化学专业英语之第三至第八副族元素

化学专业英语之第三至第八副族元素

GROUPS IIIB—VIIIB ELEMENTS

Group I-B includes the elements scandium, yttrium, lanthanum, and actinium1, and the two rare-earth series of fourteen elements each2—the lanthanide and actinide series. The principal source of these elements is the high gravity river and beach sands built up by a water-sorting process during long periods of geologic time. Monazite sand, which contains a mixture of rare earth phosphates, and an yttrium silicate in a heavy sand are now commercial sources of a number of these scarce elements.

Separation of the elements is a difficult chemical operation. The solubilities of their compounds are so nearly alike that a separation by fractional crystallization is laborious and time-consuming. In recent years, ion exchange resins in high columns have proved effective. When certain acids are allowed to flow down slowly through a column containing a resin to which ions of Group III B metals are adsorbed, ions are successively released from the resin3. The resulting solution is removed from the bottom of the column or tower in bands or sections. Successive sections will contain specific ions in the order of release by the resin. For example .lanthanum ion (La3+) is most tightly held to the resin and is the last to be extracted, lutetium ion (Lu3+) is less tightly held and appears in one of the first sections removed. If the solutions are recycled and the acid concentrations carefully controlled, very effective separations can be accomplished. Quantities of all the lanthanide series (except promethium, Pm, which does not exist in nature as a stable isotope) are produced for the chemical market.

The predominant group oxidation number of the lanthanide series is +3, but some of the elements exhibit variable oxidation states. Cerium forms

cerium( III )and cerium ( IV ) sulfates, Ce

2 (SO

4

)

3

and Ce(SO

4

)

2

, which

are employed in certain oxidation-reduction titrations. Many rare earth compounds are colored and are paramagnetic, presumably as a result of unpaired electrons in the 4f orbitals.

All actinide elements have unstable nuclei and exhibit radioactivity. Those with higher atomic numbers have been obtained only in trace amounts. Actinium (

89

Ac), like lanthanum, is a regular Group IIIB element.

Group IVB Elements

In chemical properties these elements resemble silicon, but they become increasingly more metallic from titanium to hafnium. The predominant oxidation state is +4 and, as with silica (SiO

2

), the oxides of these elements occur naturally in small amounts. The formulas and mineral names

of the oxides are TiO

2, rutile; ZrO

2

, zirconia; HfO

2

, hafnia.

Titanium is more abundant than is usually realized. It comprises about 0.44%of the earth's crust. It is over 5.0%in average composition of first analyzed moon rock. Zirconium and titanium oxides occur in small percentages in beach sands.

Titanium and zirconium metals are prepared by heating their chlorides with magnesium metal. Both are particularly resistant to corrosion and have high melting points.

Pure TiO

2

is a very white substance which is taking the place of white

lead in many paints. Three-fourths of the TiO

2

is used in white paints, varnishes, and lacquers. It has the highest index of refraction (2.76) and the greatest hiding power of all the common white paint materials.

TiO

2

also is used in the paper, rubber, linoleum, leather, and textile industries.

Group VB Elements: Vanadium, Niobium, and Tantalum

These are transition elements of Group VB, with a predominant oxidation number of + 5. Their occurrence is comparatively rare.

These metals combine directly with oxygen, chlorine, and nitrogen to form oxides, chlorides, and nitrides, respectively. A small percentage of vanadium alloyed with steel gives a high tensile strength product which is very tough and resistant to shock and vibration. For this reason vanadium alloy steels are used in the manufacture of high-speed tools and heavy machinery. Vanadium oxide is employed as a catalyst in the contact process of manufacturing sulfuric acid. Niobium is a very rare element, with limited use as an alloying element in stainless steel. Tantalum has a very high melting point (2850 C) and is resistant to corrosion by most acids and alkalies.

Groups VIB and VIIB Elements

Chromium, molybdenum, and tungsten are Group VIB elements. Manganese is the only chemically important element of Group VIIB. All these elements exhibit several oxidation states, acting as metallic elements in lower

oxidation states and as nonmetallic elements in higher oxidation states. Both chromium and manganese are widely used in alloys, particularly in alloy steels.

Group VIIIB Metals

Group VIIIB contains the three triads of elements. These triads appear at the middle of long periods of elements in the periodic table, and are members of the transition series. The elements of any given horizontal triad have many similar properties, but there are marked differences between the properties of the triads, particularly between the first triad and the other two. Iron, cobalt, and nickel are much more active than members of the other two triads, and are also much more abundant in the earth's crust. Metals of the second and third triads, with many common properties, are usually grouped together and called the platinum metals.

These elements all exhibit variable oxidation states and form numerous coordination compounds.

Corrosion

Iron exposed to the action of moist air rusts rapidly, with the formation of a loose, crumbly deposit of the oxide. The oxide does not adhere to the surface of the metal, as does aluminum oxide and certain other metal oxides, but peels off .exposing a fresh surface of iron to the action of the air. As a result, a piece of iron will rust away completely in a relatively short time unless steps are taken to prevent the corrosion. The chemical steps in rusting are rather obscure, but it has been established that the rust is a hydrated oxide of iron, formed by the action of both oxygen and moisture, and is markedly speeded up by the presence of minute amounts of carbon dioxide5.

Corrosion of iron is inhibited by coating it with numerous substances, such as paint, an aluminum powder gilt, tin, or organic tarry substances or by galvanizing iron with zinc. Alloying iron with metals such as nickel or chromium yields a less corrosive steel. "Cathodic protection" of iron for lessened corrosion is also practiced. For some pipelines and standpipes zinc or magnesium rods in the ground with a wire connecting them to an iron object have the following effect: with soil moisture acting as an electrolyte for a Fe — Zn couple the Fe is lessened in its tendency to become Fe2+. It acts as a cathode rather than an anode.