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.

Chemical Reaction

Chemical Reaction, process by which atoms or groups of atoms are redistributed, resulting in a change in the molecular composition of substances. An example of a chemical reaction is formation of rust (iron oxide), which is produced when oxygen in the air reacts with iron.

The products obtained from a given set of reactants, or starting materials, depend on the conditions under which a chemical reaction occurs. Careful study, however, shows that although products may vary with changing conditions, some quantities remain constant during any chemical reaction. These constant quantities, called the conserved quantities, include the number of each kind of atom present, the electrical charge, and the total mass.

See Chemical Symbols: Periodic Table of Elements

In order to discuss the nature of chemical reactions, certain basic facts about chemical symbols, nomenclature, and the writing of formulas must first be understood. All substances are made up of some combination of atoms of the chemical elements. Rather than full names, scientists identify elements with one- or two-letter symbols. Some common elements and their symbols are carbon, C; oxygen, O; nitrogen, N; hydrogen, H; chlorine, Cl; sulfur, S; magnesium, Mg; aluminum, Al; copper, Cu; silver, Ag; gold, Au; and iron, Fe.

Most chemical symbols are derived from the letters in the name of the element, most often in English, but sometimes in German, French, Latin, or Russian. The first letter of the symbol is capitalized, and the second (if any) is lowercase. Symbols for some elements known from ancient times come from earlier, usually Latin, names: for example, Cu from cuprum (copper), Ag from argentum (silver), Au from aurum (gold), and Fe from ferrum (iron). The same set of symbols in referring to chemicals is used universally. The symbols are written in Roman letters regardless of language.

Symbols for the elements may be used merely as abbreviations for the name of the element, but they are used more commonly in formulas and equations to represent a fixed relative quantity of the element. Often the symbol stands for one atom of the element. Atoms, however, have fixed relative weights, called atomic weights, so the symbols often stand for one atomic weight of the element.

The atomic weights (atomic wt.) of the elements (see Chemical Elements) are average atomic weights of the elements as they occur in nature. Every chemical element consists of atoms the weights of which vary because of varying numbers of neutrons in their nuclei. Atoms of the same element that differ in weight are called isotopes of the element. An isotope's weight may be indicated by a superscript to the left of the abbreviation that indicates the total number of nucleons (protons plus neutrons) in the nucleus. See Atom.

An electrically neutral atom has equal numbers of protons and electrons. Electrically charged atoms and groups of atoms are called ions. When an atom is electrically charged—that is, when it has lost or gained one or more electrons, and thereby become an ion—that state may be indicated by a superscript to the right of the symbol, as in H+, Mg++, or Cl-. The symbol H+ indicates a singly positive hydrogen ion, Mg++ a doubly positive magnesium ion, and Cl- a singly negative chlorine ion. See Ionization.

The atomic number of an element is equal to the number of protons in the nucleus of an atom of the element. All isotopes of a particular element have the same number of protons in their nuclei. The atomic number is sometimes indicated by a lower-left subscript. The symbol represents a uranium ion of triply positive charge (that is, an atom that has lost 3 electrons), with 92 protons and 146 neutrons (238 nucleons - 92 protons = 146 neutrons) in its nucleus, which is surrounded by 89 electrons (92 - 3 = 89).