Why do some solids conduct electricity and others don't? Band theory gives the answer: when atoms come together in a solid, their individual electron energy levels merge into continuous bands of allowed energy, separated by forbidden gaps where no electron can exist. Whether a solid conducts, insulates, or does something in between comes down to how these bands are arranged.
- Conductors — the valence band and the next (conduction) band overlap, or the valence band itself is only partially filled. Either way, electrons have empty states immediately available to move into, so they conduct freely. Metallic conductivity actually decreases with rising temperature, because increased lattice vibration scatters the moving electrons more.
- Insulators — a large forbidden gap separates a completely filled valence band from an empty conduction band. Electrons simply don't have enough thermal energy to jump the gap, so essentially no conduction happens (diamond is the standard example).
- Semiconductors — a small forbidden gap sits between the two bands. At room temperature, some electrons do have enough thermal energy to jump the gap, giving conductivity that's intermediate between a conductor and an insulator — and, unlike a metal, semiconductor conductivity increases with temperature, because more electrons gain enough energy to cross the gap as it heats up.
Doping deliberately introduces a controlled impurity into a pure semiconductor (like silicon or germanium, both group-14 elements) to boost its conductivity in a chosen direction:
- n-type semiconductor — doped with a group-15 element (phosphorus, arsenic, antimony). A group-15 atom has one more valence electron than silicon needs for its four covalent bonds, so that extra electron is free to conduct — giving conduction by negative charge carriers (hence "n"-type).
- p-type semiconductor — doped with a group-13 element (boron, aluminium, gallium). A group-13 atom has one fewer valence electron than silicon needs, leaving an electron hole — a missing bond that behaves like a positive charge carrier as neighbouring electrons hop in to fill it, shifting the hole along (hence "p"-type).
Magnetic properties classify solids by how their electrons respond to an external magnetic field — and the key distinguishing question is always: are the electrons paired, and if not, do they stay aligned once the field is removed?
- Diamagnetic — all electrons are paired; the substance is weakly repelled by a magnetic field (NaCl, benzene).
- Paramagnetic — some electrons are unpaired, so the substance is weakly attracted, but the alignment is random and disappears the instant the field is removed (O₂). …