The regulation game is played on a table that measures 9 ft by 5 ft (2.74 m by 1.52 m) and stands 2.5 ft (76 cm) from the floor. A transverse net 6 in. (15.25 cm) high divides the surface, which is generally dark in color, edged with white stripes, and halved longitudinally (for doubles play) by another white stripe. The celluloid ball is hollow, seamless, and about 1.5 in. (3.81 cm) in diameter, with a weight of .1 oz (2.8 grams); the racket, or bat, is a wooden paddle with a handle 3 in. (7.62 cm) long and a round blade about 6.5 in. (16.5 cm) long, covered with rubber.

In the service (unlike tennis) the ball must bounce once before clearing the net and again bounce before being struck by the receiver. After the service (only one is allowed, not two as in tennis), the returns should go over the net without bouncing on the near surface. A point is scored when a service does not land properly in play or when a player fails to return the ball properly. Each player in turn serves consecutively five times until the winning score of 21 is reached. (If the score is tied at 20-all, play must continue until a 2-point margin is won.) In doubles matches partners rotate in units of five consecutive services, and the server must deliver the ball into the diagonally opposite box.

Table tennis originated in the late 19th cent. with cork or rubber balls. First popular in England, it spread to several European countries and to the United States in the early 20th cent. The International Table Tennis Federation was founded (1926) to standardize the rules and equipment. The U.S. Table Tennis Association was founded in 1933.

Primarily a recreational sport in the United States, table tennis is a major competitive sport in Asia and parts of Europe. In 1971 the sport achieved a great measure of publicity when, while touring Japan, a U.S. table tennis team was invited to play in China, thereby initiating the first officially sanctioned Sino-American cultural exchange in almost twenty years. Table tennis took its place on the Olympic program in 1988.

There are 18 vertical columns, or groups, in the standard periodic table. At present, there are three versions of the periodic table, each with its own unique column headings, in wide use. The three formats are the old International Union of Pure and Applied Chemistry (IUPAC) table, the Chemical Abstract Service (CAS) table, and the new IUPAC table. The old IUPAC system labeled columns with Roman numerals followed by either the letter A or B. Columns 1 through 7 were numbered IA through VIIA, columns 8 through 10 were labeled VIIIA, columns 11 through 17 were numbered IB through VIIB and column 18 was numbered VIII. The CAS system also used Roman numerals followed by an A or B. This method, however, labeled columns 1 and 2 as IA and IIA, columns 3 through 7 as IIIB through VIB, column 8 through 10 as VIII, columns 11 and 12 as IB and IIB and columns 13 through 18 as IIIA through VIIIA. However, in the old IUPAC system the letters A and B were designated to the left and right part of the table, while in the CAS system the letters A and B were designated to the main group elements and transition elements respectively. (The preparer of the table arbitrarily could use either an upper-or lower-case letter A or B, adding to the confusion.) Further, the old IUPAC system was more frequently used in Europe while the CAS system was most common in America. In the new IUPAC system, columns are numbered with Arabic numerals from 1 to 18. These group numbers correspond to the number of s, p, and d orbital electrons added since the last noble gas element (in column 18). This is in keeping with current interpretations of the periodic law which holds that the elements in a group have similar configurations of the outermost electron shells of their atoms. Since most chemical properties result from outer electron interactions, this tends to explain why elements in the same group exhibit similar physical and chemical properties. Unfortunately, the system fails for the elements in the first 3 periods (or rows; see below). For example, aluminum, in the column numbered 13, has only 3 s, p, and d orbital electrons. Nevertheless, the American Chemical Society has adopted the new IUPAC system.

The horizontal rows of the table are called periods. The elements of a period are characterized by the fact that they have the same number of electron shells; the number of electrons in these shells, which equals the element's atomic number, increases from left to right within each period. In each period the lighter metals appear on the left, the heavier metals in the center, and the nonmetals on the right. Elements on the borderline between metals and nonmetals are called metalloids.

Group 1 (with one valence electron) and Group 2 (with two valence electrons) are called the alkali metals and the alkaline-earth metals, respectively. Two series of elements branch off from Group 3, which contains the transition elements, or transition metals; elements 57 to 71 are called the lanthanide series, or rare earths, and elements 89 to 103 are called the actinide series, or radioactive rare earths; a third set, the superactinide series (elements 122-153), is predicted to fall outside the main body of the table, but none of these has yet been synthesized or isolated. The nonmetals in Group 17 (with seven valence electrons) are called the halogens. The elements grouped in the final column (Group 18) have no valence electrons and are called the inert gases, or noble gases, because they react chemically only with extreme difficulty.

In a relatively simple type of periodic table, each position gives the name and chemical symbol for the element assigned to that position; its atomic number; its atomic weight (the weighted average of the masses of its stable isotopes, based on a scale in which carbon-12 has a mass of 12); and its electron configuration, i.e., the distribution of its electrons by shells. The only exceptions are the positions of elements 103 through 118; complete information on these elements has not been compiled. Larger and more complicated periodic tables may also include the following information for each element: atomic diameter or radius; common valence numbers or oxidation states; melting point; boiling point; density; specific heat; Young's modulus; the quantum states of its valence electrons; type of crystal form; stable and radioactive isotopes; and type of magnetism exhibited by the element (paramagnetism or diamagnetism).

Lanthanide Series
(Rare Earth Elements)

Actinide Series
(Radioactive Rare Earth Elements)

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Game similar to lawn tennis that is played on a tabletop with wooden paddles and a small, hollow, plastic ball. The object is to hit the ball so that it goes over the net and bounces on the opponent's half of the table in such a way as to defeat the opponent's attempt to reach and return it. Both singles and doubles games are played. A match consists of the best of any odd number of games, each game being won by the player or team who first reaches 11 points or who, after 10 points each, gains a two-point lead. Invented in England in the early 20th century, it soon spread throughout the world. Since the mid-1950s, East Asian countries have dominated the sport. It has been an Olympic sport for both men and women since 1988.

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Article of furniture used in the Western world since at least the 7th century BC, consisting of a flat slab of stone, metal, wood, or glass supported by trestles, legs, or a pillar. Though tables were used in ancient Egypt, Assyria, and Greece, only during the Middle Ages, with the growing formality of life under feudalism, did tables increasingly take on social significance. Tables with attached legs appeared in the 15th century. The draw top was invented in the 16th century, making it possible to double the table length. Increasing contact with the East in the 18th century led to increasing specialization in the design of occasional tables.

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Organized array of all the chemical elements in approximately increasing order of their atomic weight. The elements show a periodic recurrence of certain properties, first discovered in 1869 by Dmitry I. Mendeleyev. Those in the same column (group) of the table as usually arranged have similar properties. In the 20th century, when the structure of atoms was understood, the table was seen to precisely reflect increasing order of atomic number. Members of the same group in the table have the same number of electrons in the outermost shells of their atoms and form bonds of the same type, usually with the same valence; the noble gases, with full outer shells, generally do not form bonds. The periodic table has thus greatly deepened understanding of bonding and chemical behaviour. It also allowed the prediction of new elements, many of which were later discovered or synthesized. For an illustration of the periodic table, see chemical element.

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Inlet, Atlantic Ocean, forming the harbour of Cape Town, South Africa. It is 12 mi (19 km) long and 8 mi (12 km) wide. Although less sheltered than other bays along the coast, it became a port of call for ships traveling to India and the East because of the availability of fresh water. The shore was permanently settled by the Dutch in 1652.

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