You have seen that there are two classes of materials as defined by their electric properties: insulators and conductors. FIGURE 22.4 looks inside an insulator and a metallic conductor. The electrons in the insulator are all tightly bound to the positive nuclei and not free to move around. Charging an insulator by friction leaves patches of molecular ions on the surface, but these patches are immobile.

FIGURE 22.4 A microscopic look at insulators and conductors.

In metals, the outer atomic electrons (called the valence electrons in chemistry) are only weakly bound to the nuclei. As the atoms come together to form a solid, these outer electrons become detached from their parent nuclei and are free to wander about through the entire solid. The solid as a whole remains electrically neutral, because we have not added or removed any electrons, but the electrons are now rather like a negatively charged gas or liquid—what physicists like to call a sea of electrons—permeating an array of positively charged ion cores.

The primary consequence of this structure is that electrons in a metal are highly mobile. They can quickly and easily move through the metal in response to electric forces. The motion of charges through a material is what we will later call a current, and the charges that physically move are called the charge carriers. The charge carriers in metals are electrons.

Metals aren’t the only conductors. Ionic solutions, such as salt water, are also good conductors. But the charge carriers in an ionic solution are the ions, not electrons. We’ll focus on metallic conductors because of their importance in applications of electricity.