Ch8 the atom

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Text of Ch8 the atom

  • 1. THE ATOMTHE ATOM

2. D e v e l o p i n g o u rD e v e l o p i n g o u r m o d e l o f am o d e l o f a n a t o mn a t o m 3. D e m o c r i t u sD e m o c r i t u s A Greek philosopher - in the year 400A Greek philosopher - in the year 400 B.C. described the atom as the smallestB.C. described the atom as the smallest particle of a substance.particle of a substance. He used the wordHe used the word atomos (which meansatomos (which means not to cut)not to cut) to describe the smallestto describe the smallest possible particle of matter.possible particle of matter. Neither Plato nor Aristotle accepted theNeither Plato nor Aristotle accepted the atomic concept.atomic concept. 4. J o h n D a l t o nJ o h n D a l t o n Developed theDeveloped the first modernfirst modern atomic theory inatomic theory in 1803. Dalton is1803. Dalton is referred to as thereferred to as the father of modernfather of modern atomic theory.atomic theory. 5. Daltons Atomic TheoryDaltons Atomic Theory 1) All matter is made of atoms. Atoms are1) All matter is made of atoms. Atoms are indivisible and indestructible.indivisible and indestructible. 2) All atoms of a given element are identical in2) All atoms of a given element are identical in mass and propertiesmass and properties 3) Compounds are formed by a combination of3) Compounds are formed by a combination of two or more different kinds of atoms, whichtwo or more different kinds of atoms, which combine in whole number ratios.combine in whole number ratios. 4) Atoms can neither be created or destroyed.4) Atoms can neither be created or destroyed. 6. Daltons Atomic TheoryDaltons Atomic Theory Modern atomic theory is, of course, a little moreModern atomic theory is, of course, a little more involved than Dalton's theory but the basics ofinvolved than Dalton's theory but the basics of Dalton's atomic concept remains valid.Dalton's atomic concept remains valid. We cetainly know today that atoms can be destroyedWe cetainly know today that atoms can be destroyed by nuclear reactions and by bombarding the nuclei ofby nuclear reactions and by bombarding the nuclei of atoms in high speed atomic accelerators (that bust theatoms in high speed atomic accelerators (that bust the nuclei apart), but not by chemical reactions.nuclei apart), but not by chemical reactions. There are different kinds of atoms (that haveThere are different kinds of atoms (that have different masses) for a particular element that aredifferent masses) for a particular element that are known asknown as isotopes,isotopes, but isotopes of an element havebut isotopes of an element have the same chemical properties.the same chemical properties. Many heretofore unexplained chemical phenomenaMany heretofore unexplained chemical phenomena were quickly explained by Dalton with his theory.were quickly explained by Dalton with his theory. Dalton's theory quickly became the theoreticalDalton's theory quickly became the theoretical foundation in chemistry.foundation in chemistry. 7. CATHODE RAY TUBECATHODE RAY TUBE DISCOVERED IN LATE 1800sDISCOVERED IN LATE 1800s Cathode-ray tubes contain a pair of metal plates sealed into a glass tube that has been partially evacuated. If the residual pressure of the gas is small enough, the glass at the end of the tube across from the cathode will glow when the tube is connected to a series of batteries. 8. J.J. THOMSON AND THEJ.J. THOMSON AND THE DISCOVERY OF THE ELECTRONDISCOVERY OF THE ELECTRON In 1897, J. J. Thomson foundIn 1897, J. J. Thomson found that the cathode rays can bethat the cathode rays can be deflected by an electric field,deflected by an electric field, as shown at right. Byas shown at right. By balancing the effect of abalancing the effect of a magnetic field on a cathode-magnetic field on a cathode- ray beam with an electricray beam with an electric field, Thomson was able tofield, Thomson was able to show that cathode "rays" areshow that cathode "rays" are actually composed ofactually composed of particles. This experimentparticles. This experiment also provided an estimate ofalso provided an estimate of the ratio of the charge to thethe ratio of the charge to the mass of these particles.mass of these particles. The cathode rays also can be deflected by an electric field in a direction which suggests they are negatively charged. From: http://chemed.chem.purdue.edu/genchem/history/electron.html 9. J.J. THOMSON AND THEJ.J. THOMSON AND THE DISCOVERY OF THE ELECTRONDISCOVERY OF THE ELECTRON Thomson found the sameThomson found the same charge-to-mass ratiocharge-to-mass ratio regardless of the metal used to make the cathode andregardless of the metal used to make the cathode and the anode. He also found the same charge-to-mass ratiothe anode. He also found the same charge-to-mass ratio regardless of the gas used to fill the tube. He thereforeregardless of the gas used to fill the tube. He therefore concluded that the particles given off by the cathode inconcluded that the particles given off by the cathode in this experiment are a universal component of matter.this experiment are a universal component of matter. Although Thomson called these particlesAlthough Thomson called these particles corpusclescorpuscles,, the namethe name electronelectron, which had been proposed by, which had been proposed by George StoneyGeorge Stoney several years earlier for theseveral years earlier for the fundamental unit of negative electricity, was soonfundamental unit of negative electricity, was soon accepted.accepted. From: http://chemed.chem.purdue.edu/genchem/history/electron.html 10. THOMSONS PLUM PUDDINGTHOMSONS PLUM PUDDING MODEL OF THE ATOMMODEL OF THE ATOM Since it was known at the time that atoms are neutral,Since it was known at the time that atoms are neutral, there must be a positive charge to offset the negativethere must be a positive charge to offset the negative charge of the electrons. Thecharge of the electrons. The plum pudding modelplum pudding model ofof the atom was proposed by J. J. Thomson (thethe atom was proposed by J. J. Thomson (the discoverer of the electron in 1897).discoverer of the electron in 1897). The plumThe plum pudding model was proposed in 1906pudding model was proposed in 1906 by Thomsonby Thomson before the discovery of the atomic nucleus. In thisbefore the discovery of the atomic nucleus. In this model, the atom is composed of electrons (whichmodel, the atom is composed of electrons (which Thomson still called corpuscles), surrounded by a soupThomson still called corpuscles), surrounded by a soup of positive charge to balance the electron's negativeof positive charge to balance the electron's negative charge, like plums surrounded by pudding. Thecharge, like plums surrounded by pudding. The positive pudding was supposedly massless, thus sincepositive pudding was supposedly massless, thus since the electron had a mass of 1/1840 of a hydrogen atom,the electron had a mass of 1/1840 of a hydrogen atom, there would have to be 1840 electrons embedded in thethere would have to be 1840 electrons embedded in the positive pudding to make up a hydrogen atom.positive pudding to make up a hydrogen atom. 11. Millikans 1909 Oil Drop ExperimentMillikans 1909 Oil Drop Experiment 12. MILLIKANS OIL DROP EXPERIMENTMILLIKANS OIL DROP EXPERIMENT AND THE CHARGE AND MASS OF ANAND THE CHARGE AND MASS OF AN ELECTRONELECTRON Millikan determined the fundamental charge ofMillikan determined the fundamental charge of an electron to be -1.60 x 10an electron to be -1.60 x 10-19-19 Coulombs.Coulombs. He also determined the mass of an electron toHe also determined the mass of an electron to be 9.11 x 10be 9.11 x 10-31-31 Kg (about 1/1840 the mass of aKg (about 1/1840 the mass of a hydrogen atom).hydrogen atom). 13. ERNEST RUTHERFORD AND THEERNEST RUTHERFORD AND THE DISCOVERY OF THE ATOMICDISCOVERY OF THE ATOMIC NUCLEUSNUCLEUS Ernest Rtherford is the BritishErnest Rtherford is the British physicist who inphysicist who in 1911 determined1911 determined that the atom had a small densethat the atom had a small dense nucleus.nucleus. Starting sometimeStarting sometime around 1909, Rutherford beganaround 1909, Rutherford began to notice that alpha particlesto notice that alpha particles would not always behave inwould not always behave in accordance to theaccordance to the plum puddingplum pudding model of an atom when fired atmodel of an atom when fired at a piece of gold foil. Thesea piece of gold foil. These observations stimulated furtherobservations stimulated further research that was eventuallyresearch that was eventually published in 1911 and has beenpublished in 1911 and has been known ever since asknown ever since as Rutherford's Gold FoilRutherford's Gold Foil ExperimentExperiment ((http://myweb.usf.edu/~mhight/goldfoil.htmlhttp://myweb.usf.edu/~mhight/goldfoil.html).). 14. RUTHERFORD (cont.)RUTHERFORD (cont.) Rutherford bombarded gold foil with radioactiveRutherford bombarded gold foil with radioactive particles (alpha particles-positively charged). Heparticles (alpha particles-positively charged). He expected the particles to pass right through (and mostexpected the particles to pass right through (and most of them did) and was surprised that a few of the alphaof them did) and was surprised that a few of the alpha particles were scattered at high angles.particles were scattered at high angles. From this work Rutherford determined that the alphaFrom this work Rutherford determined that the alpha particles were deflected by a positively charged denseparticles were deflected by a positively charged dense nucleus within a gold atom. So atoms must have anucleus within a gold atom. So atoms must have a small, dense, positively charged nucleus.small, dense, positively charged nucleus. 15. RUTHERFORDS GOLD FOILRUTHERFORDS GOLD FOIL EXPERIMENTEXPERIMENT 16. RUTHERFORDSRUTHERFORDS INTERPRETATION OF RESULTSINTERPRET