I sometimes come across the statement that the free electron Fermi gas model best describes the electronic properties (e.g. electron heat capacity and electron thermal conductivity) of metals that are 'simple'. What does a 'simple' metal mean? And why is the free electron gas model more suitable for describing the electronic properties of these metals?
[Physics] Free electron gas model and electrons in real metals
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The free electron model is surprisingly good at predicting the properties of electrons in metals, and this implies that the electrons really are nearly free. However when you look more closely there is of course an interaction with the lattice. This is modelled using the (rather predictably named) nearly free electron model.
The conduction electrons are delocalised, so you shouldn't think of them as little balls bouncing off the ion cores. The spatial extent of their wavefunction is typically far greater than the lattice repeat, hence the relatively weak interactions. However interactions with the lattice are responsible for electrical resistance and thermal conductivity, and at very low temperatures for superconductivity. However note that these aren't interactions between a single electron and a single ion core, but rather interactions between electron waves and lattice waves (phonons).
Even in liquid metals, the current is mostly carried by electrons, ions are just too heavy in comparison to play a major role in conduction (though the situation is different in polar liquids).
The difference between electrons in a solid (e.g. copper wire in your phone) and in a liquid metal is that the scattering rate in a liquid is very high, so usually the resistance goes up, though for some poor metals such as Bismuth, this isn't true.
Usually metals have large bandwidths (on the scale of electron volts), and so even for temperatures above 1000K you can expect the bonding of electrons to be the delocalized and mostly in tact. In this sense, the conduction is not too different from an amorphous metal, where electrons are delocalized, but there is strong disorder scattering. The timescales for liquids and amorphous metals is quite different with regards to ionic motion, but as far as being delocalized electronically, they are rather similar.
Long story short, the electrons in liquid metals are generally delocalized, just like when they are in solids. The difference is in the electron-ion scattering interactions, which are typically stronger for liquid metals to their high temperature and lack of ordering. Experimental proof for this claim is that even liquid metals obey the Drude form for the optical response (although you need to adjust electron-ion interactions appropriately), which means the electrons are nearly free.
Best Answer
To expand a bit on my (potentially ephemeral) comment, I'll add a bit more background. One good blanket reference is Ashcroft and Mermin's Solid State Physics book, but lets just focus on what 'simple' means for a metal.
For this purpose, there are two classic papers to consider. The first is J.C. Slater's Reviews of Modern Physics 1934, and the second is J.C. Slater's Physical Review 1934. The first is a long and complete discussion, while the second is focused on calculating the band structure of sodium.
Lets step back a bit first. What would a 'free electron gas' (FEG) behave like? Every electron would have $E = p^{2}/2m$ ($E = \frac 1 2 mv^{2}$), so the energy would be parabolic with electron momentum. Further, no direction in the lattice would be favored more than any other, since it is 'free' of the constraints of atomic positions. Well, in the Phys. Rev. paper, Slater calculates the band structure of sodium following along from Wigner and Seitz's theory to get Bloch functions (essentially).
So, what are the results? Quoting from the Phys. Rev paper:
So, a rigorous wave function calculation of the band structure results in a free-electron looking band structure. Further,
In other words, there are some slight issues near the Brillouin zone boundaries, but it looks awfully free-electron-like.
Turning to Ashcroft and Mermin, they discuss the band structures of metals in Chapter 15. A brief quote:
Except for those contacts, the rest of the surfaces for Cu, Ag, and Au are shown to be nearly circular (free-electron like) in Figure 15.5 in A&M. As for aluminum (more below), they say:
(Note that the three electrons per atom does lead to other weirdness, including a positive high-field Hall coefficient, but that is a discussion for another day.)
So, in some metals (alkali metals and the noble metals, all having single electrons to consider for conduction), the behavior of the electrons in the crystal is similar to that of 'free' electrons. For other metals, this stops being the case, the $E$ vs $p$ band structure gets weirder, you get decidedly non-spherical Fermi surfaces (and even disconnected Fermi pockets), and life becomes harder. Or at least not very 'simple'.
As an added bonus, a comment from @Arham points to a (slightly!) more recent paper, Zhibin Lin et al., 2008 which uses DFT modeling to look at the electronic heat capacity which is, of course, also closely related to the 'free-electron'-ness and the Fermi surface. Now, the paper's major focus is on far-from-equilibrium processes, so when the show how well Al matches a FEG while the noble metals perhaps not so much, that is in regions far from the Fermi surface at room temperatures. Still, the fact that Al is essentially a FEG 5eV or more out from the Fermi surface is quite remarkable.
(As a side note, the figures in Lin et al. on the density of states are very nice in that they show the positions of the lower-lying d-levels for Cu, Ag, and Au, showing how they are responsible for the colors of Cu and Au, and, if we could see just a bit further into the UV, Ag as well. Those who argue that the color of Au has something to do with relativistic electrons should spend time looking over these figures.)
In other comments, the question was further expanded to ask why a metal might not be well described by a free electron gas. Ultimately, this comes down to the band structure ($E$ vs $p$ in all directions), which comes out of the atomic electronic configuration and the crystal structure of the solid. One good example (used by me in an answer over on Chemistry SE) would be iron. It crystallizes in a bcc crystal structure and has d-electrons and an atomic magnetic moment. The band structure is discussed in J. Callaway and C.S. Wang, Physical Review B 1977, and is fairly ugly - multiple isolated Fermi surfaces exist, and they exist separately for spin-up vs spin-down. One cannot begin to describe the Fermi surface as free-electron like - it just does not work. Most bcc crystalline metals have similarly ugly Fermi surfaces, whether they are magnetic or not.