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I have been reading a chapter in my physics text. In it there is Rutherford's gold foil experiment.

There it is written that doubly charged helium particles (alpha particles) were projected onto very thin gold foil. Most of the alpha particles passed straight through, but some of the alpha particles bounced back because positive particles (protons) in the nucleus repelled them. Positive and positive always repel each other.

If charge was the reason some of the alpha particles bounced back, then why didn't the electrons (negative) of the gold atoms which are much smaller then protons get attracted to the alpha particles? Positive and negative attract.

I would have thought the doubly charged alpha particles would be attracted to the negative electrons.

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2 Answers 2

up vote 8 down vote accepted

They did.

The alpha particles are more than seven thousand times heavier than an electron.

Also, the alpha particles have a lot of energy, around 5MeV, and the first ionization energy of gold is about 9 eV.

Because of these, alphas can hit quite a few electrons without being affected too much. Think about a speeding school bus and a watermelon :)

(References: http://dept.astro.lsa.umich.edu/~cowley/ionen.htm and http://en.wikipedia.org/wiki/Alpha_particle#Energy_and_absorption)

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2  
I was hoping the links would refer to speeding school busses and watermelons. ;-) –  Michael May 1 at 14:41
    
Unfortunately they are just boring scientific stuff :D –  netom May 1 at 17:04

Primarily because of the fact that a nucleus is much, much bigger than electrons are, and electrons are widely dispersed in an atom, also, gold foil is neutral in overall charge hence it doesn't deviate the trajectory unless the alpha particles come quite close to the foil. The probability of approaching a nucleus is very low compared to just passing through the empty space, this is even lesser for the electron.

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