Just Ask Antoine!
Energy & change
The quantum theory
Electrons in atoms
The periodic table
Acids & bases
History of chemistry
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.
- 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 email@example.com