Date: Thu Dec 21 12:08:08 2000
Posted By: Samuel Silverstein, faculty, physics, Stockholm University
Area of science: Chemistry
ID: 972658148.Ch
Message:
Jesse,
There are several ways that you can measure the mass of an atom. The most
direct way is with a
mass spectrometer.
This works by adding charge to the atom by adding or removing electrons
(since electrons have much lower mass than protons or neutrons, this does
not significantly change the atom's mass). Then you accelerate the atom and
fire it through a magnetic field towards a position detector. By observing
how much the atom's trajectory is bent by the magnetic field, you can
calculate its mass.
Before this method was discovered, scientists calculated the masses of atoms
and molecules indirectly, using
Avagadro's number . I performed a version of the
experiment described in this link in an undergraduate chemistry class. The
way it works is:
- Find a substance (such as oil) that spreads itself into a thin layer
one molecule thick when poured on water.
- Estimate the molecular weight of the substance, based on how many atoms
of different kinds are in it. The oil used in the linked pages above has a
molecular weight of 226, meaning that each molecule would have 226 protons
and neutrons.
- Avagadros number is defined as the number of protons and neutrons that
would make up 1 gram. So one mole of oil would be 226 grams.
- Measure the volume of a mole of oil.
- Pour the oil onto some water, and measure the area.
- From the volume and area, you can now estimate the thickness of the
one-molecule-thick layer of oil. This gives you a size estimate for one oil
molecule.
- Now that you know the size of one oil molecule, and the volume of one
mole of molecules, you can now calculate how many molecules are in a mole.
Did you follow all that? Anyway, scientists have used this and similar
methods to determine that Avagadro's number is 6.02x10^23. This means that
if you take an element like carbon which has an atomic weight of 12, and you
measure out 12 grams of it, then you have 6.02x10^23 atoms. It's an indirect
way of weighing the atom, but it works well.
I hope this helps.
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