The units in which the mass of an atom are expressed are atomic
mass units. At one time, the lightest atom was assigned a
mass of 1 amu and the mass of any other atom was expressed in
terms of this standard. Today atomic mass units are defined
in terms of the 12C isotope, which is assigned a
mass of exactly 12.000... amu.
Isotopes are atoms of the same element with
different numbers of neutrons, such as the 20Ne
and 22Ne isotopes of neon or the 35Cl
and 37Cl isotopes of chlorine.
The atomic weight of an element is the weighted
average of the atomic masses of the different isotopes of an
element. Naturally occurring carbon, for example, is a
mixture of two isotopes, 12C (98.89%) and 13C
(1.11 %). Individual carbon atoms therefore have a mass of
either 12.000 or 13.03354 amu. But the average mass of the
different isotopes of carbon is 12.011 amu.
The molecular weight of a compound is the sum of
the atomic weights of the atoms in the molecules that form
Example: The molecular weight of the sugar molecule found
in cane sugar is the sum of the atomic weights of the 12
carbon atoms, 22 hydrogen atoms, and 11 oxygen atoms in a C12H22O11
|12 C atoms = 12(12.011) amu =
|22 H atoms = 22(1.0079) amu =
|11 O atoms = 11(15.9994) amu =
has a molecular weight of 342.299 amu. A mole of C12H22O11
would have a mass of 342.299 grams.This quantity is known as
the molar mass, a term that is often used in place of
the terms atomic weight or molecular
The term mole literally means a small mass. It is
used as the bridge between chemistry on the atomic and
macroscopic scale. If the mass of a single 12C
atom is 12.000 amu, then one mole of these atoms would have a
mass of 12.000 grams. By definition, aa mole of any
substance contains the same number of elementary particles as
there are atoms in exactly 12 grams of the 12C
isotope of carbon.
Example: A single 12C atom has a mass of 12
amu, and a mole of these atoms would have a mass of 12 grams.
A mole of any atoms has a mass in
grams equal to the atomic weight of the element. The term
mole can be applied to any particle: atoms, a mole of atoms,
a mole of ions, a mole of electrons, or a mole of molecules.
Each time we use the term, we refer to a number of particles
equal to the number of atoms in exactly 12 grams of the 12C
isotope of carbon.
Avogadro's number (or Avogadro's constant) is the
number of elementary particles in a mole of any substance For
most calculations, four significant figures for Avogadro's
constant are enough: 6.022 x 1023.
A mole of any substance contains
Avogadro's number of elementary particles. It doesn't matter
whether we talk about a mole of atoms, a mole of molecules, a
mole of electrons, or a mole of ions. By definition, a mole
always contains 6.022 x 1023 elementary particles.
The mole is a powerful tool, which enables chemists armed
with nothing more than a table of atomic weights and a
balance to determine the number of atoms, ions or molecules
in a sample.
Example: Let's calculate the number of C12H22O11
molecules in a pound of cane sugar.
We need two pieces of information to do this calculation.
We need to know the number of grams in a pound and the
molecular weight or molar mass of this sugar. A pound of
sugar has a mass of 453.6 grams and one mole of this sugar
has a mass of 342.3 grams.
A pair of unit factors can be constructed from the
molecular weight or molar mass.
By paying attention to the units during the calculation,
it is easy to choose the correct unit factor to convert grams
of sugar into moles of sugar.
We then use Avogadro's number to determine the number of C12H22O11
molecules in our sample.