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Unit 3 The Atomic Theory

Explore the historical development of the atomic theory from Democritus to modern scientists like Dalton, Thomson, and Rutherford. Learn about the structure of atoms, conservation laws, and key experiments in atomic theory evolution.

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Unit 3 The Atomic Theory

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  1. Unit 3 The Atomic Theory Pre-Assessment – Draw a colorful model of what you think the structure of the atom looks like. This is your model from past science classes and what you know the structure of the atom to be.

  2. Origin of the term “Atom” The word “atom” comes from the Greek word atomos. The word atom means Unable to be cut, something that cannot be divided further.

  3. Ancient Theories of the Atom Democritus’ (450 BC) “all matter that makes up the world around us is actually composed of tiny invisible particles. (atomos)”

  4. Antoine Lavoisier Elements as a simple substance cannot be broken down. (1777) From this he devised a theory of the formation of chemical compounds from elements. Law of Conservation of Matter – Matter is neither created nor destroyedin a chemical reaction.

  5. *was beheaded at the height of the French Revolution. Law of Conservation of Matter:

  6. 18th Century Theories:Proust • Law of Constant Composition: • (1799) Joseph Louis Proust found that a given compound always contains the same elements in the same proportions by mass. A given compound always contains the same elements in the same proportions by mass.

  7. Modern Theories of the Atom John Dalton (1805) used what was known about the atom to establish the Atomic Theory of Matter

  8. Dalton’s Theory 4 postulates #1 – Each element is composed of extremely small particles called atoms.

  9. Dalton’s Theory #2 – All atoms of a given element are identical , but they differ from those of any other element.

  10. Dalton’s Theory #3 – Atoms are neither created nor destroyed in any chemical reaction.

  11. Dalton’s Theory #4 – A given compound always has the same relative numbers and kinds of atoms.

  12. Unit 3 The Atom Theory Wednesday 10/21 Intro the Atom with video and outlining Chapter 3. Thursday 10/22 Guiding Readings on JJ Thomson and Ernest Rutherford for Performance Task Friday 10/23 Discussion on JJ Thomson and Ernest Rutherford Monday 10/26 Performance Task #1 on Gold Foil Experiment Wednesday 10/28 Subatomic Particles

  13. What was the view of the model before the Cathode Ray Tube Experiment?

  14. JJ Thomson’s ModelThe Plum Pudding Model Diffuse cloud of (+) charge with (-) charges throughout.

  15. He discovered the first subatomic particle – the electron J.J. Thomson (IN 1896)

  16. http://highered.mcgraw-hill.com/sites/0072512644/student_view0/chapter2/animations_center.html#http://highered.mcgraw-hill.com/sites/0072512644/student_view0/chapter2/animations_center.html# The stream of electrons was attracted to the + pole of the magnet and repelled by the (-) pole of the magnet. Using the charge to mass ratio he was able to calculate the mass to being 2000x lighter than a hydrogen atom (smallest atom known).

  17. Summarize JJ’s Work Because the particles in the cathode ray tube were much lighter than the smallest known element he concluded that these are electrons that are (-) charged. He knew that positive charges also had to be present so he came up with the Plum Pudding Model.

  18. Robert Millikan - 1909 • He measured the charge of an electron. http://highered.mcgraw-hill.com/sites/0072512644/student_view0/chapter2/animations_center.html#

  19. New Zealand Scientist Ernest Rutherford(1871-1937) Had performed experiments to find that atoms were made of positively charged particles moving around a very tiny positively charged nucleus.

  20. Rutherford’s Gold Foil experiment He did an famous experiment whereby he shot a stream of alpha particles at a thin piece of gold (Au) foil expecting most of the particles to be deflected-instead most of them passed directly through the foil and onto the photographic film

  21. If JJ Thomson’s model was correct Rutherford’s Model

  22. http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/ruther14.swfhttp://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/ruther14.swf Rutherford experiment animation Rutherford’s Atomic Structure Model + =positively charged particles - = electrons

  23. Comparison of Atomic Models Dalton’s Model Thomson’s Model Rutherford’s Model

  24. YOU WILL NEED: • NOTEBOOK • PERIODIC TABLE

  25. Protons (positive) are bigger than electrons. The part of an atom that gives an element its identity. Neutrons (neutral) are slightly bigger in mass than a proton. Electrons (negative) charge, have a mass of close to zero-negligible

  26. Atomic Mass Units (amu): roughly equal to the mass of a proton or neutron. **the mass of an atom is measured in amu’s 1 amu = 1.66x10-24g

  27. Every element has a unique atomic number which is the number of protons in the atom. • Elements atomic number is just above the chemical symbol on periodic table. • Atoms are electrically neutral-means number of protons always equal number of electrons.

  28. An Element’s atomic number also indicates number of electron in its atoms. Electron Number

  29. Ions Ions are atoms that have lost or gained one or more electrons giving them apositiveor negative charge!

  30. If an ion gains an electron it has a negative charge. If an ion loses an electron it has a positive charge. For example: Na (Sodium), has originally 11 electrons but when an electron is lost it becomes a positive ion. Na+

  31. A neutral magnesium atom (atomic number=12) has 12 protons/electrons. If it loses 2 electrons it becomes an ion with a charge of 2+. Number of protons 12 Number of electrons - 10 Charge of Ion 2+

  32. Ion Electrons Protons 19. Cu²+ 20. As3- 29 33 27 36

  33. Isotopes are atoms that have the same number of protons but different numbers of neutrons • Most elements in the first two rows of the periodic table have at least 2 isotopes with one being more common than the other • In nature, elements are almost always found as a mixture of isotopes Isotopes

  34. Differences between isotopes? • Isotopes react in the same way as others of the same element

  35. Atomic mass: weighted average of the masses of the existing isotopes of an element. Atomic Number/ number of protons Atomic mass

  36. What was Dalton’s 2nd Postulate? • Was it correct? Hmmm

  37. Isotopes of Hydrogen • The most common isotope of hydrogen has no neutrons at all • There's also a hydrogen isotope called deuterium, with one neutron, and another, tritium, with two neutrons.

  38. To identify an isotope more specifically, chemists add a number after the elements name. ex. Carbon-11 Carbon-12 Carbon-14 • This number is called the isotope’s mass number and is the sum of the isotope’s number of protons and neutrons. • What is the atomic mass on your Periodic Table? • Which of the 3 isotopes of Carbon is the most abundant?

  39. 141 Mass number Ba2+ Complete Chemical Symbols 56 Charge Atomic number Number of protons = Atomic # Number of neutrons = Mass # - Atomic # Charge = Atomic # – Number of electrons

  40. Complete the Table 90

  41. WARM UP • What is the atomic mass unit of a proton? Neutron? Electron? • What is the atomic mass of an element represent?

  42. Complete the Table 90

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