Matter & Energy

Matter is composed of atoms or groups of atoms called molecules. The arrangement of particles in a material depends on the physical state of the substance. In a solid, particles form a compact structure that resists flow. Particles in a liquid have more energy than those in a solid. They can flow past one another, but they remain close. Particles in a gas have the most energy. They move rapidly and are separated from one another by relatively large distances.


Ionization, formation of electrically charged atoms or molecules. Atoms are electrically neutral; the electrons that bear the negative charge are equal in number to the protons in the nucleus bearing the positive charge. When sodium combines with chlorine, for example, to form sodium chloride, each sodium atom transfers an electron to a chlorine atom, thus forming a sodium ion with a positive charge and a chloride ion with a negative charge. In a crystal of sodium chloride the strong electrostatic attraction between ions of opposite charge holds the ions firmly in place and close together. When sodium chloride is melted, the ions tend to dissociate because of their thermal motion and can move about freely. If two electrodes are placed in molten sodium chloride and an electric potential is applied, the sodium ions migrate to the negative electrode and the chloride ions migrate to the positive electrode, causing a current of electricity to flow. When sodium chloride is dissolved in water, the ions are even more free to dissociate (because of the attraction between the ions and the solvent), and the solution is an excellent conductor of electricity. Solutions of most inorganic acids, bases, and salts conduct electricity and are called electrolytes; solutions of sugar, alcohol, glycerine, and most other organic substances are poor conductors of electricity and are called nonelectrolytes. Electrolytes that give strongly conducting solutions are called strong electrolytes (for example, nitric acid, sodium chloride); electrolytes that give weakly conducting solutions are called weak electrolytes (for example, mercuric chloride, acetic acid).


The Swedish chemist Svante August Arrhenius was the first to recognize that substances in solution are in the form of ions and not molecules, even when no electrical potential is applied. In the 1880s he stated the hypothesis that when an electrolyte goes into solution it is only partly dissociated into separate ions, and that the amount of dissociation depends on the nature of the electrolyte and the concentration of the solution. Thus, according to the Arrhenius theory, when a given quantity of sodium chloride is dissolved in a large amount of water, the ions dissociate to a greater degree than when the same quantity is dissolved in less water. A different theory of the dissociation of electrolytes, developed by the Dutch physicist Peter Debye, has been generally accepted since 1923. The so-called Debye-Hückel theory assumes that electrolytes are completely dissociated in solution. The tendency of ions to migrate and thus conduct electricity is retarded by the electrostatic attraction between the oppositely charged ions and between the ions and the solvent. As the concentration of the solution is increased, this retarding effect is increased. Thus, according to this theory, a fixed amount of sodium chloride is a better conductor when dissolved in a large amount of water than when dissolved in a smaller amount, because the ions are farther apart and exert less attraction upon one another and upon the solvent molecules. The ions are not infinitely free to migrate, however. The dielectric constant of the solvent (see Dielectric) is also important in the conductance of a solution; ionization is most marked in a solvent such as water, with a high dielectric constant. See Atom; Electrochemistry.


When a rapidly moving particle, such as an electron, an alpha particle, or a quantum of radiant energy, collides with a gas atom, an electron is ejected from the atom, leaving a charged ion. The ions render the gas conductive (see Electricity). The amount of energy necessary to remove an electron from an atom is called the ionization energy. The principle of ionization of gases by various types of radiation is used in the detection and measurement of radiation (see Particle Detectors) and in the separation and analysis of isotopes in the mass spectrometer. The atmosphere always contains ions that are produced by ultraviolet light and cosmic radiation (see Ionosphere).

A gas that is composed of nearly equal numbers of negative and positive ions is called a plasma. The atmospheres of most stars, the gas within the glass tubing of neon advertising signs, and the gases of the upper atmosphere of the earth are examples of plasmas. A gas becomes a plasma when the kinetic energy of the gas particles rises to equal the ionization energy of the gas. When this level is reached, collisions of the gas particles cause a rapid cascading ionization, resulting in a plasma. If the necessary energy is provided by heat, the threshold temperature is from 50,000 to 100,000 K and the temperatures for maintaining a plasma range up to hundreds of millions of degrees. Another way of changing a gas into a plasma is to pass high-energy electrons through the gas. See also Energy.

Nuclear physicists believe that a plasma contained within a closed magnetic field will enable them to harness the vast energy of thermonuclear fusion for peaceful purposes. In the conceptual stage is a plasma-driven rocket motor for propelling vehicles in deep space. See Nuclear Energy; Space Exploration.