Conductor and insulator of electricity

Hi, just happen to teach on conductor of electricity. I do not know why I always have a concept the aluminium is not a conductor of electricity. But when I search the net, a few website say that is a conductor of electric. Anyway just curious and find out why some material cannot conduct electricity but some can?

The reason is thatin conductive materials, the outer electrons in each atom can easily come or go, and are called free electrons. In insulating materials, the outer electrons are not so free to move.
All metals are electrically conductive.

Electric current is the uniform motion of electrons through a conductor.

In other types of materials such as glass where atoms' electrons have very little freedom to move around. While external forces such as physical rubbing can force some of these electrons to leave their respective atoms and transfer to the atoms of another material which form the static electricity which is an unmoving, accumulated charge formed by either an excess or deficiency of electrons in an object.

For electrons to flow continuously (indefinitely) through a conductor, there must be a complete, unbroken path for them to move both into and out of that conductor.

Here will be some of the example of conductor and insulator.

Conductors: silver, copper, gold, aluminum, iron, steel, brass (copper & Zinc), bronze (copper & Tin), mercury, graphite, dirty water, concrete

Insulators: glass, rubber, oil, fiberglass, porcelain, ceramic, quartz, (dry) cotton, (dry) paper, (dry) wood, plastic, air, diamond, pure water.