In electron matter, atomic nuclei provide mass and positive charge. All the other properties of this form of matter derive from its electrons. At low temperature and pressure, electrons bind to nuclei ( which have radii thousands of times smaller than even the most tightly bound electron). At low temperature and pressure, most electrons bind to only one atom. Those which are not so constrained form the basis for a scientific discipline known as "chemistry". But electron matter is not limited to low-temperature, low-pressure environments.

At high temperature, kinetic energy available in collisions can detach electrons from atoms, producing free electrons and positive ions. How heavily ionized atoms are depends on the atom's atomic number (Z) and on temperature. Since temperature and average particle kinetic energy are related by <KE> = (3/2)*kB*T, where kB is Boltzmann's constant and T is absolute temperature. The value of kB is 8.617E-05 eV/K, so <KE> = 2.585E-04*T or T = 3900*<KE>. Ionization of a specific electron from a specific atom will become significant at around 1/3 T as computed above because average KE means average - hotter particles are abundant. Hydrogen becomes heavily ionized around 20000 K, as evidenced by the weakening of H lines in the spectra of type B stars. Hydrogen also becomes completely ionized at this point. Fully ionizing atoms becomes harder and harder as Z increases, to the point that fully stripping a uranium nucleus requires temperatures on the order of 10 billion Kelvin.

Whether nuclei are partially or fully ionized, plasma remains electron matter. Their size is enormous compared to nuclei, so the coupling of their mutual repulsion and their attraction to nuclei leads to a plasma which does not immediately dissipate due to mutual proton repulsion or to mutual electron repulsion, yet does not collapse immediately into an electron-coated blob (something comparable to this does occur deep in the crusts of neutron stars). The large electrons repel each other at ranges where the average distance from a negative charge to a negative charge is less than the distance from a negative charge to a positive charge (remember the adage: looks like a wave, acts like a particle). Although electromagnetic fields govern the bulk static and dynamic properties of a plasma, electron-electron repulsion controls properties at the microscopic level.

Electron matter also shifts from atoms to ions under pressure even at low temperature. The higher pressure is, the more densely packed are the particles of any substance. (We only think of liquids and solids as incompressible because earth is a low-pressure environment.) Electrons and nuclei have to mix, regardless, so the distance between nuclei goes down as pressure goes up. That means a given electron can respond to the positive charge of more than one nucleus. When this happens, the electron must be in an unoccupied quantum state. Fortunately, the number of such states quickly becomes enormous as the number of nuclei involved gets large. Instead of discrete states, electrons are distributed quasi-continuously in a band of energies. Note that all electrons in such a band must be in unique states. At high pressure, the Maxwell-Boltzmann statistics which describe the properties of an ensemble of particles do not apply. Fermi-Dirac statistics are required. When applied to dense matter,