus was found by Rutherford in scattering charged particles from a gold foil. He observed that occasionally the charged particle was deflected through a large angle indicating it had hit something hard The nucleus of any atom is composed of protons and neutrons. The protons have positive electric charge so repel each other. If they are close enough the strong force binds them together and the nucleus is stable The nucleus 4/7/2016 Physics 214 Spring 2016 17
Atoms Atoms are made up with a central nucleus of protons and neutrons surrounded by a number of electrons equal to the number of protons. The notation we use is 2 He 4 2 is the atomic number = number of protons (and electrons) 4 is the mass number = number of protons + neutrons Note atomic mass is the actual mass of the nucleus in atomic mass units with Carbon set to be 12 units and atomic mass in general is not an integer H 2 O gives the number of atoms required to make a molecule water = 2 Hydrogen plus 1 Oxygen In a nuclear reaction energy, charge, and atomic mass are conserved. Atoms can decay Atoms can join together to form new atoms or molecules Chemistry is determined by the electrons 4/7/2016 Physics 214 Spring 2016 18
3E-05 Cloud Chamber As charged particles pass through a gas electrons are knocked out of the atom and the ions produced can act as seeds for a super saturated liquid to condense. This leaves a visible track of bubbles of liquid. Cosmic rays are bombarding the earth continuously and radioactive decays also produce charged particles which can be observed in a cloud chamber. If the chamber is in a magnetic field the tracks are curved The first observation of the positron (anti particle of the electron) was made in a cloud chamber 4/7/2016 Physics 214 Spring 2016 19
Isotopes For a given number of protons there is a nucleus that is most stable for a particular number of neutrons. Isotopes are when for the same number of protons the number of neutrons is different from the most stable configuration. Since the number of electrons is the same the chemical properties are identical but the nucleus can be unstable. The larger the difference between ideal and the isotope the more unstable the nucleus becomes. Nuclei have been made artificially that do not exist in nature. 4/7/2016 Physics 214 Spring 2016 20
Radioactive decay The general rule is that if a lower energy configuration exists for the same set of particles then they will try to change into that configuration. Each radioactive element has a characteristic half life, that is the time for half the sample to decay. Radioactive elements are clocks which tick at a specific rate. The amount remaining after N half lives is ½ x ½ x ½.. Half lives vary from billions of years to a fraction of a second. Those in the range of greater than thousands of years are used to date objects on earth back to it s formation 4.6 billion years ago C 14 5730 years I 129 1.6 million U 238 4.47 billion http://www.physics.purdue.edu/class/applets/phe/lawdecay.htm 4/7/2016 Physics 214 Spring 2016 21
Types of nuclear decays A proton can change into a neutron and emit a positron (anti particle of electron) 1e 0 Atomic number decreases by 1 mass number is the same A neutron can change into a proton and emit an electron -1e 0 Atomic number increases by 1 mass number is the same A nucleus can emit an alpha particle which is 2 neutrons and 2 protons (actually a helium nucleus) 2He 4 Atomic number decreases by 2 mass number decreases by 4 Photons can be emitted No change in Atomic number or mass number. 4/7/2016 Physics 214 Spring 2016 22
What is the daughter? When an element decays it usually changes to a new element. The notation we use is 3 Li 7 where 3 is the number of protons, called the atomic number, and 7 is the number of protons plus neutrons, called the mass number. Any decay or nuclear reaction conserves charge and mass number. 2He 4 + 7 N 14 = 8 O 17 + 1 H 1 nuclear reaction 88Ra 226 = 86 Rn 222 + 2 He 4 nuclear decay 4/7/2016 Physics 214 Spring 2016 23