Conductors, insulators, and electron Flow

From basic chemistry, we all know that different types of atoms have different energy levels or different degrees of freedom to move around. In metals, the outer shell atoms are loosely bound and they move chaotically in between the space of the atoms. In room temperature they are so loosely bound or unbound that they can move freely and leave their respective atoms. And they can float around the space between the adjacent atoms. So they are called free electrons.

In non-metals or insulators such as glass, the electrons of the atoms are heavily bound or they have a very little freedom to move. But if external energy or force is applied such as rubbing or application of heat then the electrons are excited enough to move and leave their respective atoms and transfer to the atoms of any other material. But they do not have the ability to move between atoms within that material like metals. 

Conductivity is the measure of relative mobility of electrons within a material.

Conductivity depends on various feature of an atom. Such as, the type of the atom in a material, the atomic mass, the formation of the atoms etc. The materials which has a high mobility of electrons are called conductors. On the other hand, the materials with low electron mobility are called insulators.

Few common examples of conductors and insulators are given below:

Conductors:  silver, copper, gold, aluminum, iron, steel etc.

Insulators: glass, rubber, oil, asphalt, fiberglass, porcelain, ceramic, quartz etc.