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Metalloid - Metalloid

Chemical element with relatively weak metallic and non-metallic properties
  13 14 15 16 17
2 B.
boron
C.
carbon
N
nitrogen
O
oxygen
F.
fluorine
3 Al
aluminum
Si
silicon
P
phosphorus
S.
sulfur
Cl
chlorine
4 Ga
gallium
Ge
Germanium
As
arsenic
Se
selenium
Br
bromine
5 In
Indium
Sn
Tin
Sb
antimony
The
Tellurium
I
iodine
6 Tl
Thallium
Pb
lead
Bi
bismuth
Po
polonium
At
Astatine
 
Generally recognized (86-99%): B, Si, Ge, As, Sb, Te
Detected irregularly (40–48%): Po, At
Less generally recognized (24%): Se
Rarely recognized (8-10%): C, Al
(All other elements mentioned in less than 6% of the sources)

Recognition status of some elements in the p-block of the periodic table as metalloids. Percentages are average frequencies in the lists of metalloids. The stepped line is a typical example of the arbitrary metal-non-metal dividing line found in some periodic tables.

A Metalloid is a type of chemical element in which the properties predominate between those of metals and non-metals or which are a mixture of these. There is no standard definition of a metalloid and no complete agreement on what elements are metalloids. Despite a lack of specificity, the term continues to be used in the chemistry literature.

The six commonly recognized metalloids are boron, silicon, germanium, arsenic, antimony, and tellurium. Five elements are classified less often: carbon, aluminum, selenium, polonium and astatine. In a standard periodic system, all eleven elements are located in a diagonal area of ​​the p-block, which extends from boron in the upper left to astatine lower right. Some periodic tables contain a dividing line between metals and non-metals, and the metalloids may be near that line.

Typical metalloids have a metallic appearance, but are brittle and only fair electrical conductors. Chemically, they mostly behave like non-metals. You can form alloys with metals. Most of their other physical and chemical properties are medium in nature. Metalloids are usually too brittle to have structural uses. They and their compounds are used in alloys, biological agents, catalysts, flame retardants, glasses, optical memories and optoelectronics, pyrotechnics, semiconductors and electronics.

The electrical properties of silicon and germanium enabled the development of the semiconductor industry in the 1950s and the development of solid-state electronics from the early 1960s.

The term Metalloid related originally on non-metals. Its more recent meaning as a category of elements with intermediate or hybrid properties spread between 1940 and 1960. Metalloids are sometimes referred to as semimetals, a practice that was discouraged as the term was used Semi-metal has a different meaning in physics than in chemistry. In physics, it refers to a certain type of electronic band structure of a substance. In this context, only arsenic and antimony are semimetals and are widely recognized as metalloids.

Definitions

Judgment based

A metalloid is an element that has a preponderance of properties between or made up of metals and non-metals and is therefore difficult to classify as metal or non-metal. This is a generic definition based on metalloid attributes that are consistently cited in the literature. The difficulty of categorizing is a key attribute. Most elements have a mixture of metallic and non-metallic properties and can be classified according to which properties are more pronounced. Only those elements at or near the edges, in which neither the metallic nor the non-metallic properties predominate with sufficient clarity, are classified as metalloids.

Boron, silicon, germanium, arsenic, antimony and tellurium are widely recognized as metalloids. Sometimes, depending on the author, one or more of selenium, polonium, or astatine are added to the list. Boron is sometimes excluded alone or with silicon. Sometimes tellurium is not considered a metalloid. The inclusion of antimony, polonium and astatine as metalloids has been questioned.

Other elements are occasionally classified as metalloids. These elements include hydrogen, beryllium, nitrogen, phosphorus, sulfur, zinc, gallium, tin, iodine, lead, bismuth, and radon. The term metalloid has also been used for elements that have metallic luster, electrical conductivity and are amphoteric, such as arsenic, antimony, vanadium, chromium, molybdenum, tungsten, tin, lead and aluminum. The p-block metals and non-metals (such as carbon or nitrogen) that can form alloys with metals or change their properties have also been sometimes viewed as metalloids.

Criteria-based

element IE
(kcal / mol)
IE
(kJ / mol)
DE Band structure
boron 191 801 2,04 semiconductor
silicon 188 787 1,90 semiconductor
Germanium 182 762 2.01 semiconductor
arsenic 226 944 2.18 Semi-metal
antimony 199 831 2.05 Semi-metal
Tellurium 208 869 2.10 semiconductor
average 199 832 2.05
The elements commonly recognized as metalloids and their ionization energies (IE); Electronegativities (EN, revised Pauling scale); and electronic band structures (most thermodynamically stable forms under ambient conditions).

There is neither a generally accepted definition of a metalloid nor a division of the periodic table into metals, metalloids and non-metals. Hawkes questioned the feasibility of establishing a specific definition, noting that anomalies can be found in several attempted constructs. The classification of an element as a metalloid has been described by Sharp as "arbitrary".

The number and identity of the metalloids depends on which classification criteria are used. Emsley recognized four metalloids (germanium, arsenic, antimony and tellurium); James et al. listed twelve (Emsley plus boron, carbon, silicon, selenium, bismuth, polonium, moscovium and livermorium). Such lists contain an average of seven items. Individual classification agreements usually have something in common and vary in the poorly defined margins.

A single quantitative criterion like electronegativity is commonly used, metalloids with electronegativity values ​​of 1.8 or 1.9 to 2.2. Further examples are the packing efficiency (the volume fraction in a crystal structure occupied by atoms) and the Goldhammer-Herzfeld criterion. The generally recognized metalloids have packaging efficiencies between 34% and 41%. The Goldhammer-Herzfeld ratio, roughly equal to the cube of the atomic radius divided by the molar volume, is a simple measure of how metallic an element is, with the detected metalloids having ratios of about 0.85 to 1.1 and an average of 1.0 be. Other authors have relied on atomic conductivity or the mass coordination number, for example.

Writing on the role of classification in science, Jones noted that "[classes] are usually defined by more than two attributes". Masterton and Slowinski used three criteria to describe the six elements that are commonly recognized as metalloids: metalloids have ionization energies around 200 kcal / mol (837 kJ / mol) and electronegativity values ​​close to 2.0. They also said that metalloids are typically semiconductors, although antimony and arsenic (semi-metals from a physical point of view) have electrical conductivities close to that of metals. Selenium and polonium are not suspected of being in this scheme, while the status of astatine is uncertain.

In this context, Vernon suggested that a metalloid is a chemical element which in its standard state (a) has the electronic band structure of a semiconductor or a semimetal; and (b) an average first ionization potential "(say 750-1,000 kJ / mol)"; and (c) an intermediate electronegativity (1.9-2.2).

Area of ​​the periodic table

Distribution and detection status
of elements classified as metalloids
1 2 12 13 14 15 16 17 18
H.     He
Li Be B. C. N. Ö F. No
N / A Mg Al Si P. S. Cl Ar
K. Approx. Zn Ga Ge How Se Br Kr
Rb Sr. CD in the Sn Sb Te I Xe
Cs Ba Ed Tl Pb Bi Po At the Marg
Fr. Ra Cn Nh Fl Mc Lv Ts Above
 
Common (93%) to rare (9%) as
Metalloid recognized: B, C, Al, Si, Ge, As, Se, Sb, Te, Po, At
Very rare (1-5%): H, Be, P, S, Ga, Sn, I, Pb, Bi, Fl, Mc, Lv, Ts
Sporadic: N, Zn, Rn

Periodic table extract with groups 1–2 and 12–18 as well as a dividing line between metals and non-metals. Percentages are mean frequencies of occurrence in the list of metalloid lists. Elements recognized sporadically show that the metalloid mesh is sometimes very wide; Although they are not included in the list of metalloid lists, there are isolated references to their designation as metalloids in the literature (as cited in this article).

place

Metalloids lie on either side of the dividing line between metals and non-metals. This can be found in various configurations in some periodic tables. Elements in the lower left of the line generally show increasing metallic behavior. Elements at the top right indicate increasing non-metallic behavior. Shown as a regular staircase, elements with the highest critical temperature for their groups (Li, Be, Al, Ge, Sb, Po) lie directly below the line.

The diagonal positioning of the metalloids is an exception to the observation that elements with similar properties tend to appear in vertical groups. A related effect can be seen in other diagonal similarities between some elements and their lower right neighbors, notably lithium-magnesium, beryllium-aluminum, and boron-silicon. Rayner-Canham has argued that these similarities extend to carbon-phosphorus, nitrogen-sulfur, and three rows of D-blocks.

This exception arises from competing horizontal and vertical trends in nuclear charge. Over time, the nuclear charge increases with the atomic number, as does the number of electrons. The additional attraction to external electrons with increasing nuclear charge generally outweighs the shielding effect of having more electrons. With some irregularities, atoms therefore get smaller, the ionization energy increases and there is a gradual change in character over a period of time from strongly metallic to weakly metallic, too weakly nonmetallic to strongly nonmetallic elements. When going down a main group, the effect of increasing the nuclear charge is generally offset by the effect that additional electrons are further away from the nucleus. Atoms generally get larger, the ionization energy decreases, and the metallic character increases. The net effect is that the position of the metal-non-metal transition zone shifts to the right when descending in a group and analogous diagonal similarities can be seen elsewhere in the periodic table, as mentioned earlier.

Alternative treatments

Elements that border the metal-non-metal dividing line are not always classified as metalloids. Binary classification can make it easier to establish rules for determining the types of bonds between metals and non-metals. In such cases, instead of worrying about the marginal nature of the items in question, the authors concerned focus on one or more attributes of interest to make their classification decisions. Your considerations may or may not be made explicit and can sometimes seem arbitrary. Metalloids can be grouped with metals; or viewed as non-metals; or treated as a sub-category of non-metals. Other authors have suggested classifying some elements as metalloids. "Emphasizes that properties change gradually rather than abruptly as you move across or down the periodic table". Some periodic tables distinguish elements that are metalloids and have no formal dividing line between metals and non-metals. Instead, metalloids appear in a diagonal band or diffuse area. The most important consideration is to explain the context for the taxonomy being used.

properties

Metalloids usually look like metals, but behave largely like non-metals. Physically, they are shiny, brittle solids with medium to relatively good electrical conductivity and the electronic band structure of a semi-metal or semiconductor. Chemically, they mostly behave like (weak) non-metals, have medium ionization energies and electronegativity values ​​as well as amphoteric or weakly acidic oxides. You can form alloys with metals. Most of their other physical and chemical properties are medium in nature.

Compared to metals and non-metals

The characteristic properties of metals, metalloids and non-metals are summarized in the table. The physical properties are listed in the order of easy determination. The chemical properties range from general to specific to descriptive.

Physical property Metals Metalloids Non-metals
Form solid; some liquids at or near room temperature (Ga, Hg, Rb, Cs, Fr) solid mostly gaseous
Look shiny (at least when freshly broken) glittering several colorless; others colored or metallic gray to black
elasticity typically elastic, ductile, malleable (if solid) brittle brittle when firm
Electric conductivity good to high medium to good poor to good
Band structure metallic (Bi = semi-metal) are semiconductors or exist, if not (As, Sb = semimetallic), in semiconducting forms Semiconductor or insulator
Chemical property Metals Metalloids Non-metals
General chemical behavior metallic non-metallic non-metallic
Ionization energy relatively low mean ionization energies, which are usually between those of metals and non-metals quite high
Electronegativity usually low Have electronegativity values ​​close to 2 (revised Pauling scale) or in the range 1.9–2.2 (Allen scale) high
When mixing
with metals
Give alloys can form alloys ionic or interstitial compounds are formed
Oxides lower basic oxides; higher oxides become increasingly acidic amphoteric or weakly acidic angry

The table above reflects the hybrid nature of metalloids. The properties of Shape, appearance and Behavior when mixing with metals are more like metals. elasticity and general chemical behavior are more like non-metals. Electrical conductivity, band structure, ionization energy, electronegativity and Oxides lie in between.

General uses

The focus of this section is on the identified metalloids. Elements that are less commonly recognized as metalloids are usually classified as either metals or non-metals. Some of them are listed here for comparison purposes.

Metalloids are too brittle to have structural uses in their pure form. They and their compounds are used as (or in) alloy components, biological agents (toxicological, nutritional and medical), catalysts, flame retardants, glasses (oxidic and metallic), optical storage media and optoelectronics, pyrotechnics, semiconductors and electronics.

Alloys

The British metallurgist Cecil Desch wrote early in the history of intermetallic compounds that "certain non-metallic elements are able to form compounds with a clearly metallic character with metals, and these elements can therefore be incorporated into the composition of alloys". In particular, he associated silicon, arsenic and tellurium with the alloying elements. Phillips and Williams suggested that compounds of silicon, germanium, arsenic, and antimony with B-metals "are probably best classified as alloys".

Alloys with transition metals are well represented among the lighter metalloids. Boron can be used with such metals of the composition M n B form intermetallic compounds and alloys if n > 2. Ferroboron (15% boron) is used to introduce boron into steel; Nickel-boron alloys are components of welding alloys and case hardening compositions for the mechanical engineering industry. Silicon alloys with iron and aluminum are widespread in the steel and automotive industries. Germanium forms many alloys, especially with the coin metals.

The heavier metalloids continue the theme. Arsenic can form alloys with metals, including platinum and copper; It is also added to copper and its alloys to improve corrosion resistance and appears to give the same benefit when added to magnesium. Antimony is known as an alloy former, also in the case of the coin metals. The alloys include tin (a tin alloy with up to 20% antimony) and metal (a lead alloy with up to 25% antimony). Tellurium alloys easily with iron as ferrotellurium (50–58% tellurium) and with copper in the form of copper tellurium (40–50% tellurium). Ferrotellurium is used as a stabilizer for carbon in cast steel. Of the non-metallic elements that are less commonly recognized as metalloids, selenium - in the form of ferrous selenium (50–58% selenium) - is used to improve the machinability of stainless steels.

Biological means

Arsenic trioxide or white arsenic , one of the most toxic and widely used forms of arsenic. The antileukemic properties of white arsenic were first described in 1878.

All six elements commonly recognized as metalloids have toxic, dietetic, or medicinal properties. Arsenic and antimony compounds are particularly toxic; Boron, silicon and possibly arsenic are essential trace elements. Boron, silicon, arsenic, and antimony have medicinal uses, and germanium and tellurium are believed to have potential.

Boron is used in insecticides and herbicides. It is an essential trace element. As boric acid, it has antiseptic, antifungal, and antiviral properties.

Silicon is contained in Silatran, a highly toxic rodenticide. Long-term inhalation of quartz dust leads to silicosis, a deadly disease of the lungs. Silicon is an essential trace element. Silicone gel can be used on severely burned patients to reduce scars.

Germanium salts are potentially harmful to humans and animals if ingested on a prolonged basis. There is interest in the pharmacological effects of germanium compounds, but not yet an approved medicine.

Arsenic is notoriously toxic and can also be an essential element in ultratrace amounts. During World War I, both sides used "arsenic-based sneezing and vomiting agents ... to force enemy soldiers to remove their gas masks before burning mustard or phosgene at them salvo in a second." It has been used as a pharmaceutical agent since ancient times, including for Treating syphilis before developing antibiotics. Arsenic is also a component of melarsoprol, a medicine used to treat human African trypanosomiasis, or sleeping sickness. In 2003, arsenic trioxide (under the trade name Trisenox) was reintroduced for the treatment of acute promyelocyte leukemia, a cancer of the blood and bone marrow. Arsenic in drinking water, which causes lung and bladder cancer, has been linked to reductions in breast cancer mortality.

Metallic antimony is relatively non-toxic, but most antimony compounds are toxic. Two antimony compounds, sodium stibogluconate and stibophen, are used as antiparasitic drugs.

Elemental tellurium is not considered particularly toxic. Two grams of sodium tellurate can be fatal if administered. People exposed to low levels of tellurium in the air emit a foul and persistent garlic-like odor. Tellurium dioxide has been used to treat seborrheic dermatitis. Other tellurium compounds were used as antimicrobial agents prior to the development of antibiotics. In the future, antibiotics may need to be replaced with compounds that have become ineffective due to bacterial resistance.

Of the elements less commonly recognized as metalloids, beryllium and lead are known for their toxicity. Lead arsenate has been used extensively as an insecticide. Sulfur is one of the oldest fungicides and pesticides. Phosphorus, sulfur, zinc, selenium and iodine are essential nutrients, aluminum, tin and lead can be. Sulfur, gallium, selenium, iodine, and bismuth have medicinal uses. Sulfur is a component of sulfonamide medications that is still widely used for conditions such as acne and urinary tract infections. Gallium nitrate is used to treat the side effects of cancer. Gallium citrate, a radiopharmaceutical, makes it easier to image inflamed areas of the body. Selenium sulfide is used in medicated shampoos and to treat skin infections such as tinea versicolor. Iodine is used as a disinfectant in various forms. Bismuth is a component of some antibacterial agents.

Catalysts

Boron trifluoride and trichloride are used as catalysts in organic synthesis and electronics; The tribromide is used to produce diborane. Non-toxic boron ligands could replace toxic phosphorus ligands in some transition metal catalysts. Silicon dioxide sulfuric acid (SiO 2 OSO 3 H) is used in organic reactions. Germanium dioxide is sometimes used as a catalyst in making PET plastic for containers; Cheaper antimony compounds like trioxide or triacetate are more commonly used for the same purpose, despite concerns about antimony contamination in food and beverages. Arsenic trioxide was used in the production of natural gas to aid in the removal of carbon dioxide, as were selenic acid and telluric acid. Selenium acts as a catalyst in some microorganisms. Tellurium, its dioxide and its tetrachloride are powerful catalysts for the air oxidation of carbon above 500 ° C. Graphite oxide can be used as a catalyst in the synthesis of imines and their derivatives. Activated carbon and alumina have been used as catalysts to remove sulfur contaminants from natural gas. Aluminum doped with titanium has been identified as a replacement for expensive noble metal catalysts used in the manufacture of industrial chemicals.

Flame retardants

Compounds of boron, silicon, arsenic and antimony have been used as flame retardants. Boron in the form of borax has been used as a flame retardant for textiles since at least the 18th century. Silicon compounds such as silicones, silanes, silsesquioxane, silicon dioxide and silicates, some of which have been developed as alternatives to more toxic halogenated products, can greatly improve the flame retardancy of plastic materials. Arsenic compounds such as sodium arsenite or sodium arsenate are effective flame retardants for wood, but have been used less frequently due to their toxicity. Antimony trioxide is a flame retardant. Aluminum hydroxide has been used as a flame retardant made from wood fibers, rubber, plastic and textiles since the 1890s. Aside from aluminum hydroxide, the use of phosphorus-based flame retardants - for example in the form of organophosphates, for example - now outperforms the other main types of retardation. These use boron, antimony or halogenated hydrocarbon compounds.

Glass formation

The oxides B 2 O 3 , SiO 2 , GeO 2 , As 2 O 3 and Sb 2 O 3 easily form glasses. TeO 2 forms a glass, but this requires a "heroic quench rate" or the addition of an impurity; otherwise the crystalline form results. These compounds are used in chemical, domestic, and industrial glassware and optics. Boron trioxide is used as a glass fiber additive and is also a component of borosilicate glass, which is often used for laboratory glassware and household ovenware because of its low thermal expansion. Most common glassware is made from silicon dioxide. Germanium dioxide is used as a glass fiber additive as well as in infrared optical systems. Arsenic trioxide is used in the glass industry as