I understood most of the rutherford experiment except how did rutherford know the properties of alpha particles when we didn’t know about protons and neutrons .
In: Chemistry
Rutherford knew the properties of alpha particles but he didn’t know about their internal structure (because neutrons and protons had yet to be discovered.) But he knew they had an atomic weight of 4. So from that he deduced that they were similar to a helium atom with no electrons, and proved this in 1907 – he collected alpha particles in a glass tube, then added an electric discharge. The spectrum of light emitted by the resulting gas matched that of helium – so, if alpha particles + electrons make helium gas, well then alpha particles must have a positive charge
He invented the concept of a small core with most of the mass and positive charge to explain the results. That idea that that core was protons and neutrons came later. Not sure how much later though.
We knew how to detect ionizing radiation. We also knew, that when we put a stream of radiation in an electromagnetic field, it will split into three streams. One positively charged, one negatively and one uncharged. These were called alpha, beta and gamma. Even without knowing their internal structure we could determine the charge and mass of alpha and beta, by observing how they behave in a field.
We knew they were positively charged, and we knew they were emitted from certain radioactive materials.
Rutherford discovered the alpha particles 9 years before his gold foil experiment in an attempt to figure out what the structure of atoms was like. This man was working on it for years.
We can tell it’s positively charged and has more mass than an electron by running it through an electric field and noting it deflects the opposite direction compared to an electron, which we had decided was negative.
The concept of charges was known and Benjamin Franklin had already assigned the positive-negative convention (inconveniently, as it turned out). It is rather simple to determine the sign of the charge and the mass of the particle.
In a known electromagnetic field, a charged particle will move one way or the other depending on the sign of the charge of the particle. Electrons were already discovered so if the particle moves in the same direction as the electron, it must have the same charge as the electron.
From how much the particle moves (ie amount of deflection), the charge/mass ratio can be determined. This would be a relatively simple use of Newtonian equations of motion combined with Maxwell’s laws.
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