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A History of Atomic Theory Atomic Models What is a model ? detailed, 3-D representation of an object - typically on smaller scale than original model airplane model car clothing model Let’s Take a Trip Through Time! It Started with the Greeks DEMOCRITUS • Lived in Greece 2500 years ago (460-370 BC) • “Father of modern science” Demacritus said: “All matter is made of atoms that are tiny, indestructible and indivisible.” Democritus Greek banknote with Democritus and drawing of atom Where did Democritus get his ideas for small, indestructible, indivisible atoms? Atoms are small… new rings old ring EVIDENCE: old gold rings wear away slowly, getting thinner and thinner, but you never see gold atoms on your finger, so gold atoms must be very small! Atoms are indestructible… EVIDENCE: • Earth has been around long time • Mountains wash away but rocks continue to exist • New plants grow where old plants die Atoms are indivisible… EVIDENCE: None This was HYPOTHESIS made by Democritus to explain nature as he saw it. What did Democritus think atoms looked like? Appearance of atoms assumed based on behavior: • Liquids pour • Solids are hard and rigid Liquids pour. Why? Greek idea of liquid atom If liquid atoms were like little balls they would roll out when you tip the container Liquids require a container Solids are rigid. Why? cocklebur plant Velcro® fastener cockleburs stick on clothing and each other Velcro hooks and loops little hooks on cockleburs Greek idea of solid atom Aristotle had different ideas Greek banknote and coin picturing Aristotle ARISTOTLE: • famous Greek philosopher, born 384 BC • student of Plato (another famous philosopher) • teacher of Alexander the Great (who later conquered the world) Aristotle’s idea of matter Aristotle did not believe Democritus’s idea of atoms was correct Aristotle believed all matter made from four elements: Earth Air Fire Water Aristotle’s idea of matter Aristotle was more famous than Democritus, so people believed him, even though he was wrong! Democritus’s idea of atomos (atoms) was lost for nearly 2000 years until John Dalton brought it back in 1803 Atom idea lost for ~2000 years John Dalton, New Atom,1803 Born in England, 1766 Studied chemistry, physics, and color blindness Brought back Democritus’s idea of an indivisible atom Color blindness Can you see a number in this box? If not, you may be color blind. (More males are color blind than females) Start writing - Foldable information Dalton’s Billard Ball Model Dalton’s Theory of Atoms Five parts (postulates): # 1: Elements are made of extremely small particles called atoms So according to Dalton’s theory: atoms are considered to be like billiard balls # 2: Atoms of given element are identical in size, mass, & other properties; Atoms of different elements are different in size, mass, & other properties (later found not to be exactly correct) #3: Atoms cannot be subdivided, created, or destroyed (later proved wrong) #4: Atoms of different elements combine in simple whole-number ratios to form chemical compounds (H2O 2:1 ratio H:O) #5: In chemical reactions, atoms are combined, separated and/or rearranged Ex. of a chemical reaction: + + + 2H2O + 2Na 2NaOH + H2 Dalton’s Chemical Symbols We use different symbols today Thomson’s Plum Pudding Model J.J. Thomson English physicist (1856-1940) won Nobel Prize in 1906 1897: discovered ELECTRON - studied cathode rays using Crooke’s tube - showed atoms were divisible Crooke’s tube cathode rays: (stream of electrons) move from metal cathode (on left) to cross-shaped anode (on right) cross-shaped shadow ↓ cathode ↓ cross-shaped anode ↑ cross-shaped shadow is cast on glass (on right) shows … electrons are particles smaller than an atom and have negative charge Cathode Ray Tube Experiment thought cathode rays were streams of particles smaller than atoms magnetic fields deflect beam of charged particles JJ Thomson’s discovery of (-) charged electrons proved that atoms were divisible! Thomson’s “Plum pudding” atom ENGLISH PLUM PUDDING – electron raisin Thomson’s Plum Pudding Model: (+) charge evenly spread out while… (-) charge found in small bits – like raisins in plum pudding (or chocolate chips in cookies) More About Charge… opposite charges attract, like charges repel RULE: Matter is NEUTRAL (no charge) so… for every one (-) charged electron there must be one something of (+) charge What do we mean by charge? MATTER IS NEUTRAL (no charge): • (+) charges equal (-) charges • charges cancel out each other ELECTRONS are negatively charged (-1) PROTONS are positively charged (+1) ELECTRICAL CHARGE - property of matter: • # electrons greater or less than # protons batteries have (+) and (-) ends Rutherford’s Nuclear Model Ernest Rutherford 1871-1937 • 1908: won Nobel Prize (Chemistry) • 1920: discovered proton discovered that most of mass of atom is found in the center (nucleus) Rutherford’s Experiment – 1911 Rutherford’s exp’t: animation Rutherford’s Gold Foil Experiment Rutherford tested Thomson’s plum pudding model of atom: • sent tiny radioactive alpha (a) particles, like bullets, towards thin sheet of gold foil Rutherford gold foil experiment expected a particles to pass straight through, like this… most a particles went straight through without hitting anything, BUT a few were deflected, this means a particles must have hit something: really heavy OR (+) charged gold foil experiment scattering results showed: Plum pudding model of atom was INCORRECT, so… Rutherford devised new model that fit his results… proposed Nuclear Model Rutherford concluded: • all atoms have dense, positive (+) center (center contains most of mass of atom) • atom is mostly empty space (except very tiny (–) electrons; but didn’t say where are) electrons (–) / nucleus / [protons (+)] Problems with Rutherford’s Model did not speculate how electrons arranged around (+) center to prevent (-) electrons from being attracted to (+) nucleus, electrons must orbit nucleus like Earth orbits sun but… e- need energy to orbit nucleus & Rutherford didn’t address this name nucleus came later So how BIG is the nucleus compared to the entire atom? If atom is as big as football stadium… the nucleus is smaller than a flea on 50-yard line! If atom is as big as a period at the end of a sentence in textbook, it would have the mass of 70 cars! . So how big is an atom? most atoms are 1-2 angstroms across 1 Å = 1 X 10-10 m 6 Billion Cu atoms in a line = less than 1 meter! Can we “see” an atom? NO! James Chadwick 1891-1974 worked with Rutherford 1932: discovered neutron 1935: Nobel Prize (Physics) Bohr’s Planetary Model Niels Bohr 1885 - 1962 created quantized atom model, 1915 worked on Manhattan Project won Nobel Prize in 1922 Bohr’s model: e- only move from one energy level to another in atom: • e- absorb energy when jump from lower to higher levels • e- emit energy when jump from higher to lower levels Bohr’s new atomic model addressed issue of atom’s nucleus surrounded by electrons: : e- have definite orbits e- orbiting at different energy levels e- energy levels are quantized e- only move by jumping levels (n = 1, n = 2, etc.) electrons: • absorb energy when they jump away from nucleus • emit energy when they jump towards nucleus Bohr’s Planetary Model • electrons travel only in specific orbits • each orbit has definite energy • inner most orbit (n=1): least energy • outer most orbit (n=7): most energy • atoms emit radiation when e- jumps from outer orbit to inner orbit (photons of light) • e – in outermost orbits determine atom’s chemical properties Modern Wave Mechanical Model Erwin Schrödinger Austrian scientist (1887-1961) • 1933: won Nobel Prize (Physics • calculated wave model of hydrogen atom (1926) • Today’s atomic model called: • modern model • quantum mechanical model • wave model • cloud model Schrödinger’s wave model of atom: • e- behaves as: energy wave AND as matter particle If we could see an electron’s path it might look like this “cloud” (light also behaves as particle and wave) Einstein had predicted that energy and matter were related in his equation E = mc2 Modern/Wave/Cloud Model electron’s energy is quantized (specific values) electrons in probability zones called “orbitals”, not orbits - location cannot be pinpointed e- considered as both particles & waves electrons move around nucleus at speed of light (3 x 108m/sec) Orbitals A Missing Particle – The Neutron 1932: James Chadwick • discovered last major piece of atom: neutron (“neutral”, no charge) 1935: won Nobel Prize (Physics) • neutron& proton weigh about same while electron weighs almost nothing (1891-1974) The Modern Model an atom is: • mostly empty space • nucleus: most of atom’s mass • nucleus contains protons & neutrons • electrons in energy levels around nucleus • electrons jump between levels, emitting & absorbing energy as jump Atomic Theory JJ Thomson Democratus 400 BC 1803 John Dalton 1904 Niels Bohr 1910 Ernest Rutherford 1913 1926 Schroedinger / Heisenberg The development of atomic theory represents the work of many scientists over approximately 100 years Next Atomic Theory ? Which one of you will discover something new to add to the atomic model of the atom and win a Nobel Prize in the future?