Molecule

Molecule, smallest unit of a substance that shows all the chemical properties of that substance. A molecule is a group of atoms that are bound tightly together by strong chemical bonds called covalent bonds. Every molecule has a definite size. If a molecule is broken up into its atoms or into smaller groups of atoms by chemical processes, these pieces will not behave like the original molecule. A molecule can contain atoms of the same element or atoms of different elements. A substance made up of molecules that include two or more different chemical elements is called a molecular compound. An example of a molecular compound is water. Water is made of molecules that contain two hydrogen atoms and one oxygen atom. See also Atom.

Many substances on Earth are made of molecules. Millions of molecules join together to make up the cells in humans or in any other plant or animal. The food we eat, the air we breathe, the clothes we wear, and the wood, paint, and carpeting that we use in homes are all made of molecules. Millions of different molecules exist in nature or can be made by chemists. The nature of each molecule depends on the atoms that it contains and how they link to each other. For example, the oxygen that animals require is made of molecules that have two oxygen atoms bound together. If one oxygen atom binds to a carbon atom, the molecule is instead the poisonous gas carbon monoxide.

Scientists study molecules and their structures so they can better understand why substances behave the way they do. For example, molecular structure helps explain why water boils at a high temperature. Scientists and manufacturers also use their knowledge of molecules and molecular structures to make substances with desirable properties. Plastics, for instance, are laboratory-made substances that consist of enormous molecules containing thousands of atoms. By manipulating the molecular structure of plastics, chemists have created materials that stretch better, resist fading, or can be used in microwave ovens without melting. Similarly, pharmaceutical chemists use their knowledge of molecular structure to develop new drugs that more effectively ease pain or fight disease. The discovery of the structure of deoxyribonucleic acid (DNA), the molecule that contains the genetic blueprint for living organisms, opened the door to tremendous advances in medicine and industry. Knowledge of the structure of DNA has enabled physicians to understand and treat certain genetic diseases. Moreover, by manipulating DNA structure, scientists have been able to modify—or genetically engineer—organisms, creating, for example, bacteria that produce valuable drugs (see Genetic Engineering).

Ion

Ion (chemistry), particle formed when a neutral atom or group of atoms gains or loses one or more electrons. An atom that loses an electron forms a positively charged ion, called a cation; an atom that gains an electron forms a negatively charged ion, called an anion. Atoms can be converted to ions by radiation such as X rays or light of sufficient energy. This kind of radiation is thus called ionizing radiation. See Ionization.

Ionization

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).

RESEARCH

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.

IONIZATION IN GASES

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.