Why J.J Thomson use gold in his experiment for atomic model?

waqarrashid33

any can answer this?
that why J.J Thomson strike gold by alpha particles in his experiment by which he propose model for atomic structure?

Vanadium 50

It was Ernest Rutherford. (Actually, Geiger and Marsden)

Gold can be made very thin.

bcrowell

Yes, and in addition we can see with hindsight that the experiment was more likely to give a detectable number of scattering events at back angles if the Z was large. Gold has a high Z.

Actually, I'd be interested to know more about the motivation for the experiment. Both the raisin cookie model and the planetary model of the atom existed at the time, but the planetary model was not taken seriously because it would collapse due to radiation. From what I've read, I think Rutherford had the raisin cookie model firmly in mind. He assigned the project to Marsden, apparently expecting it to be a fairly boring negative result that would allow a green grad student to get some experience with the relevant techniques. They did not know the exact atomic number of gold, but they had some idea that it was somewhere around 100. Because they weren't thinking in terms of the planetary model, they wouldn't think of 100 as an estimate of the charge of a nucleus, they'd think of it as the charge of the "dough" in the raisin cookie.

It would be interesting to know whether they even recognized atomic number as a possible variable of interest to study, or simply used gold because gold is very ductile and therefore easy to make thin foils out of by rolling. You can also make thin foils by evaporation, but I don't know if that technique had been developed then, or whether the resulting thicknesses would have been usable with their crude setup. It would be interesting to know whether they tried any other substances before they tried gold.

Vanadium 50

I just read both papers. In fact, they looked at materials other than gold as well.

The targets were thin, but not unusually so.

bcrowell

Cool! Any insight into what Rutherford actually thought he was testing?

mgb_phys

You need a film on it's own not on a substrate and gold leaf is the most common thin film lying around in a physics lab.

I always thought Rutherford suggested it just to give Marsden practice in using the instrument - there is a good description of the lab setup/personalities and politics in the book "the fly in the cathedral"

bcrowell

You can make self-supporting films by evaporation. The standard technique is that you evaporate the substance onto a glass surface, then lower it into water, floating the film off of the glass. Then you mount the film on a frame.

Astronuc

Some insight here
http://galileo.phys.virginia.edu/cla...cattering.html


Gold is very maleable (and not too chemically reactive) and can be formed in thin film without annealing. Back in 1906, it was easiest to form gold into thin foils as opposed to other metals which harden with repeated cold work.

bcrowell

Interesting. They say the initial observation of scattering was with mica, not gold. They also say he was surprised that the electric field was strong enough to make even a tiny amount of scattering, and that he understood that it couldn't be due to scattering by electrons. So although he apparently expected the result to be negative, he did seem to have some hint that something strange was going on, and he did have physical reasons for wanting to try a higher Z -- it would give a higher electric field, and he was already surprised that the field was high enough to give 1-2 degrees.

Before they did the now-famous version of the experiment, they had also observed that they got scattering at 1-2 degrees with "heavy metals." So it sounds like they may have had at least some idea that it depended on Z. Mica is a broad term; some micas have some fairly high-Z elements in them.

They seem to have been assuming that angles as large as 1-2 degrees had to be happening because of multiple scattering. Under that interpretation, I guess it would make sense to use a thicker foil. If they were only interested in scattering angles greater than 90 degrees, then I suppose they didn't even have to use a thin foil, although using a thick one would have made it impossible to check that anything at all was happening at angles less than 90 degrees.

waqarrashid33

through my study i reach to the idea that
GOLd has a great Z so it,s nucleas is heavy and when alpha rays strike it so it is rebound due to great mass of nucleas.also the charge on the outer of atoms i.e electrons is great so it can repell alpha rays very well.
thank you guy,s

waqarrashid33

sorry charge on nucleas is greater and so it can repell the alpha ray