Classify solids on the basis of their electrical conductivities.

(N/A) Solids exhibit electrical conductivities ranging from $10^{-20}$ to $10^{7} \ \Omega^{-1} \ m^{-1}$. They are classified into three categories:
$(i)$ Conductors: Solids with conductivities ranging between $10^{4}$ to $10^{7} \ \Omega^{-1} \ m^{-1}$. Metals have conductivities in the order of $10^{7} \ \Omega^{-1} \ m^{-1}$ and are good conductors.
$(ii)$ Insulators: Solids with very low conductivities ranging between $10^{-20}$ to $10^{-10} \ \Omega^{-1} \ m^{-1}$. Examples include wood and plastics.
$(iii)$ Semiconductors: Solids with conductivities in the intermediate range from $10^{-6}$ to $10^{4} \ \Omega^{-1} \ m^{-1}$. Their conductivity is mainly due to defects or impurities. The conduction of electricity in metals and semiconductors is explained by Band Theory.
Certain transition metal oxides show variations in electrical properties:

Electrical Property	Transition Metal Oxides
Metals-like oxides	$TiO, CrO_{2}, ReO_{3}, VO$
Metal-to-insulator behavior at certain temperatures	$Ti_{2}O_{3}, VO_{2}, V_{2}O_{3}$
Insulators-like oxides	$MnO, CuO, FeO$

$ReO_{3}$ has an appearance and conductivity similar to copper.