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ATOM Early Thoughts Aristotle-- Continuous theory Greeks matter is made up of particles--4 elements 4 elements --air--fire--water-- earth Matter can be divided indefinitely Democritus --Discontinuous theory Matter can not be divided indefinitely Atoms-indivisible Rich -Paradis ATOMIC THEORIES Dalton’s Atomic Theory • All elements are composed of individual atoms. •All atoms of a given element are identical • Atoms of different elements are different. •Compounds are formed by the combination of atoms of different elements (Law of Definite Proportions) Reactions are rearrangements of atoms Sphere Uniform density Fire ball Cathode Tube Cathode ray Negative electrode -cathode rays are a form of radiation that contain an electrical charge -rays move from negative to positive -rays are negative (electrons) Positive electrode hi gh vol tage QuickTime™ and a decompressor are needed to see this picture. JJ Thomson’s Model used the cathode ray tube to show one of the smaller units (particle) that make up the atom Thomson theorized that an atom contains small negatively charged particles -------electrons His model is the “Plum Pudding” Negative electrons float around Bulk of the atom is positive Still uniform density Rutherford’s Gold Foil Experiment Alpha emitter (positvie charge) screen Hypothesis Because the atom has uniform density, the alpha particles will pass straight through with little to no deflection. Like a bullet through a stick of butter. Gold foil Rutherford’s Gold Foil Experiment Results Conclusion The atom does not have uniform density. Because the alpha particles passed through, there must be empty space. Because the alpha particles were bounced back, they must have hit something dense. Because the alpha particles were deflected, the dense part must have a positive charge. QuickTime™ and a decompressor are needed to see this picture. >Click Me< Rutherford’s model Dense positive center Mostly empty or unoccupied space Electrons float around randomly in the empty space Chadwicks -discovered the neutron -when doing studies, he found that the atom often weighed twice as much as he anticipated with the number of protons -he determined that there must be some other particle with the same mass as a proton but had no charge -neutrons are located in the dense positive center (nucleus) Particle Symbol Charge Mass Location nucleons proton p +1 1 amu nucleus neutron n 0 1 amu nucleus -1 1/1836 mass of a proton electron e Mass number = # of protons + #of neutrons Atomic number =#of protons Protons And neutrons Nuclear charge # of Protons shell A Z X Calculate the number of protons, neutrons, and electrons. K Se 39 79 19 34 P- 19 P- 34 N- 20 N- 45 E- E- 19 Rewrite: 34 When an atom is neutral (no charge), the number of electrons will equal the protons. I-131 Mass # I 53 131 P- 53 N- 78 E- 53 Ion -has a charge (pos or neg) Gain electrons = negative ion 35 17Cl - P- 17 N- 18 E- 18 Lose electrons = positive ion 40 2+ 20Ca P- 20 N- 20 E- 18 Isotope -Atoms with the same atomic number, but different mass number -same bottom, different top -differ in the number of neutrons C 6 14 C 12 6 Atomic Mass -is the weighted average of all naturally occurring isotopes. -based on the percent abundance and mass 1. 2. 3. 4. Write percent as decimal. Multiply decimal by the mass #. Add them up. Units will be atomic mass units (amu) 75% Cl-35 25% Cl-37 .75 x 35 = 26.25 .25 x 37 = 9.25 35.50 amu Bohr Model •proposed a model of the atom showing a dense nucleus with electrons found in surrounding orbits. •Bohr model shown shows a nucleus surrounded by electrons in circular orbits. •Each electron must posses just the right amount of energy to keep it in place around the nucleus. energy levels principal energy levels shells 2 2(n ) Energy level Electron configuration the distribution of the electrons in an atom 2-8-8 1st level 2nd level 3rd level Energy level 1 2 3 4 5 6 7 Max # of e’s 2(12) 2(22) 2(32) 2(42) 2(52) 2(62) 2(72) 2 8 18 32 50 72 98 In theory We will assume only 32 per level kernel represents atom from the nucleus to the valance shell valence electrons are electrons in the outer energy level of the atom quantum-- a discrete amount of energy -- each energy level represents a specific quanta -- to move from one level to another, you must use that much energy Ground state all electrons are in the lowest possible level will follow the pattern: 2-8-18 Pattern on periodic table Excited state when an electron absorbs energy and jumps to a higher level can’t stay there pattern skips 2-7-1 NOT on periodic table WAS: 2-8 (moved 1 e- to the next level) Atoms can not stay excited, when electrons drop to a lower level, energy is released in the form of light(photon). Each element gives off a specific color and spectrum. continuous spectrum ROYGBIV •white light through a prism or a diffraction gradient •shows all the colors is the visible spectrum •diffraction gradient separates the light bright line spectra •each element gives off their own bright line spectrum •each line represents an electron dropping to a lower level •fingerprint of an element Modern Theory -- Wave (Quantum) Mechanical Model electron cloud electrons don’t travel in fixed paths but are located in regions of space called orbitals we don’t know the exact location of the electron but can guess where it is most likely located QuickTime™ and a based on probability decompressor are needed to see this picture.