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 Elements

Chemical Elements are substances that cannot be decomposed or broken into more elementary substances by ordinary chemical means. Elements were at one time believed to be the fundamental substances but are now known to consist of a number of different elementary particles. More than 100 chemical elements are known to exist in the universe, although several of these, the so-called transuranium elements, have not been found in nature, and can only be produced artificially. See also Chemistry.

Chemical elements are classified as metals and nonmetals. The atoms of metals are electropositive and combine readily with the electronegative atoms of the nonmetals. A group of elements called metalloids, intermediate in properties between the metals and the nonmetals, are sometimes considered a separate class. When the elements are arranged in the order of their atomic numbers (a number proportional to the net positive charge on the nucleus of an atom of an element), elements of similar physical and chemical properties occur at specific intervals (see Periodic Law). These groups of elements with similar physical and chemical properties are called families, examples of which are the alkaline earth metals, rare earth elements, halogens, and the noble gases.

The unit for atomic weight of the elements is one-twelfth of the weight of the carbon-12 atom, which is arbitrarily set at 12. The atomic number, weight, and chemical symbol of each of the known elements are given in the accompanying table. See articles on each element.

When two atoms have the same atomic number but different atomic weights, they are said to be isotopes. Many natural isotopes are known for some elements, whereas other elements occur in only one isotopic form. Hundreds of synthetic isotopes have been made. Some natural isotopes, and many synthetic ones, are unstable (see Radioactivity).

The heavy transuranium elements are produced in particle accelerators by bombarding atomic nuclei with charged atomic nuclei or nuclear particles to form a heavier element. These superheavy elements are radioactive and decay into more stable, lighter elements rapidly.