Name: Dan E.
Date: 2001 - 2002
Today is "mole day" and I was asked what a mole is base
upon. My textbooks tell me that it is based on the number of atoms in 12
grams of carbon-12. Then I was asked "Why 12 grams?" and I have no
answer. Do you know why 12 grams of carbon and not 10 grams or 1 gram or
100 grams, a nice round number?
Sure, it is simple. You want a gram per mole and an atomic mass unit (amu)
to be the same. A carbon-12 atom contains 6 protons, 6 neutrons, and 6
electrons. Protons and neutrons have about the same mass, and electrons
have a much smaller mass. Defining a mole as 12 grams of carbon-12, then,
ensures the equivalency.
The reason carbon-12 was specified instead of protons or hydrogen atoms or
something else has to do with the way molecular masses are accurately
measured, with a mass spectrometer. Carbon-12 is an important nucleus to
mass spectroscopists! The reason it is necessary to specify a particular
atom is that atomic masses aren't exactly simple sums of the masses of the
electrons, protons, and neutrons they contain. The reason for this is that
the binding energy of the atom draws off some of the mass of the components,
according to the famous formula E = mc^2.
Richard E. Barrans Jr., Ph.D.
PG Research Foundation, Darien, Illinois
Hi, Dan !!
Let us suppose you have three different masses :
6 kg, 12 kg and 24 kg. To compare them,
lets choose 12 kg. Related to 12 kg, we you
have the relationship between them : 0.5, 1, 2.
Almost the same happened between the atoms : carbon
was chosen as the standard and its mass was
by definition said to be 12 unities of atomic mass(=12u).
1/12 of the carbon mass is - accordingly to that - the unity
"u" of mass. If you have another atom, lets say, Calcium,
and you wish to compare with this unity "u", then you
can say later that : Calcium is equivalent to 40u or -
in other words - has 40 times mores mass than 1/12
of the carbon mass.
Your question - than - could be : why is that the
carbon was said to have 12u and NOT 13u or 14u ??
Well, the carbon has 6 electrons around the nucleus
and the nucleus contains 6 protons and 6 neutrons.
Some other carbon atoms may contain more than
6 neutrons, like - for instance - the famous C14
(radioactive - used to measure the age of fossils).
But lets forget C13, C14, etc and think of C12.
C12 is precisely the chosen atom. And 1/12 of its
mass is "u", the so called "atomic mass unity" (=a.m.u.)
Atoms of the same element, lets say, Calcium, contain
different number of neutrons in its nucleus. So, it is
necessary to compare each isotope (atom with different
number of neutrons in the nucleus) with the "a.m.u.".
Finally, it is necessary to get the average of the measures.
And you say then : well, the average atomic mass of
Calcium is 40,078.
And the SAME happens to molecules !!! In other words,
you speak of how many times a given molecules has
more mass than a "a.m.u.". And here you can see
the connection between mole and carbon-12.
The definition of the mole is tied in with the determination of Avogadro's
6.02...*10^23 things. My point is that 1 mole is equal to 6.02...*10^23
atoms, molecules, horses or chickens. The current definition of Avogadro's
constant, and hence the mole, is the number of atoms (on the atomic scale)
or mass (on the laboratory) contained in 12 gm (exactly) of the 12 isotope
of carbon. This is a fairly recent definition. Previously it was based on
the mass of the mixture of naturally isotopic masses of carbon, oxygen, or
hydrogen. However, we now know that these compositions change somewhat
depending upon the source of the sample (Why that is is another interesting
question.). Why the modern definition has been accepted is explained in a
very interesting website:
The nice round number is the mass of a proton (or neutron) which are the
particles in the nucleus of all atoms that account for most of the mass of the
atom. These particles have a mass of 1 gram per mole (to a few significant
figures). Since carbon-12 has 6 protons and 6 neutrons in the nucleus it has
Atomic mass comes mainly from the contents of the nucleus (to several
significant figures). Both neutrons and protons have approximately the same
mass. Thus an amount of matter scale (such as a mole) that is linked to the
number of neutrons and protons in the nucleus makes sense. Carbon-12 has 6
protons and 6 neutrons in the nucleus, thus 1 mole of carbon-12 has a mass of
exactly 12 grams (by definition).
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Update: June 2012