History of Mole Day: Avogadro’s Number

HISTORY OF MOLE DAY: AVOGADRO’S NUMBER

Today, October 23, at 6:02 a.m., Mole Day begins. 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?

Amadeo Avogadro. Image: Wikimedia

Lorenzo Romano Amedeo Carlo Avogadro, also known as 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 the field of chemistry.

Contemporary French chemist Joseph Louis Gay-Lussac was experimenting with combining volumes, which intrigued Avogadro. Gay-Lussac developed a law stating that when two volumes of gas react to form a third gas, the ratio of the volumes of the reactants and the resulting product is a simple whole number.

Illustration: Two volumes of Hydrogen gas combine with one volume of Oxygen to form two volumes of water vapor.

Example:

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 did not yet exist. 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.)

Stanislao Cannizzaro

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) are 1/12 of the mass of, for example, Carbon-12.

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, 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?

The esteemed American Chemical Society has a dedicated Mole Day site on this topic. And they also tell Mole Day jokes, like “Finding NeMole,” as they make coffee in their Erlenmeyer flasks.

In high school, I won my school’s American Chemical Society Chemistry Award. It 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 avocado.

Bill Petro, your friendly neighborhood historian
billpetro.com