atomic weights

[modeller]

There is a great deal of information about elements in the periodic table but most of it compares one element with another to obtain the data . However I haven't been able to find out how the first atomic weight was obtained back in the 18/19 th centuries. Can anyone help ?

[jake-the-peg]

Milikan's experiment!

It's mentioned in the above link.

It's relatively easy to get the charge to mass ratio of an ion like a proton or an electron ( throw them through a magnetic field and see how much they bend) but to get the actual real mass you have to be determine the actual charge.

Millikan's experiment involves charged oil drops between two charged plates

http://www.goalfinder.com/product.asp?producti d=69

It's a tricky experiment and requires awesome experimental skill.

You end up with the charge of an electron, which is the same as the charge on the proton from which you can get the absolute mass by throwing things through a magnetic field

[Bert]

This is what I remember from school chemistry, 45 years ago. Atomic weights are only relative. Once somebody found out that hydrogen appeared to have the lightest atom (from density measurements) it was arbitrarily given the atomic weight of 1. When many atomic weights of other elements proved to be other than integer numbers, compared with hydrogen (later explained by the discovery of isotopes), the basis for atomic weights was changed, first to one-sixteenth of the atomic weight of oxygen (it may have been oxygen-16, I'm not sure), then to one-twelfth the atomic weight of the carbon-12 atom, in order to make atomic weights more nearly integers.

[Bert]

Here's something else I remember. Millikan's experiment determined the charge on an electron. He apparently found that his oil droplets would have a charge of e, (the smallest charge that he found) or 2e, or 3e, or 4e, etc, but never 1.4e or 1.75e, etc. This established that electric charge was discrete, not continuous. However, I heard later that, when his results were examined closely, some results did show fractional charges, but that he had chosen to ignore these results, either because he thought they were spurious (from experimental error), or because they spoiled the theory.

[jake-the-peg]

In any experiment where you have a large number of results you will occasionally get spurius results.

These are normally down to experimental error but occasionally mask some really important behaviour of the real world.

The ability ( call it instinct or luck) to be able to distinguish these two cases is what seperates a good scientist from a great one.

In the case of Millikan we know the charge on the electron is discrete and that he was right to put down his spurious results as errors.

Einstein took the puzzling results of the Michalson-Moreley experiment and discovered special relativity

[modeller]

Thank you all, this has helped to clear up my basic question . I recently read an article that gave what appeared to be an actual weight and not one based on hydrogen .