How do electrons move through conductors?

Question

I am a bit confused about how electric charge or current moves through different conductors like steel, silicon, and even water. Is water classified as a conductor? And how exactly does the current move through the atoms of these materials? Could you please explain this?

Answer 1

Electrons are bounded to atoms in different ways depending on the material and, in case of solids, how the different atoms organize in space. Atoms store these electrons in energy states which are separated by finite and large energy differences. The electrons with the highest quantum numbers (and therefore more loosely bounded to the atoms) are the ones who participate in the conduction process. These loose electrons have some probability of, being sited on one atom, move to the next one. Now it all depends if the next atom has an empty state with an energy close to the original electron's energy to store the electron (conductors) or if the only available states have a large gap compared to the original electron's energy (insulators). Applying an electric filed will create a current of electrons through these scatterings between states in different localized atoms if there is no gap or won't be able to create a current if the gap is large.

In case of water, which is a conductor but not a solid, the picture is somehow similar but the conduction happens between states of molecules with less spatial structure. Hence the properties of the current are different; in particular it is less polarized and goes in all directions.

Answer 2

As you know, to each atom a number of electrons is bound such that the total charge of the atom is zero or close to zero. You can picture this like the solar system: a compact object in the center (the nucleus) with other objects (the electrons) orbiting this center$^\dagger$. The electrons that are orbitting in the outer shell feel less of the nucleus, because the electrons in the inner shells are negatively charged and this cancels out some of the positive charge in the nucleus.

Depending on where you are in the periodic table, the outer electrons are more strongly or more loosely bound to the nucleus. In metals these electrons are so loosely bound that they are essentially detached from the atom. They have enough kinetic energy to move freely from one atom to the next. In fact, you can view these outer electrons as moving in a straight line with just small perturbations due to the fact that are atoms everywhere.

Pure water is an insulator although it might not seem so. The outer electrons are tightly confined to be within the molecule, which means they can't hop between different molecules. So why is water normally a conductor? Everyday water has a lot of ions dissolved in it. When you apply a voltage these ions will move and that's what conducts current.

Sillicon is a semiconductor, which means they normally are a poor conductor. Their are electrons are half-way between being tightly bound and loosely bound$^{\dagger\dagger}$. At room temperature there are some electrons which can move freely, but not enough to make it very conductive. By "doping" the material, which means adding atoms of other materials, we can introduce atoms which have either a very loosely bound electron in their outer shell, or atoms that are missing one electron in their shell. The former are good at donating electrons to the material and the latter are good at accepting electrons. When these dopants are placed correctly in the material and an electric field is applied, a current develops that is much bigger than you would expect without the dopants.

$\dagger$ This model is wrong for a number of reasons. Still, it is a good mental model to have if you don't want to go into quantum mechanics.

$\dagger\dagger$ The proper expression would be to say that silicon has a band gap right at the conductive part of the energy levels.