Chemistry Reference
In-Depth Information
So now you are convinced that we need a really large number to repre-
sent a group of atoms, and you are convinced that we are better off using
scientific notation when dealing with these large numbers, but are you still
wondering why we use that specific large number? The masses of the ele-
ments on the periodic table were calculated using carbon-12 as the base or
standard. The mass of one atom of that particular isotope of carbon was
set at 12 atomic mass units (u). There are 6.02 × 10
23
atomic mass units (u)
in one gram. This conversion allows us to use the masses listed on the peri-
odic table for both atomic mass and molar mass. Atomic mass, which is the
mass of one atom, is measured in atomic mass units (u). Molar mass, which
is the mass of one mole of atoms or molecules, is measured in grams. We
can use the periodic table to find the number that goes with the units for
each of these quantities.
Atomic and Molar Masses
Element and
Symbol
Atomic Mass—
Mass of 1 Atom
Molar Mass—
Mass of 6.02 × 10
23
Atoms
Carbon (C)
12.0 u
12.0 g
Helium (He)
4.00 u
4.00 g
Copper (Cu)
63.5 u
63.5 g
Potassium (K)
39.1 u
39.1 g
This is the point where some students might say, “Hey, I thought that
the mole is always 6.02 × 10
23
. How can the molar masses of carbon and
helium be different?” Remember: A mole always represents 6.02 × 10
23
items, but the mass of these items can certainly be different. You wouldn't
expect a dozen eggs to have the same mass as a dozen cars. It is the number
of items in a mole that is always the same, not the mass, weight, or volume
of those items.
The periodic table can also be used to calculate the molar mass of mol-
ecules and formula units as well. If you can add up the mass of all of the
atoms in a molecule to find the molecular mass in atomic mass units, the
molar mass of the same molecular compound will have the same value
with the unit, grams (g). Following are some examples:
