Electrons orbit in concentric circuits around the nucleus of the atom. Each orbit is called a **shell**.
Each shell can accomodate a maximum number of electrons. The shells are designated letters and filled in sequence moving out from the shell nearest to the nucleus.
The outer shell called the **valence shell** and the number of electrons it contains, the **valence**. This part of the electron is the most important from the point of view of electricity because it is from here that electrons can escape the atom and where electrons from other atoms may join.
> The farther the valence shell is from the nucleus, the less attraction the nucleus has on each valence electron. Thus the potential for the atom to gain or lose elections increases if the valence shell is not full and is located far enough away from the nucleus.
The **conductivity** of a material is an expression of its capacity to channel electrical charge. Where electrical charge is the _flow of free electrons from one atom to another_. The **insularity** of a material is the opposite: its _capacity to resist the flow of electrical charge_.
Electrons in the valence shell can gain energy from external forces. If these electrons gain enough energy, they can leave the atom and become **free electrons**, moving randomly from atom to atom.
We call materials that contain a large number of free electrons **conductors**. In contrast **insulators** are atoms that stabilize themselves by absorbing valence electrons from other atoms to fill their valence shells, eliminating the free electrons.
### Semiconductors
Semiconductive materials are midway between conductors and insulators: they are neither good conductors or insulators but can be altered to function in the manner of either.
Ionization is the process of atoms gaining and losing electrons.
When an atom has an equal number of protons and electrons it is said to be **electrically balanced**.
A balanced atom that receives one or more extra electrons gives way to an overall negative charge. An atom in this state is a **negative ion**. Conversely if a balanced atom loses one or more electrons it becomes positively charged and is thus called a **positive ion**.
> The process of ionization is a constitutive part of the flow of current but not the only part. We also must factor the force that triggers ionization and the resistance that impedes it.
