COUNTING ATOMS

When we want to know how many atoms of a substance are in a sample of the substance that we can see, counting the atoms individually is not practical.  First of all, atoms are so small that we cannot see them individually with our eyes.  Second, we would be counting for a very, very, very long time.  Here is an example for you to think about.  The "lead" of a pencil is made of pure carbon (graphite).  Think about the size of the dot on a "i" that you would write on your paper with a pencil.  Not very big.  The number of atoms of carbon in that dot is roughly equal to the population of the world - about 6 billion!  It would take a very long time to count that high.  And that is just for the number of atoms in the dot on an "i" on your paper!

So, you can see that we have a problem if we try to count atoms individually.  Instead, chemists have come up with a system that is much easier for us to use.  They (and we) use a unit for counting the number of atoms in a sample of matter.  This unit is called the MOLE.  One mole is equal to 6.022 x 1023 atoms of any element.  Using the term "mole" is similar to using the word "dozen".  We all know that a dozen means 12.  Well, a mole means 6.022 x 1023.

Now, that is all well and good.  But we, as humans, still cannot see individual atoms.  How can we count them?  The chemists came up with a way that we could measure moles.  Chemists decided that one mole of any element was equal to the average atomic mass of that element in grams.  They call it the "molar mass" of the element.  For example, the average atomic mass of carbon on the Periodic Table is 12.011.  Therefore, 12.011 grams of carbon is equal to one mole of carbon.  And, one mole of carbon is equal to 6.022 x 1023 atoms of carbon.  So, 6.022 x 1023 atoms of carbon have a mass of 12.011 grams.

1 mole = 6.022 x 1023 atoms or molecules = molar mass of element or compound

DIVIDE BY MOLAR MASS (MM)                   DIVIDE BY 6.022 X 1023
------------------------------------>&nbssp;          <--------------------------------------
GRAMS                                            MOLES                                           ATOMS OR
<------------------------------------         ------------------------------------> MOLLECULES

MULTIPLY BY MOLAR MASS (MM)             MULTIPLY BY 6.022 X 1023