Sodium chloride crystals

(table salt) form a lattice of sodium ions (Na+) and chlo­ride ions (CI"). These ions are held together by the electrostatic attraction be­tween positive and negative forces. This is known as ionic bonding. The structure is a face-centered cube (A) with ions at each corner and at the centers of each face (B). The ion at the center (Q— sodium or chloride—is closest to six ions of the other type arranged octahe-drally around it. This octahe­dron sits in the center of the lattice (D).

Bluejohn,seen here carved into an ornate vase, is a unique banded form of the mineral fluorite (calcium flu­oride). It is found only in Derbyshire, England.

Fact entries

Fluorinewas first isolated in 1886 by the French chem­ist Henri Moissan (1852-1907). It occurs mainly in the mineral fluorite, after which it is named. At. no. 9; at. mass 18.9984; m.p. -219.62° C;b.p. -188.14° C.

Chlorinewas discovered by the Swedish chemist Karl Scheele (1742-1786) in 1774. It was named later after its greenish-yellow color. Chloros is Greek for greenish-yellow. At. no. 17; at. mass 35.453; m.p. -100.98° Qb.p. -34.6° C.

Brominewas discovered independently in 1826 by the French chemist Antoine Balard (1802-1876) and the German chemist Carl Lowig. Its name is derived from bromos, the Greek word for stench. At. no. 35; at. mass 79.904; m.p. -7.25°C;b.p. 59.47° C.

Iodinewas discovered by the French chemist Bernard Courtois (1777-1838) in 1811. It is named after the violet color of its vapor. The Creek word iodes means violet. At. no. 53; at. mass 126.905; m.p. 113.5° Qb.p. 184° C

Astatineis an artificial ele­ment, first prepared at the University of California by Emilio Segre and others in 1940. Its 30 isotopes are ra­dioactive, it was named after the Greek astatos, meaning unstable. At. no. 85; at. mass (the most stable isotope) 210.

The rare gasesoccupy Group 8A of the periodic table. They react only with fluorine and chlorine, halo­gens from Group 7A.

The rare gases

The rare gases form a group of six elements-helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn). They are also known as the noble gases, inert gases, or ar-gonons. They occur naturally and are found in the atmosphere. All the gases are colorless, tasteless, and odorless. They are monoatomic. This means they exist as single atoms, rather than as molecules of two or more atoms. For example, oxygen in the air occurs in molecules composed of two atoms of oxygen. Each of the rare gases, however, occurs as a single atom in the air.

These gases are remarkable for their un-reactivity. Under normal conditions, they do not enter into chemical combination with other elements to form compounds. This is be­cause the outer shells of their atoms have the maximum possible number of electrons. As a consequence, the atoms have no strong attrac­tion for electrons from other atoms, nor do they readily give up electrons to other atoms.

The rare gases lost their claim to total inert­ness, however, in the early 1960's, when sev­eral compounds of krypton, xenon, and radon were prepared by treating them with fluorine and chlorine. Today, these same three noble gases are combined with fluorine and oxygen to form compounds.

Because of their unreactivity, the noble

gases have important industrial uses. Except for radon, which is highly radioactive, all of them are used as light sources in incandescent and gaseous-discharge lamps. These lamps in­clude electric light bulbs and fluorescent light fixtures. The rare gases are also used in mak­ing metals and alloys, in chemical processing, and in nuclear reactors. In each of these cases, the rare gas "blankets" a material that would react violently if exposed to air or nitrogen, the most commonly used "blanket."

Helium

Although helium is relatively rare on earth, it is one of the most common elements in the uni­verse. All stars—including our sun—are made up of hydrogen and helium. A star's energy is produced by the fusion (melting together) of two hydrogen atoms to form a helium atom. This is the same process that produces the en­ergy in a hydrogen bomb.

Helium has several important uses. It is em­ployed in the fuel systems of rockets. In indus­try, helium is used in heliarc welding (a type of electric arc welding). It protects metal from corroding while it is being welded. Helium, being less dense than air, is used to inflate bal­loons and airships as an alternative to highly inflammable hydrogen. It is also used to pre-

A neon signconsists of a glass tube (A) containing neon gas at low pressure. Positive ions, striking an electrode, generate second­ary electrons. A neutral neon atom (B) has ten elec­trons. A secondary electron excites an outer electron (Q, making it jump to a higher energy level. When the ex­cited electron jumps back, its excess energy is emitted as red light (D).

The AD500,a modern semirigid airship is seen here flying over the Eiffel Tower in Paris. It is filled with helium gas, which is second only to hydrogen in lightness. Instead of venting (and therefore losing valu­able gas) to reduce buoy­ancy in order to descend, the AD500 uses engine-driven ducted fans to drive the ship downward.

Major groups of elements: The rare gases 57

vent chemicals from reacting with other ele­ments during storage, handling, and transpor­tation.

A mixture of helium and oxygen sometimes makes breathing easier for persons with asthma or other breathing difficulties. A simi­lar mixture has been developed to produce safer underwater diving systems. Breathing or­dinary compressed air causes nitrogen to dis­solve in the blood, which can give divers "the bends" (nitrogen narcosis) if they surface too rapidly. Artificial air made by mixing helium and oxygen overcomes this problem. The only side effect is the squeaky "Donald Duck" voice quality caused by breathing the mixture.

From neon to radon

Neon is used chiefly for filling lamps and lumi­nous sign tubes. When an electric current is passed through a tube containing neon at a low pressure, the gas emits a bright reddish-orange light. By using various gas mixtures, it is possible to produce the wide range of col­ors of neon signs. Neon light can penetrate fog and is, therefore, used in many airport beacons.

Argon is mixed with 10 per cent nitrogen to fill electric light bulbs. The argon helps keep the incandescent filament from evaporating, giving the bulb a longer working life. Argon is also used in arc welding in a manner similar to helium.

Krypton, in combination with argon, fills most fluorescent lamps. It is also used in some electronic tubes. In luminous sign tubes, kryp­ton imparts a greenish-yellow color to the light.

Xenon is the gas in the tube of a photogra­pher's electronic flash gun and in the high-power electric lamps used in some light-

houses. It is also used in other powerful lamps and in certain instruments used in the study of nuclear particles.

Radon is a radioactive, heavy gas produced by the radioactive decay of radium in the earth's crust. It is released by soil and rocks. There are at least 28 known isotopes (forms) of radon—3 in nature and 25 produced artificially in nuclear reactions. Radon has been used in medicine for treating tumors. However, if in­haled in large quantities, it can cause lung can­cer.

Date: 2015-12-11; view: 1677