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During photoelectric effect when an electron absorbs a photon having energy greater than the threshold energy,it is ejected from the metal So when the metal continuously gets photons then could all the electrons from the metal get ejected?

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So when the metal continuously gets photons then could all the electrons from the metal get ejected?

The answer to this particular question is simply, unequivocally, no.

It is inconceivable that all the electrons could be removed while leaving something recognizable as the metal (object). Recall that it is the (outermost, valence) electrons that determine chemical properties.

If hypothetically, by some unknown mechanism, all of the electrons were removed, only the positively charged nuclei of the atoms would remain, all (strongly) repelling each other and with no way to form chemical bonds.

Other answers have addressed the fact that the metal becomes positively charged as electrons are ejected and collected thus attracting electrons back to the metal so this answer is just to address the quoted question.

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One general property of metals is that they are great conductors. This means that there are a large number of free electrons that are not tightly bound to the atoms in the metal. When a photon ejects an electron from the metal, there are a myriad of free electrons available to de-excite and become bound in the atom in the vacancy left by the ejected electron.

If the number of incident photons is large enough and there are not any free electrons left to fill the vacancies, the metal ascertains a net positive charge. In such a case, the work function, which is the minimum energy required to eject an electron from the metal, increases, because the electrons are more tightly bound. Hence, the energy of the incident photons would need to increase.

I am unsure if there has been any experiments that use incident photon energies far higher than the work function, but I imagine that a if an electron is ejected, the Coulomb force between the negative electron and positive atom would be far greater than the kinetic energy of the electron, and hence the electron would return to the atom, but I am not entirely sure.

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  • $\begingroup$ So you mean the electrons never actually get out of the atom? $\endgroup$
    – Rima
    Apr 13, 2015 at 4:40
  • $\begingroup$ In general, no, because of the large amount of free electrons moving around in the metal that take up the spaces left by the ejected electrons. $\endgroup$
    – user77629
    Apr 13, 2015 at 4:53
  • $\begingroup$ But the electrons get ejected,right?? $\endgroup$
    – Rima
    Apr 13, 2015 at 5:06
  • $\begingroup$ Yes. A photon would hit a bound electron, and eject it (provided the photon had the right energy). This would leave a 'gap' in the atom, and this gap is quickly closed up by a 'free electron' which jumps back into the gap. $\endgroup$
    – user77629
    Apr 13, 2015 at 5:13
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    $\begingroup$ But if the free electons are no more. Then?? $\endgroup$
    – Rima
    Apr 13, 2015 at 5:14
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Think of it like this:

When an electron is ejected from the metal surface and moves out to infinity the metal body acquires a positive charge +e. This is exactly like bringing a positive charge from infinity to the metal. Then another electron is ejected, and this is similar to bringing another +e charge to the metal. Since it is a metal, the charge will be residing on the surface. So you are basically building a positively charged shell of the shape of the metal(say it is sphere). Then the total energy required to construct a shell with charge equal to n electrons (which is equal to the energy required to eject n electrons) is just the self energy of a charged spherical shell.

$$U = \frac{Q^2}{8\pi \epsilon_0 r}$$

So as you keep increasing the Q on the sphere by ejecting you will need more and more energy(U + bandgap + workfunction) to eject more electrons. Eventually you will run out of free electrons and the electrons will be so tightly bound to the nucleus that your light source will need to be extremely powerful. So powerful in fact that it may generate temperatures that will generate temperatures so high that your metal block will start melting.

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