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Counting Atoms

Counting Atoms. How do we know how many we have if they are so small?. Relative Mass Scale. In the Bean Counting Lab, we set up a scale to help “count” large amounts of something, without having to actually count them Because that can be a pain in the butt, as you saw yesterday

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Counting Atoms

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  1. Counting Atoms How do we know how many we have if they are so small?

  2. Relative Mass Scale • In the Bean Counting Lab, we set up a scale to help “count” large amounts of something, without having to actually count them • Because that can be a pain in the butt, as you saw yesterday • By using “groups” of something as counting units, counting becomes a lot easier for larger numbers • A dozen eggs (15 dozen vs. 180 eggs) • Reams of paper (2 reams vs. 1000 sheets)

  3. Atomic Mass • Atoms are REALLY small (duh!) • Proton Mass: 1.6726510-24 g • Neutron Mass: 1.6749510-24 g • Electron Mass: 9.1095310-28 g • Using these masses when talking about atoms would be cumbersome and make calculations difficult • A relative mass scale for atoms has been developed for this purpose

  4. Average Atomic Mass(Atomic Weight) • The value on the periodic table for most elements is the average atomic mass • It is a weighted average that takes into account all naturally occurring isotopes

  5. Using Average Atomic Mass • The relative scale for counting atoms has been based off of the standard Carbon-12 isotope • The number of atoms contained in exactly 12.000 grams of Carbon-12 is used as the standard counting unit • Just like the number of beans in the mass of the smallest bean was used as your counting unit

  6. Using Average Atomic Mass • The number of atoms contained in 12.000 g of the Carbon-12 isotope is 6.0221023 • Avogadro’s Number • The same number of atoms is contained in a mass equal to the Average Atomic Mass (in grams) for each element • This value is often times referred to as the Molar Mass of an element

  7. The Mole • The mole is the SI base unit for counting • It is similar to a dozen (12 items) or a gross (144 items) or a ream (500 items) in that it always contains the same NUMBER of items • 6.0221023 items

  8. Using the Mole • The mole can be used in conversion factors because: • 1 mol = 6.0221023 things • 1 mol = (Average Atomic Mass or Molar Mass) grams of any element • 6.0221023 atoms/molecules = (Average Atomic Mass or Molar Mass) grams of any element • These equalities can be used to set up conversion factors that can be used to count atoms (without actually counting them…)

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