I'm not asking a question like "Is the wood conductive?". No. I'm asking what properties do they have to have to be good conductors. Theoretically I mean.
[Physics] How to know what materials are good conductors of electricity
conductorselectricityelectronsmaterial-sciencephysical-chemistry
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Material Science – Why Materials Better at Conducting Electricity are Also Better at Conducting Heat
See http://en.wikipedia.org/wiki/Thermal_conductivity In metals, I think it generally has to do with the higher valence band electron mobility, but it gets more interesting elsewhere.
In metals, thermal conductivity approximately tracks electrical conductivity according to the Wiedemann-Franz law, as freely moving valence electrons transfer not only electric current but also heat energy. However, the general correlation between electrical and thermal conductance does not hold for other materials, due to the increased importance of phonon carriers for heat in non-metals. As shown in the table below, highly electrically conductive silver is less thermally conductive than diamond, which is an electrical insulator.
The short answer is: Because metals are really absorptive (which comes from the fact that the nearly free electrons in the metal follow the oscillations of the radiation thereby depleting its energy), but some only in part of the visible range.
The reflectivity of a material is given by the Fresnel equations in terms of the index of refraction. They describe the angle dependency and further tell you that the higher the difference in the index of refraction the more light will be reflected at the interface.
It is important to understand, that the index of refraction in general can be a complex number. The imaginary part of the index of refraction describes the absorption of the material, while the (well known) real part describes the usual "optical density" causing refraction. So there are two possibilities for a material to reflect strongly: Either because it has a large real part of the index of refraction (like diamond) or because it absorbs light strongly (like metals). The latter effect can also be seen with lines written using a dark overhead transparency marker: they reflect in the colour range that does not pass through.
So, the reflection on the surface of metals is mainly due to the imaginary part of the index of refraction (that is, the absorptivity). For coloured metals like copper or gold the so called "plasma frequency" of the metal above which the metal begins to loose its strong absorptivity is in the visible range (or in the near UV). Therefore such metals only reflect a portion of the spectrum, well you get a tinted reflection.
The other materials (plastic, glass, apples) have one thing in common: they have a relatively low absorptivity (while for metals the wave only enters a few nanometers, the other materials range from transparent to waves entering at least several micrometers; the absorption caused by pigments in the material is typically much weaker than the one in metals). This means that the reflection is caused by the change of the real part of the index of refraction. As most materials are only slightly dispersive in the optical range, this means that all frequencies are reflected more or less equally, therefore the reflection is not tinted.
Best Answer
To be a good conductor, they need a pathway for electrons to flow. And the electrons must be as weakly bound to the nucleus as possible.
Most metals have their valence electrons delocalised in a "sea". These sea electrons are weakly bound, and can easily move from here to there. An outside electron can easily be injected here, and mingle with the others, starting the current chain-reaction. This makes all metals good conductors. Out of these, mercury is the worst conductor (has to do with the sea being ineffective--not sure;will look it up and see)
Graphite is a goodish conductor since the unpaired electrons in the sheet structure are delocalised. An outside electron can again join the delocalised cloud, and start the current chain reaction--along the sheet, though. Current flow perpendicular to the sheets is harder.
Semiconductors do not exactly have an electron sea--all the electrons are used up in forming weakish bonds. These electrons can be displaced, causing current. We can also dope the semiconductor--this changes the electron density and interesting stuff happens.
Electrolytic solutions have ions. Instead of electrons moving, these ions move, causing current.
Insulators/bad conductors have all of their electrons bound in the nucleus or used in forming strong bonds.
I think that covers all the cases...