The following excerpt is from Young and Freedman 13th edition:
Most metals are good conductors, while most nonmetals are insulators. Within a solid metal such as copper, one or more outer electrons in each atom become detached and can move freely throughout the material, just as the molecules of a gas can move through the spaces between the grains in a bucket of sand. The other electrons remain bound to the positively charged nuclei, which themselves are bound in nearly fixed positions within the material. In an insulator there are no, or very few, free electrons, and electric charge cannot move freely through the material. Some materials called semiconductors are intermediate in their properties between good conductors and good insulators.
So what I understand is, conduction of electricity is simply movement of electrons through a body. As the excerpt states, for some reason not yet explained in the book, metals do not cause much problems to give up a couple of their outer electrons (which makes them become positive ions). This results in free electrons in the body, which makes the body a conductor.
On the other hand, the excerpt states that there are no or very few free electrons in an insulator, which AFAIK is due to reluctance of insulators to give up a couple of their outer electrons.
If what is written up to here are correct, then why doesn't an insulator conduct electricity when it is negatively charged? (let's assume the insulating body is composed of a single type of atom for simplicity) The reason I think why it should do is: When a body is negatively charged, it has an excess of electrons. Then numerically, every atom in the body is able to own as many electrons as their number of protons. If every atom own exactly the same number of electrons as their protons, then there will be free electrons in the body. Hence, by definition, the body will be a conductor.
But AFAIK this is no the case. Then one possibility is, atoms in the insulator keep excess electrons and become negative ions when the body is negatively charged. If that's true, why this is the case? Why do insulators keep electrons more than their number of protons?