EasyChemistryHigh School (11–12)Structure of the Atom › Subatomic Particles and Atomic ModelsConceptual
In Thomson's tube, the charge to mass ratio of the rays was the same for hydrogen, air and carbon…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
In Thomson's tube, the charge to mass ratio of the rays was the same for hydrogen, air and carbon dioxide, and for cathodes of iron, copper and aluminium. In Goldstein's tube, the ratio of the positive rays changed with the gas. What does each result tell us?
Need a reminder? The key ideaShow
Cathode rays are electrons: negative, the same in every gas, with . Millikan found , so . Canal rays are positive, with depending on the gas. The neutron has no charge. In a field , a charge feels .
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. Cathode rays. The same ratio everywhere means the same particle comes from every atom: the electron is part of all atoms.
Step 2. Canal rays. Positive rays are what is left when atoms lose electrons. Different gases leave different positive ions, with different masses.
Step 3. The lightest. Hydrogen gives the largest ratio, so its positive ion, the proton, is the lightest.
Answer: electrons are universal; positive ions depend on the gas, and hydrogen's is the proton.
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