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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: mana on February 05, 2016, 11:12:42 AM

Title: cathodic ray can cange molecules to cation?
Post by: mana on February 05, 2016, 11:12:42 AM

hi all
as you know in Milikan experiment and thomson experiment (for considering atomic structure), cathodic ray in a vacuum lamp can interact with molecules of the gas in the lamp and change them to cation by giving electron,
 my questions are:
 1-cathodic ray has negative charge itself how it can absorb electron (which is negative too)?
 2-how Milikan and Thomson calculated q/m, I know q and m are directly related to degree of deviation of oil drops, but I can't understand how they calculated q/m ?
3- why did thomson used cathod electrod with some holes on it? what is the role of holes?

Title: Re: cathodic ray can cange molecules to cation?
Post by: Enthalpy on February 06, 2016, 06:09:14 PM

1- This is the usual process, here a electron with some 20eV energy rips an other electron away from a molecule. Afterwards, you have two electrons and a positive ion.

It's the process that sustains an arc in significant gas pressure (not in a vacuum lamp) when the voltage is big enough. It creates more and more electrons and ions in an "avalanche".

The other process, where a molecule absorbs an electron to form a negative ion, is less frequent. It can help to quench an arc in good insulating gases.