HISTORY OF MOLE DAY: AVOGADRO’S NUMBER
Today, October 23, at 6:02 AM, begins Mole Day. As a Geek Holiday, which is part of National Chemistry Week, it is based on something you may remember from high school chemistry:
Avogadro’s Number: 6.02 x 1023
Or fully, it’s 602,214,076,000,000,000,000,000.
What better reason than at 06:02 (6.02) on October (10) 23rd (23) we celebrate Mole Day?
What’s a Mole, and who came up with it?
A mole is that number of things, like a dozen, but more. Because a mole of baseballs would cover the earth several hundred miles deep, it’s better to use “mole” to talk about atoms or molecules.
Who was Avogadro?
Lorenzo Romano Amedeo Carlo Avogadro, or Amedeo Avogadro to his friends, was born in Italy in the same year as the American Declaration of Independence, 1776. This period was a renaissance of sorts in chemistry.
Contemporary French chemist Joseph Louis Gay-Lussac was experimenting with combining volumes, which intrigued Avogadro. Gay-Lussac developed a law that when two volumes of gas combine to react with each other to create a third gas, then the ratio of the volume of the two reactants that go into it and the volume of the resulting product of that combination is a simple whole number.
Example: two volumes of Hydrogen gas combine with one volume of Oxygen to form two volumes of water vapor.
2H2 + O2 ==> 2H2O
It’s simple stoichiometry.
But how many particles do we need for a Mole?
This is where Avogadro’s genius came in: for the above law to be valid, he determined that you must have equal volumes of the two reactants — at the same temperature and pressure — to hold an equal number of particles. The problem for Avogadro was that the words and categories for molecules didn’t exist yet. Indeed, Hydrogen exists as an atom, but Oxygen is only stable as a molecule O2, not as an atom. (It has to do with electron valences.)
Fortunately, Italian chemist Stanislao Cannizzaro (1826-1910) brought Avogadro’s Law to prominence, even though the latter had already died. He and subsequent chemists derived that Atomic Mass Units (amu) is 1/12 of the mass of, for example, Carbon-12.
This common isotope of Carbon has 6 protons, 6 neutrons, and 6 electrons. Since electrons have negligible mass, 1/12 (1 divided by 6 protons + 6 neutrons) of a Carbon-12 atom is very close to the mass of just a single proton or neutron.
Still with me?
The atomic weights of elements, like Carbon-12, as I mentioned, are expressed in terms of Atomic Mass Units. From our example above, Hydrogen has an atomic weight of 1.00784 amu.
Chemists created a relationship between the amu and the gram, which looks like this:
1 amu = 1/6.02214076 x 1023 grams
So, with a mole, or an Avogadro’s number of Carbon-12 atoms, we’d have exactly 12 grams. Now chemists can use this equation to convert between measurable grams and invisible atoms. This resulting number is named in honor of Avogadro.
Who is Promoting this Holiday?
In high school, I won my school’s American Chemistry Society Chemical Award. Chemistry was my major at University before I switched to history.
Can You Eat a Mole?
Kind of. There is a popular restaurant-bar-music venue in Ft. Collins, Colorado, called Avogadro’s Number. Try the Avo’s Club Sandwich; it comes with fresh avocadro.
P.S. Special tip of the hat to my friend for her original article here.
Bill Petro, your friendly neighborhood historian