How were atomic masses measured before mass spectrometry?

Before the mass spectrometer was invented, how did the earlier scientists determine the mass of an atom (what kind of experiment was performed) ? I realize they were able to find relative mass but I was hoping you would know how they determined actual mass. Kevin 6/23/99

	Vocabulary
	atomic weight Avogadro's number mass spectrometer mole

Here's a dollar bill. Tell me how thick it is, and you can keep it.

It's too thin to measure with these crummy rulers you've given us, but it's about 0.1 mm.

Close, but not quite. Yes?

It's about 0.045 mm thick.

How did you come up with that?

I rolled it up as tight as I could, counting the number of turns, and then measured the diameter of the roll. I divided that by the number of bill-thicknesses in the roll.

That works. Now here's a five dollar bill. Tell me how to find the mass of a copper atom and you can keep it.

Weigh an atom with a very good scale.

We don't have a scale that good. Our scales won't read masses that are much less than a milligram.

Count out a million copper atoms, weigh them, and divide the weight by a million.

OK. But that would be like like measuring the thickness of a dollar bill with a crummy ruler, because a million atoms is still too light to register on an analytical balance.

All right, a million million.

How are you going to count them out? Are you sure that's going to be enough to give you more than, say, a milligram of material?

Avogadro's number of atoms, then.

How do you put Avogadro's number of atoms on a balance pan?

Put a mole of copper on the pan.

How do you do that?

A mole of copper weighs 63.54 g.

An atom of copper weighs 63.54 g divided by Avogadro's number. 63.54 divided by 6.02×10²³ is 1.06×10^-22 g per copper atom.

But suppose you didn't know the atomic weight, or Avogadro's number-

Now you're asking two related but different questions: How to find an atomic weight, and how to find Avogadro's number. Those will cost you extra. Pay up for the copper atom first.

You've learned well, grasshopper.

Author: Fred Senese senese@antoine.frostburg.edu

General Chemistry Online! How were atomic masses measured before mass spectrometry?