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THE ATOM

THE ATOM. Part 1. Pg 65-74. The Atom:. From Philosophy to Science. Think with me…about sugar crystals, you can see that they are small crystals and every crystal is identical . You may grind these particles into a very fine powder, but each tiny piece is still sugar.

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THE ATOM

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  1. THE ATOM Part 1 Pg 65-74

  2. The Atom: From Philosophy to Science • Think with me…about sugar crystals, you can see that they are small crystals and every crystal is identical. • You may grind these particles into a very fine powder, but each tiny piece is still sugar. • If you dissolve the sugar in water, the sugar particles become virtually invisible. Pg 65-69 Sec 3-1

  3. The Atom: From Philosophy to Science • You could look at the dissolved sugar water under a microscope and you’d still not be able to see the sugar. • However, you know it is still there because you can taste it. • These kind of observations and logic patterns led ancient philoso- phers to ponder the design of the universe Pg 65-69 Sec 3-1

  4. The Atom: From Philosophy to Science • The two schools of thought philoso-phers were divided into were…is everything in the universe continuous and infinitely divisible, or is there a limit to how small you can get? • Particle theory was not a popular early opinion, but was supported as early as Democritus in ancient Greece. Pg 65-69 Sec 3-1

  5. The Atom: From Philosophy to Science • Democritus proposed that all the matter is composed of tiny particles called Atomos • These particles are indivisible • Aristotle did not believe in Democ-ritus’ atom, he was of the “matter is continuous” philosophy • Because of Aristotle’s popularity his idea was accepted as the better philosophy for 200 years Pg 65-69 Sec 3-1

  6. The Atom: From Philosophy to Science • In the 1700’s nearly all chemists accepted the modern definition of an element as a substance that is indivisible • It was also thought that elements combine to form compounds that are different in their properties than the elements that composed them Pg 65-69 Sec 3-1

  7. The Atom: From Philosophy to Science • There was controversy as to whether elements always combine in the same ratio when forming a particular compound. • In the 1790’s, chemistry was revolut-ionized by a new emphasis on quant- itative analysis because of new and improved balances Pg 65-69 Sec 3-1

  8. The Atom: From Philosophy to Science • This new technology led to the discovery of some new scientific laws • The Law of Conservation of Mass: • States that mass is neither created nor destroyed during ordinary chemical rxns or physical changes. • Which means the total mass of the reactants must equal the total mass of the products. Pg 65-69 Sec 3-1

  9. + = Carbon, C Oxygen, O Carbon Monoxide, CO Mass x Mass y Mass x + Mass y + Carbon, C Oxygen, O Carbon Monoxide, CO Mass x Mass y Mass x + Mass y Law of Conservation of Mass = Pg 65-69 Sec 3-1

  10. The Atom: From Philosophy to Science • The Law of Definite Proportions: • The fact that a chemical compound contains the same elements in exactly the same proportions by mass regardless of the size of the sample or the source of the compound • NaCl is NaCl no matter if it is table salt (small crystals) or rock salt (large crystals) Pg 65-69 Sec 3-1

  11. + 1 Carbon 2 Oxygen Law of Definite Proportions ALWAYS + = 1 Carbon 1 Oxygen Carbon Monoxide ALWAYS = Carbon Dioxide, CO2 Pg 65-69 Sec 3-1

  12. The Atom: From Philosophy to Science • The Law of Multiple Proportions: • If 2 or more different compounds are composed of the same 2 elements, then the ratio of the masses of the 2nd element combined with a certain mass of the 1st element is always a ratio of small whole numbers Pg 65-69 Sec 3-1

  13. + Carbon Oxygen Law of Multiple Proportions + = Carbon Oxygen Carbon Monoxide, 1:1 1 1 = Carbon Dioxide, 1:2 1 2 Pg 65-69 Sec 3-1

  14. Pg 65-69 Sec 3-1

  15. The Atom: From Philosophy to Science • In 1808, John Dalton proposed an explanation for each of the proposed laws • He reasoned that elements were composed of atoms & that only whole #’s of atoms can combine to form compnds • His ideas are now called the Atomic Theory of Matter Pg 65-69 Sec 3-1

  16. ELEMENT 1 ELEMENT 2 ELEMENT 3 ELEMENT 4 Pg 65-69 Sec 3-1

  17. + + Pg 65-69 Sec 3-1

  18. The Atom: From Philosophy to Science • Through these statements, evidence could be gathered to confirm or discount its claims • Not all of Dalton’s claims held up to the scrutiny of experimentation • Atoms CAN be divided into even smaller particles • Not every atom of an element has an identical mass Pg 65-69 Sec 3-1

  19. The Atom: From Philosophy to Science • Dalton’s Atomic Theory of Matter has been modified. • What remains is… • All matter is composed of atoms • Atoms of any one element differ in properties from atoms of another element Pg 65-69 Sec 3-1

  20. The Structure of the Atom • One of the disputed statements of Dalton was that atoms are indivisible • In the 1800’s it was determined that atoms are actually composed of several basic types of smaller particles • it’s the number and arrangement of these particles that determine the atom’s chemical properties. Pg 70-74 Sec 3-2

  21. The Structure of the Atom • The def. of an atom that emerged was, the smallest particle of an element that retains the chemical properties of that original element. • All atoms consist of 2 regions that contain the subatomic particles • The nucleus • The electron cloud around the nucleus Pg 70-74 Sec 3-2

  22. The Structure of the Atom • The nucleus is a very small region located near the center of the atom • In every atom the nucleus contains at least 1 proton, which is positively charged particle • Usually contains 1 or more neutral particles called neutrons Pg 70-74 Sec 3-2

  23. The Structure of the Atom • The electron cloud is the region that surrounds the nucleus • This region contains 1 or more electrons, which are negatively charged subatomic particles • The volume of the electron cloud is much larger than the nucleus Pg 70-74 Sec 3-2

  24. The Structure of the Atom • The 1st discovery of a subatomic particle took place in the late 1800’s. • electric current was passed through various gases at low pressures • The glass tubes the experiments were carried out in are called cathode ray tubes. Pg 70-74 Sec 3-2

  25. The Structure of the Atom • Investigators noticed that when electric current was passed through a cathode ray tube, the surface of the tube directly opposite the cathode glowed. • they decided that the glow was caused by a stream of particles • they called the stream a cathode ray Pg 70-74 Sec 3-2

  26. Electric Current Electric Current cathode ray cathode Cathode Ray anode Pg 70-74 Sec 3-2

  27. The Structure of the Atom • The ray traveled from the cathode to the anode when current was passed through the tube • Electric current instead flows from the anode to the cathode • Scientists began to cleverly study cathode rays and observed several remarkable characteristics. Pg 70-74 Sec 3-2

  28. The Structure of the Atom • An object placed in the path of the ray cast a shadow on the glass • A paddle wheel placed the path of the cathode ray began to spin • Cathode rays were deflected by a magnetic field • The rays were deflected away from a negatively charged object Pg 70-74 Sec 3-2

  29. The Structure of the Atom • The first 2 observations support the evidence that the cathode ray is composed of particles traveling through the depressurized gases • The second 2 observations support the evidence that the ray is composed of a substance that has a negative charge to it. Pg 70-74 Sec 3-2

  30. magnet cathode ray deflection due to magnetic field Pg 70-74 Sec 3-2

  31. Pg 70-74 Sec 3-2

  32. The Structure of the Atom • An English Physicist J.J. Thomson became the authority on cathode-rays • He measured the ratio of the charge of cathode-ray particles to their mass • He discovered the ratio was always the same regardless of the conditions His conclusion: all cathode-rays are composed of identical negatively charged particles a.k.a. electrons Pg 70-74 Sec 3-2

  33. The Structure of the Atom • Thomson’s experiments revealed that the electron has a very large charge in relation to its mass • In 1909, Robert Millikan, performed an ingenious experiment to calculate the mass of an electron • he calculated that the electron’s mass is 9.109x10-31kg Pg 70-74 Sec 3-2

  34. Pg 70-74 Sec 3-2

  35. The Structure of the Atom • Thomson’s & Millikan’s Ideas: • atoms are composed of smaller particles, and one of these compo-nents is negatively charged • atoms are neutral, so there must be an opposing (+) charge • because E’s are essentially mass-less there must be something else that accounts for the atoms mass. Pg 70-74 Sec 3-2

  36. The Structure of the Atom • These proposals led to an elemen-tary understanding of the compo-sition of the atom. • The atom might look something like a tiny chocolate chip cookie The chips would be the E’s The cookie part is (+) matter to cancel out the (-) chips Pg 70-74 Sec 3-2

  37. The Structure of the Atom • In 1886, E. Goldstein observed a cathode-ray tube and found rays traveling in the opposite direction of that of the cathode rays • The new rays are called canal rays and they are (+) charged • And the particles mass is about 2000 X’s that of the electron Pg 70-74 Sec 3-2

  38. The Structure of the Atom • In 1932, the English physicist James Chadwick discovered yet another subatomic particle. • the neutron has no electrical charge • It’s mass is nearly equal to the proton • Therefore the subatomic particles are the electron, proton, and neutron. Pg 70-74 Sec 3-2

  39. The Structure of the Atom • When subatomic particles were discovered, scientists wondered how they were put together in an atom. • This was a difficult question to resolve, given how tiny atoms are. • Most thought it likely that the atom resembled the chocolate chip cookie we discussed earlier Pg 70-74 Sec 3-2

  40. The Structure of the Atom • In 1911, Ernest Rutherford et al. provided a more detailed picture of the internal structure of the atom • In his experiment, Rutherford direct-ed a narrow beam of alpha particles at a very thin sheet of gold foil. • Alpha particles () are He atoms that have been stripped of its electrons Pg 70-74 Sec 3-2

  41. The Structure of the Atom • According to the prevailing theory, the alpha particles should have passed easily through the gold, with only a slight deflection • And mostly that’s how it happened. • However, they found 1 every 8000 particles had actually been deflected back toward the source. Pg 70-74 Sec 3-2

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  46. The Structure of the Atom • Based on the results, Rutherford suggested a new theory of the atom. • The atom is mostly empty space; • He concluded that all the (+) charge and almost all the mass is conc. in a small core in the center of the atom • The E’s surround the positively charged nucleus like planets surround the sun. Pg 70-74 Sec 3-2

  47. The Structure of the Atom • With the exception of Hydrogen, all nuclei contain 2 kinds of particles protons and neutrons • Proton has a (+) charge equal in mag-nitude to the (-) charge of an electron • Atoms are neutral because they con- tain equal #’s of protons & electrons • A neutron is electrically neutral Pg 70-74 Sec 3-2

  48. The Structure of the Atom • The nuclei of atoms of different elements differ in the # of protons they contain and therefore in the amount of (+) charge they possess. • The # of protons the atom contains determines the atom’s identity • Oxygen contains 8 protons • Fluorine contains 9 protons • Neon contains 10 protons Pg 70-74 Sec 3-2

  49. Properties of Subatomic Particles RELATIVE RELATIVE ACTUAL MASS (g) PARTICLE SYMBOL MASS CHARGE Pg 70-74 Sec 3-2

  50. The Structure of the Atom • The nucleus is composed of a coll-ection of protons, which are all (+) • Shouldn’t they repel each other; like charges and all? • When 2 protons are very close to each other, there is a strong force of attraction between them. • A similar attraction exists when neutrons are close too. Pg 70-74 Sec 3-2

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