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Platinum and Nickel How did we get here? Copper Atoms Blue- tin Red- silicon Green- lead Next Click here to find out how we see atoms like the ones shown on this page! Carbon atoms ATOMIC THEORY Next Cat Joke Scientists Ancient Greeks Democritus & Leucippus J.J. Thomson Ernest Rutherford Summary Video John Dalton Niels Bohr Atom Review Click on scientist to guide you through the PowerPoint. All green boxes are links. Use them to guide you! Cat Joke! Scientist Home Ancient Greeks (450 BC) • Believed all matter was made of: – Earth – Fire – Water – Air Scientist Home Cat Joke! Cat Joke! Scientist Home Democritus & Leucippus (300 BC) Experiment Scientist Home Experimental Conclusions New Model Experiment • They did not run any experiments. • Atoms were an invention of their minds Democritus & Leucippus Next Experimental Conclusions • Atoms are the smallest particles of matter • Defines the atom as the smallest particle • Explains the existence of elements Democritus & Leucippus Next Model Name: Atom the indivisible particle Description: Matter is made up of particles so small and indestructible that they cannot be broken down into smaller units- particles were called “atomos”, which is Greek from “indivisible” Democritus & Leucippus Model Shortcomings Model Shortcomings • No scientific view of the atom only a conceptual definition • Does not talk about subatomic particles (electrons, protons, neutrons) Democritus & Leucippus Cat Joke! Cat Joke! Democritus & Leucippus Scientist Home John Dalton (1800) Dalton Fun Facts Scroll to them Experiment Scientist Home Experimental Conclusions New Model Experiment • Dalton ran experiments with different types of gases and through deductive reasoning and experimentation he came to several conclusions. Dalton Next Experimental Conclusions 1. Pure Elements consist of particles called atoms. 2. Atoms of an element are all the same for that element. That means gold is gold and oxygen is oxygen down to the last atom. 3. Atoms of different elements can be told apart by their atomic weights. 4. Atoms of elements unite to form chemical compounds. 5. Atoms can neither be created or destroyed in chemical reaction. Dalton Next Dalton’s Model Name: The Solid Sphere Model (Clay Ball Model) Description: – Atoms are seen as solid, indestructible spheres (like billiard balls) Dalton Model Shortcomings Model Shortcomings • Atoms are not indivisible (b/c they are made up of subatomic particles) • Atoms can change from one element to another by radioactive decay (not by chemical reactions). • Not all atoms of the same element have exactly the same mass (isotopes). Dalton Cat Joke! Cat Joke! Dalton Scientist Home J.J. Thomson (1850) Experiment Scientist Home Experimental Conclusions New Model Experiment Experiment Animation Next Summarized Experiment Detailed explanation of cathode ray tube Voltage Cathode (-) • • • • • Click here anode (+) Empty glass tube (vacuum) connected to a metal cathode and anode. High voltage is applied to the tube. This caused the atoms in the cathode to get excited. Electrons stream off the cathode, producing a cathode ray. Next The cathode ray traveled toward the anode Summarized Experiment Voltage Cathode (-) anode (+) Click here When a negative charge was brought toward the cathode ray, it caused the cathode ray to move away from the negative charge. Next Summarized Experiment Voltage + Cathode (-) anode (+) Click here When a positive charge was brought toward the cathode ray, it caused the cathode ray to move toward the positive charge. Thomson Next Experimental Conclusion • Discovered the electron – Electrons have negative charges • Electrons are evenly distributed in a positive sphere. Thomson Next Thomson’s Model Name: Plum Pudding Model Description: • Atoms are solid spheres made up of a solid positive mass with tiny negative particles embedded in the positive core. • Like a chocolate chip cookie! – Cookie is positive – Chocolate chips are negative electrons Thomson Next Real Plum Pudding! Gross!! Thomson Model Shortcomings Model Shortcomings • Does not explain the existence of electrons outside the nucleus • Does not explain the role of electrons in bonding • Does not talk about neutrons, protons, or the nucleus Thomson Cat Joke! Cat Joke! Thomson Scientist Home Ernest Rutherford (1905) Experiment Scientist Home Experimental Conclusions New Model Experiment Experiment Animation Next Summarized Experiment • Sent alpha particles (+2 charged Helium atoms) at a thin layer of gold foil • He expected 100% of the particles to go through to screen • 90% went through the foil to screen, but 10% were deflected • He was surprised and wrote that this result was comparable with shooting a bullet at a piece of paper and having the shell reflected back. • The plum pudding model could not be correct. Rutherford Next Experimental Conclusion • Atoms are mostly empty space • Atoms have a dense center with a positive charge called a nucleus • Electrons move around the nucleus at a distance Detailed explanation of gold foil experiment Rutherford Next Rutherford’s Model Name: Nuclear atom Description: • Positively charged nucleus in center of atom • Electrons moving around nucleus at a distance Nucleus Rutherford Model Shortcomings Electrons Model Shortcomings • It does not explain: how negatively charged electrons keep from crashing into the positivelycharged nucleus. Rutherford Cat Joke! Cat Joke! Rutherford Scientist Home Niels Bohr (1920) Experiment Scientist Home Experimental Conclusions New Model Experiment Experiment Animation An important experiment was the study of the emission of light by excited hydrogen atoms. Next Experiment Summary Bohr Next • When atoms receive energy, electrons move way from nucleus (higher energy level) • When atoms release energy, electrons move toward nucleus (lower energy level) and release light Click for examples Energy Released Energy Added e- Absorbs energy Releases energy e- Experimental Conclusion • Electrons can only exist on definite energy levels that are specific distances from the nucleus. Bohr Next Bohr’s Model Name: Planetary Model Model Animation Description: • Positively charged nucleus in center of atom • Electrons moving around nucleus at specific energy levels (or distances from nucleus) Scientist Home Bohr Next Summary of Atomic Theory Summary Animation Scientist Home Atom Review Atom Review • Mass number = #protons + # neutrons • Atomic number = #protons= element • Charge= #protons - # electrons Mass Number 23 Atomic Number 11 +1 Charge Na Nuclear symbol Next Atom Review • Mass number = #protons + #neutrons • Average atomic mass = percent abundances of all the isotopes of an element 12 C 13 C 14 C The average atomic mass is on the periodic table: C 12.01 Next Sample Average Atomic Mass Calculation • Determine the average atomic mass of Ar if the isotopes are: 5% 79Ar and 95% 80Ar • Step 1: convert % to decimal • 0.05 and 0.95 • Step 2: multiply decimal by mass number • 0.05 X 79 = 3.95 • 0.95 x 80 = 76 • Step 3: add values together Scientist Home • 3.95 + 76 = 79.95 • Step 4: check that the value matches the value shown under Ar on the periodic table