CHEM 101 LECTURE NOTES Fall 2001 Dr. Joy Heising S 572-580
Tuesday, September 4, 2001 Chapter 2
How much does a single atom weigh?
Different elements weigh different amounts – related
to what makes them unique.
What units do we use to define the weight of an atom?
amu – units of atomic weight. (atomic mass unit)
1 amu – 1/12 the mass of 1 “carbon-12” atom.
But in the lab, scientists use grams – it’s more practical.
When scientists do experiments with substances (elements,
molecules, ionic compounds), they usually
- Use the chemical formula to plan (ratio of atoms, NaCl)
- Measure the amount needed in grams
How can we relate grams to the chemical formula?
1.0 g C – 5.0 x 1022 C atoms
1.0 g Ba – 4.4 x 1021 Ba atoms
Inconvenient to work with such large numbers all the time.
Chemists use another unit which works like a ‘dozen’:
Mole – 6.022 x 1023 particles
(atoms, molecules or formula units)
Avogadro’s number
Why do we use such a weird number?
Because it gives us an ‘easy’ relationship between
amus and grams.
