The Big Idea Behind Avogadro's Number (That Most People Miss)

This video steps backward to begin the story of Avogadro's number back in ancient Greece, similarly to how I teach the atomic history in my chemistry class at Princeton.

The development of relative masses of the elements, though, is something that I tend to skim over fairly quickly, though I might start putting a bit more emphasis on this concept in the past.

I do, however, mention - partially thanks to Steve Mould's explanation of moles - define the moles as the conversion factor between one gram and one atomic mass unit. 

I didn't know, however, that the value for Secret Number N was discovered in 1909. Now I guess I need to find out how that value was discovered more than 100 years ago.

World's Roundest Object!

Be careful when discussing this video with your students. This is about the world's roundest sphere...sphere. It's not a ball. It's a sphere.

Just saying.

'Cause I know I've said ball in front of my high school students, and it took me a while to get the boys' attention back.

(In a related aside, my wife is listening in as this video plays. She lost control at 6:22 when the narrator said "Newtons, Joules". To quote her: "heh, heh...Newton's jewels...heh, heh".)

This 'world's roundest object' is more about the basis of our metric (now the systemé international d'unités or SI) than it is about materials science. There are, however, some serious challenges involved in making a material that won't decompose, that won't get dirty, that won't wear away, that won't change over time.

Here they have created - according to the scientist at 7:25 - a single crystal of silicon with 'no voids or dislocations' and containing only one isotope of silicon, making the material just slightly less valuable than absolutely, perfectly, priceless.

Complete History of the Avogadro Number

Around 0:50 into the video, our meandering narrator mentions that he wasn't able to find a single source with correct, complete information on the history of the Avogadro Number...which makes me wish he'd provided sources for his information. Otherwise, how do we know that what he's telling us is correct?

(As an aside, I've always called it Avogadro's Number. Maybe the the and lack of possessive is a British thing. I'll admit that I don't remember how my professors at the University of Aberdeen referred to it.)

I will say that this is an amazingly thorough and likely correct history of the development of the concept and number of the moles. It goes through from Democritus through the Karlsruhe Congress and all the way to the modern measurement of Avogadro's number via a nearly perfect sphere of silicon.

I need to watch this video a few more times until I'm more familiar with all the steps in this development so I can tell the story to my students.

An Actually Good Explanation of Moles

This video is brilliant, and the title is surprisingly accurate.

I've been defining a mole for two and a half decades now, and I've never quite approached it from this angle. The mole is simply a quantity that relates the mass of different elements that react together.

I like that.

How big is a mole? (Not the animal, the other one.) Daniel Dulek

Gotta love the TED talk animations.

We use the term mole in chemistry when we're calculating how many atoms (or molecules or formula units) are in something, or how much of a substance is needed to react with another substance.

A mole is 6.022 x 10²³ of something, but that's a ridiculously large number to try to conceive of.

It's 602 200 000 000 000 000 000 000. It's HUGE, but that's okay because atoms are tiny, like really tiny.

A mole of moles

Only someone as twisted and intelligent and educated as Randal Munroe would be able to answer the question, "What would happen if you were to gather a mole (unit of measurement) of moles (the small furry critter) in one place?" and to then take the answer to a place where we get a sentence like this, "Plumes of hot meat and bubbles of trapped gases like methane—along with the air from the lungs of the deceased moles—periodically rise through the mole crust and erupt volcanically from the surface, a geyser of death blasting mole bodies free of the planet."

I love Randal Munroe.