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Download The Atom - Wunder Chem
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CHEM I - TUES, 9/8/15 Do Now Complete the worksheet as review for your quiz! Homework MEAL paragraph if not finished in class Agenda Matter Quiz SigFig Review Atomic Theory Presentation (Dimensional Analysis) MEAL paragraph APPLIED CHEM - TUES, 9/8/15 Do Now Write a brief paragraph to describe what you felt was the most difficult part of the lab. How hard was it to write the procedure on your own? Was having group mates helpful? Did your procedure work perfectly? What flaws did you notice in the procedure? Homework NA Agenda Review quiz % Yield String Thing Atomic Theory THE ATOM DEFINITION TO START Atom – smallest particle of an element that retains its identity They are tiny! Electron microscope – allows us to observe individual atoms DEMOCRITUS THE PHILOSOPHER 460-370 BC First credited with proposing the existence of an atom Indivisible and indestructible Shortcomings? Did Aristotle agree? THEN CAME JOHN DALTON 1766-1844 Experimental methods -> scientific theory Dalton’s atomic theory Matter is composed of indivisible atoms Atoms of same element are identical Combine in whole number ratios to form compounds Rxns occur when atoms separate, bond, or rearrange. Atoms of one element never become atoms of another thru chemical rxns JJ THOMSON’S EXPERIMENT Cathode Ray Experiment Gas filled glass tube fitted with electrodes Electricity -> cathode ray (travels from cathode to anode) Conclusion: electrons - negatively charged subatomic particles Further tests: Mass to charge ratio Different gases Conclusion: Electrons are part of atoms of all elements. OTHER SUBATOMIC PARTICLES But we don’t get shocked every time we touch something…so where’s the positive? Goldstein: Cathode ray (1886) Detected rays traveling in the opposite direction Conclusion: protons – positively charged subatomic particles 1840x mass of an electron Chadwick (1932) Neutrons – neutral subatomic particles ~same mass as proton THOMSON’S MODEL TO RUTHERFORD’S MODEL Thomson – ‘plum pudding’ (1897) Chocolate chips in cookie dough Gold Foil Experiment (1911) Alpha particles thru gold foil Predictions: only slight deflection Results: Most: straight thru or slight deflection Some: large deflection or ‘bounced’ back toward the source RUTHERFORD MODEL/NUCLEAR MODEL Atom is mostly empty space explains why the alpha particles could pass straight thru All the positive charge and most of mass is located in a small region explains the large deflections Nucleus - protons and neutrons Electrons are around the nucleus and account for most of the volume Still not quite right! CHEM I - WED, 9/9/15 Do Now Turn in MEAL paragraph Start on Factor Label Sheet 1-4 Homework Pages 1 and 2 of math packet Wks 4.5 Agenda Papers Back Bohr E- in the atom IT’S ALL ABOUT COLOR… In terms of atomic models, so far: Dalton (1803) = Tiny, solid particle Thomson (1897) = “Plum Pudding” model – Electrons stuck on the outside of a big positive charge Rutherford (1911) = Positively-charged nucleus with electrons moving around it Rutherford’s model of the atom not quite right Could not explain chemical properties of elements Could not explain color changes when metal is heated BOHR MODEL OF THE ATOM Niels Bohr’s model of the atom Electron found only on specific, circular paths around nucleus Each orbit has fixed energy level Hypothesis: When electrons are excited (added energy), jump into higher energy levels. When they moved back into lower energy levels - gave off light. Electrons do not exist between levels (think of rungs on a ladder) Electrons absorb and emit only certain quanta (amounts) of energy Quantum of energy = fixed amount of energy required to move from one energy level to another energy level BOHR’S MODEL Nucleus Electron Orbit Energy Levels Chapter 5 BOHR’S PLANETARY MODEL OF THE ATOM Electrons must have enough energy to keep moving around the nucleus Electrons orbit nucleus in defined energy levels, just like planets orbit the sun Each energy level assigned a principal quantum number n. Lowest energy level called ground state (n=1) Higher energy levels (n=2, 3, 4...) excited states Model worked OK for hydrogen but not so good for other elements Nucleus n=1 n=2 BOHR’S MODEL Fourth Third Second First Nucleus Chapter 5 Increasing energy Fifth Further away from the nucleus means more energy. There is no “in between” energy Energy Levels Electron starts on lowest energy level (ground state) Lowest energy level = ground state Higher energy levels = excited states Add energy to electron – moves to excited state Energy levels are not evenly spaced Energy Level 3 Energy Level 2 Energy Level 1 Nucleus Electron starts on lowest energy level (ground state) Lowest energy level = ground state Higher energy levels = excited states Add energy to electron – moves to excited state Energy Energy levels are not evenly spaced Electron returns to lower state – emits/gives off quantum of energy Energy Level 3 Energy Level 2 Energy Level 1 Nucleus Chapter 5 Bohr used this theory to explain the lines in the atomic emission spectra for hydrogen 434 nm 410 nm 656 nm 486 nm Chapter 5 Each of these lines corresponds to different energy changes WHAT CAN THE PERIODIC TABLE TELL US? Atomic number – the number of protons in an element Elements are defined by their atomic number Mass number – total number of neutrons and protons ISOTOPES! Same number of protons Different number of neutrons Used to calculate the atomic weight of an element Mass of an atom is tiny – so knowing the actual mass is a bit impractical ATOMIC MASS UNIT Isotope Carbon 12 was assigned the mass of 12 atomic mass units. So then, 1/12 of the mass of a carbon 12 atom is 1 amu. So then helium 4 has 1/3 the mass of carbon 12. Carbon has 6 protons and 6 neutrons…this accounts for the bulk of the mass…so one proton or 1 neutron has a mass of ~1 amu. ATOMIC WEIGHT Atomic weight – weighted average of the atomic masses of the isotopes of an element in a naturally occurring sample Example problem….Grades!
