MadSci Network: Chemistry
Query:

Re: how can you weigh an atom if it is so microscopic?

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:

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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