Metalloids can also be called semimetals. On the periodic table, the elements colored yellow, which generally border the stair-step line, are considered to be metalloids. Notice that aluminum borders the line, but it is considered to be a metal since all of its properties are like those of metals.
Silicon is a typical metalloid see Figure 1. It has luster like a metal, but is brittle like a nonmetal. Silicon is used extensively in computer chips and other electronics because its electrical conductivity is in between that of a metal and a nonmetal. Boron is a versatile element that can be incorporated into a number of compounds see Figure 2. Borosilicate glass is extremely resistance to thermal shock. Extreme changes in the temperature of objects containing borosilicates will not create any damage to the material, unlike other glass compositions, which would crack or shatter.
Because of their strength, boron filaments are used as light, high-strength materials for airplanes, golf clubs, and fishing rods. Sodium tetraborate is widely used in fiberglass as insulation and also is employed in many detergents and cleaners.
Arsenic has long played a role in murder mysteries, being used to commit the foul deed see Figure 3. This use of the material is not very smart since arsenic can be easily detected on autopsy.
We find arsenic in pesticides, herbicides, and insecticides, but the use of arsenic for these applications is decreasing due to the toxicity of the metal. Its effectiveness as an insecticide has led arsenic to be used as a wood preservative. Boron does not react with nonoxidizing acids.
Reduction of boric oxide with magnesium powder forms boron 95— An amorphous substance is a material that appears to be a solid, but does not have a long-range order like a true solid. Treatment with hydrochloric acid removes the magnesium oxide. Further purification of the boron begins with conversion of the impure boron into boron trichloride.
The next step is to heat a mixture of boron trichloride and hydrogen:. The crust is composed almost entirely of minerals in which the silicon atoms are at the center of the silicon-oxygen tetrahedron, which connect in a variety of ways to produce, among other things, chains, layers, and three-dimensional frameworks.
These minerals constitute the bulk of most common rocks, soil, and clays. In addition, materials such as bricks, ceramics, and glasses contain silicon compounds. It is possible to produce silicon by the high-temperature reduction of silicon dioxide with strong reducing agents, such as carbon and magnesium:.
Extremely pure silicon is necessary for the manufacture of semiconductor electronic devices. This process begins with the conversion of impure silicon into silicon tetrahalides, or silane SiH 4 , followed by decomposition at high temperatures. In this method, a rod of silicon is heated at one end by a heat source that produces a thin cross-section of molten silicon.
Slowly lowering the rod through the heat source moves the molten zone from one end of the rod to other. As this thin, molten region moves, impurities in the silicon dissolve in the liquid silicon and move with the molten region. Ultimately, the impurities move to one end of the rod, which is then cut off. This highly purified silicon, containing no more than one part impurity per million parts of silicon, is the most important element in the computer industry.
Pure silicon is necessary in semiconductor electronic devices such as transistors, computer chips, and solar cells. Like some metals, passivation of silicon occurs due the formation of a very thin film of oxide primarily silicon dioxide, SiO 2.
Silicon dioxide is soluble in hot aqueous base; thus, strong bases destroy the passivation. Removal of the passivation layer allows the base to dissolve the silicon, forming hydrogen gas and silicate anions.
For example:. Silicon reacts with halogens at high temperatures, forming volatile tetrahalides, such as SiF 4. Unlike carbon, silicon does not readily form double or triple bonds. Silicon compounds of the general formula SiX 4 , where X is a highly electronegative group, can act as Lewis acids to form six-coordinate silicon.
Antimony reacts readily with stoichiometric amounts of fluorine, chlorine, bromine, or iodine, yielding trihalides or, with excess fluorine or chlorine, forming the pentahalides SbF 5 and SbCl 5. Depending on the stoichiometry, it forms antimony III sulfide, Sb 2 S 3 , or antimony V sulfide when heated with sulfur.
As expected, the metallic nature of the element is greater than that of arsenic, which lies immediately above it in group These nonpolar molecules contain boron with sp 2 hybridization and a trigonal planar molecular geometry.
The fluoride and chloride compounds are colorless gasses, the bromide is a liquid, and the iodide is a white crystalline solid. Except for boron trifluoride, the boron trihalides readily hydrolyze in water to form boric acid and the corresponding hydrohalic acid. Boron trichloride reacts according to the equation:.
Boron trifluoride reacts with hydrofluoric acid, to yield a solution of fluoroboric acid, HBF 4 :. In this reaction, the BF 3 molecule acts as the Lewis acid electron pair acceptor and accepts a pair of electrons from a fluoride ion:. All the tetrahalides of silicon, SiX 4 , have been prepared. Silicon tetrachloride can be prepared by direct chlorination at elevated temperatures or by heating silicon dioxide with chlorine and carbon:.
It is possible to prepare silicon tetrafluoride by the reaction of silicon dioxide with hydrofluoric acid:. Hydrofluoric acid is the only common acid that will react with silicon dioxide or silicates.
This reaction occurs because the silicon-fluorine bond is the only bond that silicon forms that is stronger than the silicon-oxygen bond.
For this reason, it is possible to store all common acids, other than hydrofluoric acid, in glass containers. An early use of borax was to make perborate, the beaching agent once widely used in household detergents. Boron compound also came into the average home in the guise of food preservatives, expecialy for margarine and fish.
Boron is not present in nature in elemental form. It is found combined in borax, boric acid, kernite, ulexite, colemanite and borates. Vulcanic spring waters sometime contains boric acids. Borates are mined in US, Tibet, Chile and Turkey, with world production being about 2 million tonnes per year. Humans can be exposed to boron through fruit and vegetables, water , air and consumer products. When humans consume large amounts of boron-containing food, the boron concentrations in their bodies may rise to levels that can cause health problems.
Boron can infect the stomach, liver, kidneys and brains and can eventually lead to death. When exposure to small amounts of boron takes place irritation of the nose, throat or eyes may occur.
It takes 5 g of borc acid to make a person ill and 20 grams or more to put its life in danger. Eating fish or meat will not increase the boron concentrations in our bodies, as boron does not accumulate within the tissues of animals.
Boron is an element that occurs in the environment mainly through natural processes. Boron occurs naturally in the environment due to the release into air, soil and water through weathering.
It may also occur in groundwater in very small amounts.
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