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Atoms: The Building Blocks of Matter

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This chapter explores the philosophical ideas and scientific theories about atoms, including the differences between Democritus and Dalton, the structure of the atom, and counting atoms.

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Atoms: The Building Blocks of Matter

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  1. Chapter 3 Atoms: The Building Blocks of Matter

  2. Sect. 3-1: The Atom: From Philosophical Idea to Scientific Theory • Democritus vs. Aristotle • Atom vs. infinitely divisible • Aristotle’s idea won out for ~ 2000 yr. • Problem with both ideas…

  3. Foundations of Atomic Theory • Law of conservation of mass • Mass not created or destroyed • Law of definite proportions • Same compound always same ratio of elements • Water is always 2 H and 1 O

  4. Law of multiple proportions • Compounds of same elements combine in different ratios • Carbon dioxide (1 C : 2 O) and carbon monoxide (1 C : 1 O)

  5. Dalton’s Atomic Theory • Atoms • Atoms of an element are the same (mass, size, etc), atoms of different atoms are different • Atoms can’t be divided • Atoms combine in ratios to form compounds • Atoms combine, separate, or rearrange during a chemical reaction

  6. What’s the difference between Democritus and Dalton??? • Experimental Data!!!

  7. Modern Atomic Theory • What’s no longer held as true from Dalton’s theory? • Atoms being divisible • Atoms of same element having same mass

  8. Sect. 3-2: The Structure of the Atom • Atoms made up of: • Protons • Neutrons • Electrons

  9. Discovery of the Electron • Cathode Ray Tube experiments concluded that the “cathode rays” had mass and a negative charge http://images.google.com/imgres?imgurl=http://www.hcc.mnscu.edu/programs/dept/chem/abomb/Thomson_ExpBoth.jpg&imgrefurl=http://www.hcc.mnscu.edu/programs/dept/chem/abomb/page_id_15138.html&h=342&w=512&sz=35&hl=en&start=44&tbnid=niL-YcGGzm9sqM:&tbnh=88&tbnw=131&prev=/images%3Fq%3Dcathode%2Bray%2Btube%26start%3D36%26gbv%3D2%26ndsp%3D18%26svnum%3D10%26hl%3Den%26sa%3DN

  10. J.J. Thomson measured ratio of charge to mass for these particles • concluded particles were the same for all elements • Cathode-ray particles later named electrons

  11. Millikan determined exact mass of electron (about 1/1837 mass of hydrogen atom) • Confirms negative charge of electron • Inferences made: • must be something positive to balance negative • Atoms must have more particles due to tiny mass of electron and much larger mass of atom

  12. Discovery of Atomic Nucleus • Rutherford’s gold foil experiment http://www.dlt.ncssm.edu/TIGER/diagrams/structure/GoldFoilExperiment.jpg

  13. Significance of Rutherford’s experiment: • Nucleus of the atom • Very dense • Positive charge • Nucleus = marble, whole atom = football field

  14. Composition of the Atomic Nucleus • Protons – positive charge • Very massive compared to electrons • Determine what type of element the atom is • Neutrons – neutral (no charge) • Slightly more massive than protons • Nuclear forces – short range forces that hold protons and neutrons together in the nucleus

  15. Sect. 3-3: Counting Atoms • Atomic number = # protons • Isotopes • Same element, different masses • same # protons, different # of neutrons

  16. Mass # = protons + neutrons • Hyphen notation • Hydrogen - 3 • Nuclear symbol • 31H • # neutrons = mass # - atomic #

  17. How many of each of the following are in the carbon – 14 isotope? • Protons • Neutrons • Electrons • 6, 8, and 6 respectively

  18. Relative Atomic Masses • 1 atomic mass unit (amu) = 1/12 the mass of a carbon-12 atom • Isotopes differ in mass, but not significantly different in chemical behavior

  19. Average Atomic Masses of Elements • Weighted average of atomic masses of all naturally occurring isotopes of an element • (%of isotope) (mass isotope) + (%of isotope) (mass isotope) = weighted average

  20. Relating Mass to Numbers of Atoms • Mole – SI unit for amount of substance • Based on Carbon-12 • Abbreviated mol • Counting unit (like dozen)

  21. Avogadro’s Number - # of particles in one mol of a substance • 6.022 x 1023 particles = 1 mol • Ex: How many moles of silver are in 3.01 x 1023 atoms of silver? • 0.500 mol

  22. Molar Mass – mass of one mole of a pure substance • Units are typically reported in g/mol • What is the molar mass of Cu? • 63.55 g/mol

  23. Gram to mole conversion • Use molar mass as conversion factor • What is the mass in grams of 3.50 mol of Cu? • 222g

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