Plasma as the Fourth State of Matter

Although the term chemistry in the title of the book does not require a special introduction, the term plasma probably does. Plasma is an ionized gas, a distinct fourth state of matter. “Ionized” means that at least one electron is not bound to an atom or molecule, converting the atoms or molecules into positively charged ions. As temperature increases, molecules become more energetic and transform matter in the sequence: solid, liquid, gas, and finally plasma, which justifies the title “fourth state of matter.”

The free electric charges – electrons and ions – make plasma electrically conductive (sometimes more than gold and copper), internally interactive, and strongly responsive to electromagnetic fields. Ionized gas is usually called plasma when it is electrically neutral (i.e., electron density is balanced by that of positive ions) and contains a significant number of the electrically charged particles, sufficient to affect its electrical properties and behavior. In addition to being important in many aspects of our daily lives, plasmas are estimated to constitute more than 99% of the visible universe.

The term plasma was first introduced by Irving Langmuir (1928) because the multi-component, strongly interacting ionized gas reminded him of blood plasma. Langmuir wrote: “Except near the electrodes, where there are sheaths containing very few electrons, the ionized gas contains ions and electrons in about equal numbers so that the resultant space charge is very small.