1. Atomic Theory Avogadro’s Number and the Mole

2. Review: We have already discussed that the atomic mass unit will describe the mass of individual atoms in more easily handled terms. Remember, the Isotopic Weight is simply the mass number in amu’s.

3. Review: We also know that the average atomic weight is a value that takes into account all of the different isotopic weights that a given element may have and the percent occurrence in nature of each isotope. Remember that the equation for average atomic weight is Ave. At. Wt. = Σ (Isotopic Weight x Pct. Occurrence)

4. Moving On… This work that we have already done is good, but nobody really cares about the mass (in any units) of a single atom. Instead, we care about the total mass of LOTS of atoms…

5. A Definition: Remember that the metric unit for amount of substance is the MOLE. By definition, 1 mole of any element is equal to 6.02 x 10 23 atoms of that element. We will be free to use that definition as a “factor” in conversion calculations.

6. Example: How many atoms are equivalent to 2.75 moles of oxygen? Solution: 2.75 moles Oxygen ( 6.02 x 10 23 atoms Oxygen 1 mole Oxygen ) = 1.66 x 10 24 atoms Oxygen Note how the factor is set up to cancel the moles of Oxygen.

7. There is More !!! Suppose we counted out 1 mole of atoms of the element iron and placed them all on a balance at the same time. That would be 6.02 x 10 23 atoms of iron.

8. What would all of those Atoms weigh??? All together, the 6.02 x 10 23 atoms of iron would have a mass of 55.847 grams. Where did that number come from ?? If you look on the periodic chart, you see that this number is found in the box for iron. It is the average atomic weight for this element. This gives us a 2 nd definition for the mole….

9. Definitions of the Mole We already have the definition that states 1 mole is equal to 6.02 x 10 23 atoms. Now we get the definition that 1 mole is equal to the average atomic weight in grams. We get to use the appropriate definition we need to solve the given problem.

10. Consider this example: What mass, in grams, is equivalent to 3.80 moles of sodium? Solution: 3.80 moles Na ( 23 grams Na 1 mole Na ) = 87.4 grams Na The 23 grams of Na came from the periodic chart – it is the atomic weight that is reported on the chart. Note too how the moles cancel. Always set up factors to cancel.

11. Finally, this flowchart may help. Atoms Moles Grams Notice that the moles is in the middle and can be converted to and from the atoms and the grams. But, if you wanted to turn a measurement of grams into a measurement of atoms, you would have to convert through the moles on the way – it would take two steps. The key here is using the correct conversion for the task – pay close attention to the units in the problems.