Question:

Early scientific researchers hypothesized that electricity was an invisible fluid that could move through certain substances. Those substances "porous" to this "fluid" were called conductors, while substances impervious to this "fluid" were called insulators.

We now know what electricity is composed of: tiny bits of matter, smaller than atoms. What name do we give these tiny bits of matter? How do these particles of matter relate to whole atoms?

In terms of these tiny particles, what is the difference between the atoms of conductive substances versus the atoms of insulating substances?

Answer:

The tiny bits of matter that move through electrically conductive substances, comprising electricity, are called electrons. Electrons are the outermost components of atoms:

Although electrons are present in all atoms, and therefore in all normal substances, the outer electrons in conductive substances are freer to leave the parent atoms than the electrons of insulating substances.

Such "free" electrons wander throughout the bulk of the substance randomly. If directed by a force to drift in a consistent direction, this motion of free electrons becomes what we call electricity.