kemija

http://www.meritnation.com/ask-answer/q...

Pozdravljeni upam da je tole prava tema za moje vprašanje:)
Zanima me kako kemiki/znanstveniki vedo oz. kako so določili število elektronov,protonov in nevtronov v atomu?Z kakšnimi eksperimenti?
Tega na internetu nikjer ne najdem, najdem samo, kako se določi število posameznih delcev iz periodnega sistema, odgovora na moje vprašanje pa nikjer...

Se mi zdi, da je tole bolj vprašanje za fizike kot pa za kemike.
Drugače pa skozi preteklost so to odkrivali različni znanstveniki:

Before any concept of subatomic particles, people knew about atoms and elements via a wide array of experiments (which could be detailed, but isn't really relevant to this story). When you have a purified sample of an element, you can determine how much an atom of it weighs, and so we start with knowing a bunch of elements and their atomic weights.

Enter Dmitri Mendeleev. He notices that if you arrange the elements by increasing weight, you end up with repeating groups of elements that are chemically similar. This was nice, except people knew something wasn't quite right with it, because you had to do a couple tricks to make similarly-acting elements end up in the right groups (such as swapping the by-weight ordering of tellurium and iodine or cobalt and nickel). Once this was done, the elements gradually got numbered using their place in the table: 1 is Hydrogen, 2 is Helium, etc. The numbers didn't mean anything to anyone, they were just useful identifiers, and they were called atomic numbers.

Later, Joseph J. Thomson is working on figuring out what is going on in electric discharge tubes. He discovers that tiny particles are being discharged from the cathode in those tubes, and he managed to measure the ratio between their charge and mass. He calls those particles electrons.

Next we go to Ernest Rutherford. He had an experiment where he fired a bunch of positively-charged particles at a thin sheet of gold leaf, and he noticed that the vast majority of them made it through without interacting with anything, but a few of them got deflected. This led him to hypothesize that atoms had a dense central core of positive charge (the nucleus) and then a diffuse cloud of electrons around it. Since atoms overall usually have neutral electric charge, the number of electrons and the charge of the nucleus had to be equal.

Following on from this was Henry Moseley. He measured the wavelengths of X-rays sent off by various elements in a particular kind of experimental setup, and he determined that the wavelength was proportional to the square of the atomic number. That indicated that the atomic number actually indicated a physical quantity, which was then hypothesized as the amount of charge in the nucleus. One of the neat things this did was conclusively show where there were gaps in the periodic table: elements that should exist, but hadn't been discovered yet. In the course of time, all those gaps were filled in, once people knew what to look for.

Back to Rutherford again, a few years later. He succeeds in experimentally stripping out everything but the nucleus of a hydrogen atom, which has a charge of 1 and a weight of 1. He calls what he ends up with a proton.

Finally, we need to discover the neutron. We already know that the nucleus has a positive charge of the atomic number, and thus there are that many electrons as well, but the problem is that most atoms have a much higher weight than their atomic number, so something more than protons had to be in the atomic nucleus. A leading theory was that there were a bunch of extra proton-electron pairs just hanging around in the nucleus, but that was shown to be impossible. Enter James Chadwick, who shot a weird kind of radiation others had discovered at paraffin and other substances and measured what happened, and concluded that the radiation was in fact made up of neutrally-charged particles that had about the same mass as a proton. He's discovered the neutron.

So, now that we know everything, we can gather up our data on elements. The number of protons in an atom is the atomic number of that element. That's also the number of electrons, since atoms are neutrally-charged. Finally, the difference between the atom's weight and the atomic number is the number of neutrons.

Danes za to uporabljajo masni spektrometer.