If we heat a metal at high temperatures to melt it, its outermost electrons should also gain some energy so as to get excited to a higher state and eventually become free from the metal atom. But that doesn't happen. WHY?
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2$\begingroup$ The outermost electrons are already ‘free’ in the solid, as in not tied to a specific atom. $\endgroup$– Jon CusterCommented Jan 1 at 15:45
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6$\begingroup$ Why do you think it doesn't happen? en.wikipedia.org/wiki/Thermionic_emission $\endgroup$– Ján LalinskýCommented Jan 1 at 15:51
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$\begingroup$ en.wikipedia.org/wiki/Thermionic_emission $\endgroup$– A NejatiCommented Jan 2 at 0:08
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2 Answers
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Because the first ionization energy is much larger than the latent heat of vaporization of the metal, which is much larger than the latent heat of fusion of the metal. Consequently, for a neutral metal, continuous random distribution of the available kinetic energy in the sample by a multitude of individual molecular interactions will result in vaporization before ionization is possible.
A comment suggests thermal electron emission, but that is for a charged metal cathode. The energy required to remove extra electrons from the surface of a solid conductor is lower than latent heat of fusion. Consequently, for a charged cathode, continuous random distribution of the available kinetic energy in the sample results in electron emission before melting or vaporization.
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$\begingroup$ What will happen if the metal in the gaseous state is given energy? $\endgroup$ Commented Apr 1 at 15:29
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$\begingroup$ @AdwitKumar The metal gas will increase in temperature and eventually ionize at very high temperature. $\endgroup$– g sCommented Apr 1 at 16:02
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The minimum energy required to extract an electron from the surface of a solid is the so-called electron work function. It is pretty easy to look for the "electron work function" to find comprehensive tables of the work functions for most of the elements ( see, for instance, here ) and a few compounds. Typical values range from $2.3$ (potassium) to $6$ eV among the elements. It is then clear that when the specific energy to extract an electron is a couple of orders of magnitude larger than the typical thermal energy per particle at the melting point, there is no chance of getting surface ionization at the melting temperature.
answered Jan 1 at 21:52
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