Slide 1

Development of Atomic Theory Slide 2 Atomic Theory Poem/Song Write a song or poem: With a stanza for each theorist Dalton, Thomson, Rutherford & Bohr Mentioning their contribution to our understanding of the atom. Mentioning key evidence to support each of their findings. Include a noticeable rhythm, rhyme scheme or beat. Be prepared to read or sing in front of class. Slide 3 John Dalton British A small town school teacher at the age of 12. Introduced his atomic theory in 1803. Slide 4 Previous Findings 1. Law of Conservation of Mass Matter is neither created or destroyed in a chemical reaction. (Antoine Lavoisier) 2.Law of Definite Proportions The percentage by mass of elements in a compound is constant for any sample. Ex: H 2 O 3. Law of Multiple Proportions Compounds composed of the same two elements differ in one element by simple ratios. Ex: CO vs CO2; H2O vs H2O2 Slide 5 Daltons Atomic Theory 1.All matter is composed of tiny indivisible particles called atoms 2.All atoms of the same element are identical 3.Different elements have different types of atoms 4.Compounds are formed from simple combinations of atoms of different elements. 5.In a chemical reaction atoms are simply rearranged. *Activity: Ball & Stick Reactions Slide 6 Picture Daltons Atomic Theory Slide 7 Updates to Daltons Theory 1a. Atoms are divisible into protons, neutrons & electrons (& even smaller!). 1b. In nuclear decay they actually fall apart! 2. All atoms of a single element have the same number of protons, but not neutrons. (isotopes) 4. Compounds may be very complex! Slide 8 Cathode Rays Tape Lab Static electricity attractions & repulsions. Where do the charges originate? An evacuated glass tube when placed in an electric field Crookes observed a glowing inside. Thomson repeated Crookes experiment and did additional experiments. (-)(+) Slide 9 Thomsons Experiment #1 Setup: A cross was placed in the path of the glowing beam. (D?) Observation: A shadow appeared on the anode (+) side. (D?) Interpretation: The rays come from the cathode (-) side. Cathode (-) Anode (+) Slide 10 Thomsons Experiment #2 Setup: The cathode ray tube was placed in an electric field: (-) electrode on top, (+) electrode on bottom. (DPath?) Observation: The cathode rays were attracted towards the (+) electrode. (D?) Interpretation: Cathode rays must be negative (-). Slide 11 Slide 12 Thomsons Experiment #3 Setup: Cathode rays were placed in a magnetic field. Observation: Cathode rays are bent perpendicular to the magnetic field. Interpretation: Cathode rays are not a form of light. Slide 13 Thomsons Experiment #4 Setup: A glass wheel was placed on a level track inside the cathode ray tube. Observation: Cathode rays can rotate the glass wheel. Interpretation: Cathode rays are particles with mass. Slide 14 Thomson Experiment #5 Setup: Thomson made cathode ray tubes with a variety of different gases & metal electrodes in the tube. Observation: Every tube produced the same cathode rays. Interpretation: Cathode rays are fundamental to matter. He called cathode rays electrons! Discovered in 1897. Slide 15 Thomsons Plum Pudding Model Thomson concluded that all atoms must have negative charges and positive charges to balance them. Thomson assumed that (+) & (-) charges would be evenly distributed. Slide 16 Uses of cathode rays 1. A cathode ray tube (CRT) is widely used in research laboratories to convert any signal (electrical, sound, etc) into visual signals. These are called CRT or oscilloscopes. 2. CRT is the basic component in all television and computer screens. The signals are sent to the vertical and horizontal deflecting plates, which produce a pattern on the fluorescent screen. High energy cathode rays when stopped suddenly produce X-rays. The X- rays have many medical and research applications. Slide 17 Slide 18 Slide 19 Slide 20 Millikans Oil Drop Experiment http://www.youtube.com/watch?v=XMfYHag7 Liw&feature=related http://www.youtube.com/watch?v=XMfYHag7 Liw&feature=related Slide 21 Ernest Rutherford New Zealander Discoverer of alpha, beta & gamma radiation. Discovered nucleus of atom in 1912. Slide 22 Discovery of Alpha, Beta & Gamma Radiation Slide 23 Rutherfords Experimental Design Uranium alpha emitter. Slits to focus radiation Gold foil target. Scintillation screen of zinc sulfide to flash when hit. Slide 24 Rutherfords Prediction Positive alpha particles would go straight through or have minor deflections due to the electrons embedded in a sea of positively charged matter. Slide 25 Rutherfords Observations (1 of 2) 1.http://www.mhhe.com/physsci/chemistry/e ssentialchemistry/flash/ruther14.swfhttp://www.mhhe.com/physsci/chemistry/e ssentialchemistry/flash/ruther14.swf 2.Vary the slit width and see what happens: 3.http://micro.magnet.fsu.edu/electromag/jav a/rutherford/http://micro.magnet.fsu.edu/electromag/jav a/rutherford/ 4.http://chemteacher.chemeddl.org/services/c hemteacher/index.php?option=com_conten t&view=article&id=74http://chemteacher.chemeddl.org/services/c hemteacher/index.php?option=com_conten t&view=article&id=74 Slide 26 Rutherfords Observations (2 of 2) Slide 27 Interpreting the Results Most positive alpha particles went straight through or were slightly deflected. Therefore the atom is mostly empty space. A few positive alpha particles bounced back radically! Thus the atom must have a large concentration of positive charge! Slide 28 Rutherfords Atomic Model Slide 29 Development of the Bohr Model In 1913 Danish physicist Neils Bohr proposed a new model of the atom. Bohrs Model explained the emission and absorption patterns of light discovered by Bunsen in flames & lamps. Slide 30 Emission Lamps Slide 31 Emission Spectra Each element emits a unique set of bright line wavelengths. Slide 32 Emission Spectra of All the Elements http://chemistry.beloit.edu/bluelight/moviepa ges/em_el.htm http://chemistry.beloit.edu/bluelight/moviepa ges/em_el.htm http://jersey.uoregon.edu/vlab/elements/Ele ments.html http://jersey.uoregon.edu/vlab/elements/Ele ments.html Slide 33 Slide 34 4 Principles of the Bohr Model 1)Electrons assume only certain orbits around the nucleus. These orbits are stable and called "stationary" orbits. 2)Each orbit has an energy associated with it. The lowest energy levels are close to the nucleus. The farther from the nucleus corresponds to higher energy levels. Electrons tend to occupy the lowest energy levels available. 3)Light is emitted when an electron jumps from a higher orbit to a lower orbit. Light is absorbed when it jumps from a lower to higher orbit. 4)The quantity of energy and wavelength of light emitted or absorbed is given by the difference between the two orbit energies. (Quantum Leaps!) Slide 35 With these conditions Bohr was able to explain the stability of atoms as well as the emission spectrum of hydrogen. Line spectra correspond to quantum leaps between levels of specific energies. Violet light corresponds to high energy quantum leaps while red light corresponds to low energy. ROYGBIV Slide 36 Excited State Ground State Semi-Excited State Green light emitted Red light emitted Slide 37 Excited vs Ground States Light is absorbed when electrons jump up to higher excited energy levels. Light is emitted when electrons jump back down to their lowest energy ground state energy levels. Animated Absorption & Emission Fluorescent lights are constantly exciting gas atoms to emit light by passing a stream of electrons through the interior gas. Slide 38 The Suns Spectra Many elements can be identified by their unique lines. Helium was 1 st discovered in the Suns (Helios) spectrum Slide 39 Emission vs Absorption Slide 40