Chapter 4 – History Of The Atom

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Chapter 4 – History ofthe Atom

This is myglobe. Hisname is Globey.Democritus460 – 370 BCn The first to propose the ideaof an atomIndivisiblen Indestructiblen Atom comes from the Greekword atomos, which means“indestructible”n Everything is made of themn

First Concept of an Atom

Unfortunately n Aristotle: opposedDemocritus and said allmatter is a combo ofEarthn Airn Firen Watern n Why? BecauseDemocritus had noproof.

What aboutthisDemocritusfellow?Some dude (Plato?)Oh, don’tlistento him, he craycray!Aristotle

I may be crazy, butI’m right! Timewill tell.

Evidence for a new Atomic Theory:1.Law of conservation of mass (matter): Mass isneither created or destroyed in a chemical reaction.2.The law of Definite proportions: Chemicalcompounds always have the same proportion ofelements no matter how much of that compound youhave.2.3.Example: water is always 2 parts Hydrogen to 1 part OxygenThe law of Multiple proportions: When two elementsform compounds the ratio of their masses alwaysform small whole numbers.Example: Carbon and Oxygen combine to form CO orCO2, but never C0.5O1

John Dalton 1766-1844n Do you likemy fancypants? Youdo? They’recalled nickers.Dalton’s Atomic Theory of Matter:n Each element is composed of extremelysmall particles called atoms that arecannot be dividedn All the atoms of a given element areidentical to each other, but they differfrom those of any other elementn Atoms are neither created nordestroyed in any chemical reaction, justrearrangedn Atoms bond together to formcompounds. A given compound alwayshas the same relative numbers andkinds of atoms.

JJ Thompson 1856-1940I’m notTeddyRoosevelt. Discovered electron 1897 –Cathode Ray Experiment Electrons are negatively chargedparticles in an atom.

The Cathode Ray Tube Experimentvideo

JJ Thompson 1871-1937Very interesting, there seemsto be both positive andnegative forces workingwithin an atom.I really need a girlfriend. Plum Puddingmodel 1904 Electrons in a soupof positive chargesDiscovered isotopes1913

Plum Pudding Model

Robert Millikann In 1909 Robert Millikanmeasured the exact electroniccharge and mass of an electronin his Oil Drop Experiment.How did Icome up withthis device?MAGIC, that’show.

The Oil Drop Experimentn Millikan concluded that: The mass of an electronis 9.10939 x 10-28 g Which is 1840times smaller thanthat of the Protonn The electroniccharge on an electronis 1.60 x 10-19 CThe relative charge onan electron is -1 (relativeto a proton which is 1)video

Ernest Rutherford 1871-1937I’m kinda cool.No, really. Doyou want to seemy foilcollection?n Nuclear Model 1910n Gold Foil Experiment

Gold Foil Experiment Alpha particles scatter from the gold foil.

Rutherford’s Model (cont)n Rutherford concluded that n The Atom is mostly empty spacen Has a region called the nucleus that n is positively chargedn contains most of the mass of the atomn The electrons are very small and orbit thenucleus in fixed paths.

Rutheford’s Model (cont)This model is called the Nuclear Atom orNuclear Model

If we drew the atom to scale and made protons and neutrons acentimeter in diameter, then the electrons would be less thanthe diameter of a hair and the entire atom's diameter would begreater than the length of thirty football fields! 99.999999999999% of an atom's volume is just empty space!

“Discovery” of ProtonIf there are negative charges in the atom,there must be positive charges balancingthem out PROTONSn Ernest Rutherford gets credit for theproton because he named it.n Moseley discovered atomic numbern

Discovery of the Neutronn n In 1932, the English physicist JamesChadwick (1891–1974) confirmed theexistence of yet another subatomicparticle: the neutron.Neutrons:n Located in nucleus with protonn Have no chargen mass nearly equal to a proton.NEUTRon.Because It’sNEUTRAL, yousee. Neutral getit?

Niels Bohr 1885-1962n n n Planetary Model 1913n Nucleus surrounded byorbiting electrons atdifferent energy levelsn Electrons have definiteorbitsUtilized Planck’s QuantumEnergy theoryWorked on the ManhattanProject (US atomic bomb)I’m not bohring.Get it? BOHRing?.I’ll just be over hereexciting electrons

Bohr Model for Nitrogen

Ernst Schrödinger 1887-1961n Quantum Mechanical Model 1926n Schrodinger: Used math todescribe the likely hood that anelectron will be found in aparticular place. (probability)Electrons do not follow a fixed path.n Electrons are in electron cloudscalled “orbitals”.n The denser parts of the clouds arewhere they are more likely to befound.n Can I borrowyour cat for abit?

Quantum Mechanical Theory

Werner Heisenberg 1901-1976n n Uncertainty Principle:You cannot know the locationand the momentum of anelectron at the same time.You cannot know exactly wherean electron will be.n Just where it might be based onprobability.n I don’t know howfast I’m going but Iknow where I am.

Quantum Model of the Atom

Atomic TheoryJJThomsonDemocratus400 9131926Schroedinger/ Heisenberg

If we drew the atom to scale and made protons and neutrons a centimeter in diameter, then the electrons would be less than the diameter of a hair and the entire atom's diameter would be greater than the length of thirty football fields! 99.999999999999% of an atom's volume is just empty space! “

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