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Dalton's Model of Atom

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Science Performance Task

  1. 1. …Today we will discuss about “Dalton’s Model of Atom”
  2. 2. It was in the early 1800s that John Dalton,an observer of weather and discoverer of colorblindness among other things, came up with hisatomic theory. Lets set the stage for Daltonswork. Less than twenty years earlier, in the1780s, Lavoisier ushered in a new chemical era bymaking careful quantitative measurements whichallowed the compositions of compounds to bedetermined with accuracy. By 1799 enough datahad been accumulated for Proust to establish theLaw of Constant Composition ( also called the Lawof Definite Proportions).
  3. 3. In 1803 Dalton noted that oxygenand carbon combined to make twocompounds. Of course, each had its ownparticular weight ratio of oxygen tocarbon (1.33:1 and 2.66:1), but also, forthe same amount of carbon, one hadexactly twice as much oxygen as theother. This led him to propose the Law ofSimple Multiple Proportions, which waslater verified by the Swedish chemistBerzelius. In an attempt to explain howand why elements would combine with oneanother in fixed ratios and sometimesalso in multiples of those ratios, Daltonformulated his atomic theory.
  4. 4. The idea of atoms had been proposedmuch earlier. The ancient Greek philosophershad talked about atoms, but Daltons theorywas different in that it had the weight ofcareful chemical measurements behind it. Itwasnt just a philosophical statement thatthere are atoms because there must be atoms.His atomic theory, stated that elementsconsisted of tiny particles called atoms. Hesaid that the reason an element is pure isbecause all atoms of an element were identicaland that in particular they had the same mass.He also said that the reason elements differedfrom one another was that atoms of eachelement were different from one another; inparticular, they had different masses.
  5. 5. He also said that compoundsconsisted of atoms of different elementscombined together. Compounds are puresubstances (remember they cannot beseparated into elements by phase changes)because the atoms of different elementsare bonded to one another somehow,perhaps by hooks, and are not easilyseparated from one another. Compoundshave constant composition because theycontain a fixed ratio of atoms and each atomhas its own characteristic weight, thusfixing the weight ratio of one element tothe other. In addition he said that chemicalreactions involved the rearrangement ofcombinations of those atoms.
  6. 6. So that, briefly, is Daltons theory.With modifications, it has stood up prettywell to the criteria that we talked aboutearlier. It did not convince everyone rightaway however. Although a number ofchemists were quickly convinced of thetruth of the theory, it took about a halfcentury for the opposition to die down, orperhaps I should say die off. Daltonsmodel was that the atoms were tiny,indivisible, indestructible particles andthat each one had a certain mass, size,and chemical behavior that wasdetermined by what kind of element theywere.
  7. 7. However, only 3 or 4 pages in the thirdchapter discussed the atomic theory heproposed. In this theory, there are four basicideas...1) chemical elements are made of atoms.2) the atoms of an element are identical in theirmases3) atoms of different elements have differentmasses4) atoms only combine in small, whole numberratios such as 1:1, 1:2, 2:3 and so on.5) atoms can be neither created nor destroyed
  8. 8. This is the Law of Multiple Proportions. Simply put, whenelements combine to form compounds they do so in whole numberratios. Carbon and Oxygen can form either CO ( a 1:1 ratio) or CO2(a 1:2 ratio). It also states that if the mass of the first element isconstant the second elements masses would be in a whole numberratio.1.00g of Carbon will have combined with 1.33g in CO1.00g of Carbon will have combined with 2.66g in CO2The mass are in a 1:2 ratio Simply put if combinations are in whole number ratios, theremust be a one. This one being an atom. Here are Daltonsassumptions.All matter is composed of atoms Atoms cannot be made or destroyed All atoms of the same element are identical Different elements have different types of atoms Chemical reactions occur when atoms are rearranged
  9. 9. Dalton’s atomic model is one of thefundamentals of physics and chemistry. This theoryof atomic composition was hypothesized andpartially confirmed by the English chemist andPhysicist John Dalton. Dalton came with his Atomictheory as a result of his research into gases. Hediscovered that certain gases only could becombined in certain proportions even if twodifferent compounds shared the same commonelement or group of elements. Through deductivereasoning and experimentation, he made aninteresting discovery. His findings led him tohypothesize that elements combine at the atomiclevel in fixed ratios. This ratio would naturallydiffer in compounds due to the unique atomicweights of the elements being combined.
  10. 10. This was a revolutionary idea butfurther experimentation by himself andothers confirmed his theory. Thefindings became the basis of ofDalton’s Atomic Laws or Model. Theselaws focus on five basic theorems.First, Pure Elements consist ofparticles called atoms. Second,atoms ofan element are all the same for thatelement. That means gold is gold andoxygen is oxygen down to the last atom.Third, atoms of different elements canbe told apart by their atomic weights.Fourth, atoms of elements unite toform chemical compounds. Finally,atoms can neither be created ordestroyed in chemical reaction. Thegrouping only changes.
  11. 11. The last of Dalton’s AtomModel were at the timeconsidered true for all reactionsinvolving atoms. This was latercorrected with the discovery ofnuclear fission and fusion. So wenow know that this only holds truefor chemical reactions.
  12. 12. Like other scientific theories,Dalton’s atomic model has beenperfected over time with the researchand discoveries of other scientists. Wenow know that the atom can be dividedinto even smaller particles and we haveeven discovered the actual internalatom structure, even able to view itthrough modern technology. We nowknow that atomic weight is a product ofthe structure of the atoms as well.
  13. 13. This atomic theory made possible modernchemistry and physics. Up until Dalton’s time theatom was only considered to a philosophicalconstruct passed down by the ancient Greeks.Dalton’s ground breaking work made theoryreality. This understanding the atom helped tofuel many other discoveries such as thefundamental forces and Einstein’s theory ofrelativity. It is especially is important when onegoes into Quantum physics a discipline that looksat physics at the atomic and subatomic levels.
  14. 14. "A series of Essays read before this society and afterwards published in the 5th Vol. of their Memoirs gradually led me to the consideration of ultimateparticles or atoms & their combinations. Under the date of September 3rd, 1803, I find in my notebook Observations on the Ultimate Particles of Bodies andtheir Combinations, in which the atomic symbols I stilluse [were] introduced. On the 23rd of October the same year[I] [r]ead my Essay on the absorption of gases [by water] at the conclusion of which a series of atomic [weights] was given for 21 simple and compound elements .. . . "- John Dalton (Excerpt from paper read to Manchester Literary and Philosophical Society, 1830)
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