Rutherford alpha ray scattering experiment

Contd : Remember how sir said that we are literally taking all the stuff that actually does have mass and condensing it all into a tiny object?

and ofc back then there was no means of knowing that alpha particles were helium nuclei since they hadn't discovered nuclei. But yeah. Basically the helium NUCLEI will be more dense that gold ATOM.

I was not talking about a time at which alpha particles where thought to be helium nuclei. In fact there is no concept of nuclei at the time of Rutherford. Because the most exapted theory of atomic structure at that time was the plum pudding model. Which does not involve the idea of nuclei. So at that time alpha particles where not helium nuclei instead where just dubly charged helium atoms. Which according to plum pudding model should be having a radius equal to the radius of helium atom about 140pm .Think in the past as if you are doing the experiment when plum pudding model was widely expected
And there was not even the notion about nuclei.

we can apply plum pudding model to alpha particle by removing all the electrons(plum) in the He atom(pudding), we are left with a positive He atom(alpha particle). But we shouldn't be saying it as a nucleus.

oops i didnt check page 2

Yeah but once the alpha particle was bombarded, for doing the density calculations, if anyone did do that, they'd take or measure the radii of the alpha particle itself right? Its not like they knew that alpha particles were helium "nuclei" or even something from helium, for that matter. They'd have probably just viewed it as some doubly charged particles with high density, I suppose.

yea all right maybe they didnt think its a helium atom but how do they measure the radius of it
#confused

And another thing is alpha particle and helium atom have same mass, they could hav assumed both to be same

Things with the same mass need not always be the same.

Also, alpha particles and helium can be distinguished by deflection of alpha particles in electric and magnetic fields.
But the question of measuring the radius still remains.

Density of Gold atom is an observed phenomenon from Rutherford's experiment. The fact that the alpha particles were passing through the gold foil. Rutherford initially thought that this validated plum pudding model. However, when he saw that there are some particles deflecting, he realized that the positive charge is not distributed all over the atom, rather it is concentrated in a small region (otherwise no particle would be deflected, right?). Therefore, most of the atoms would be empty. We are not comparing the density of nucleus here, which is higher for the gold atom. Since we understand that most of the atoms are empty, we know why alpha particles would pass freely. I don't know if I answer Sreejit's question. If not, please ask again.

they didnt measure the radii of He atom till 1960s :/ , Rutherford performed this experiment in 1911, so there was no chance they could have measured the radius of a alpha particle =(
they used bond lengths to estimate size of atoms back then

Keep Guessing! wrote:Also, alpha particles and helium can be distinguished by deflection of alpha particles in electric and magnetic fields.
But the question of measuring the radius still remains.

I saw this on wiki
Rutherford's work also included measurements of the ratio of an alpha particle's mass to its charge, which led him to the hypothesis that alpha particles were doubly charged helium ions (later shown to be bare helium nuclei). In 1907, Ernest Rutherford and Thomas Royds finally proved that alpha particles were indeed helium ions. To do this they allowed alpha particles to penetrate a very thin glass wall of an evacuated tube, thus capturing a large number of the hypothesized helium ions inside the tube. They then caused an electric spark inside the tube, which provided a shower of electrons that were taken up by the ions to form neutral atoms of a gas. Subsequent study of the spectra of the resulting gas showed that it was helium and that the alpha particles were indeed the hypothesized helium ions
They knew alpha particles were somehow related to He

well , i think even if he considered atom to be a plum pudding model , he might be aiming to get an estimation of bond length between atoms beacuse if we try to think in that time , plum pudding model was accepted over worldwide so it's hard to think that he was already thinking about space between nucleus and electron . but when he saw the result , it was far away from his expectation then he might have made his assumption about new model . so it doesn't even matter about density . about size helium was known to all he might knew that helium is smaller than gold atom.

Well, since alpha particle is a helium nucleus only your answer is correct @Student1.