Metalloid

On a standard periodic table, all eleven elements are in a diagonal region of the p-block extending from boron at the upper left to astatine at lower right.

They and their compounds are used in alloys, biological agents, catalysts, flame retardants, glasses, optical storage and optoelectronics, pyrotechnics, semiconductors, and electronics.

These elements include[21] hydrogen,[22] beryllium,[23] nitrogen,[24] phosphorus,[25] sulfur,[26] zinc,[27] gallium,[28] tin, iodine,[29] lead,[30] bismuth,[19] and radon.

[31] The term metalloid has also been used for elements that exhibit metallic lustre and electrical conductivity, and that are amphoteric, such as arsenic, antimony, vanadium, chromium, molybdenum, tungsten, tin, lead, and aluminium.

On average, seven elements are included in such lists; individual classification arrangements tend to share common ground and vary in the ill-defined[43] margins.

[69] When presented as a regular stairstep, elements with the highest critical temperature for their groups (Li, Be, Al, Ge, Sb, Po) lie just below the line.

[75] Elements bordering the metal–nonmetal dividing line are not always classified as metalloids, noting a binary classification can facilitate the establishment of rules for determining bond types between metals and nonmetals.

Physically, they are shiny, brittle solids with intermediate to relatively good electrical conductivity and the electronic band structure of a semimetal or semiconductor.

[105] They and their compounds are used in alloys, biological agents (toxicological, nutritional, and medicinal), catalysts, flame retardants, glasses (oxide and metallic), optical storage media and optoelectronics, pyrotechnics, semiconductors, and electronics.

[129] During World War I, both sides used "arsenic-based sneezing and vomiting agents…to force enemy soldiers to remove their gas masks before firing mustard or phosgene at them in a second salvo.

In 2003, arsenic trioxide (under the trade name Trisenox) was re-introduced for the treatment of acute promyelocytic leukaemia, a cancer of the blood and bone marrow.

[181] Future applications for GeSbTe may include, "ultrafast, entirely solid-state displays with nanometre-scale pixels, semi-transparent 'smart' glasses, 'smart' contact lenses, and artificial retina devices.

[186] Boron carbide has been identified as a possible replacement for more toxic barium or hexachloroethane mixtures in smoke munitions, signal flares, and fireworks.

[n 21] Arsenic trisulfide As2S3 was used in old naval signal lights; in fireworks to make white stars;[189] in yellow smoke screen mixtures; and in initiator compositions.

Carbon, in black powder, is a constituent of fireworks rocket propellants, bursting charges, and effects mixtures, and military delay fuses and igniters.

[199] Silicon is the leading commercial semiconductor; it forms the basis of modern electronics (including standard solar cells)[200] and information and communication technologies.

[202] Germanium has largely been replaced by silicon in semiconducting devices, being cheaper, more resilient at higher operating temperatures, and easier to work during the microelectronic fabrication process.

Both form type III-V semiconductors (such as GaAs, AlSb or GaInAsSb) in which the average number of valence electrons per atom is the same as that of Group 14 elements, but they have direct band gaps.

[208] Cadmium telluride (CdTe) is used in solar modules for its high conversion efficiency, low manufacturing costs, and large band gap of 1.44 eV, letting it absorb a wide range of wavelengths.

[215] Boron, silicon, germanium, antimony, and tellurium,[216] as well as heavier metals and metalloids such as Sm, Hg, Tl, Pb, Bi, and Se,[217] can be found in topological insulators.

[230] "Meta-metal" is sometimes used instead to refer to certain metals (Be, Zn, Cd, Hg, In, Tl, β-Sn, Pb) located just to the left of the metalloids on standard periodic tables.

[238] Use of this latter term has more recently been discouraged by Atkins et al.[2] as it has a more common meaning that refers to the electronic band structure of a substance rather than the overall classification of an element.

[258] Analogies between boron and transition metals have been noted in the formation of complexes,[259] and adducts (for example, BH3 + CO →BH3CO and, similarly, Fe(CO)4 + CO →Fe(CO)5),[n 25] as well as in the geometric and electronic structures of cluster species such as [B6H6]2− and [Ru6(CO)18]2−.

[366] Antimony has some definite cationic chemistry,[367] SbO+ and Sb(OH)2+ being present in acidic aqueous solution;[368][n 40] the compound Sb8(GaCl4)2, which contains the homopolycation, Sb82+, was prepared in 2004.

[384] It dissolves in boiling alkalis, to give the tellurite and telluride: 3 Te + 6 KOH = K2TeO3 + 2 K2Te + 3 H2O, a reaction that proceeds or is reversible with increasing or decreasing temperature.

[397] The increase in metallic character evident in tellurium, as compared to the lighter chalcogens, is further reflected in the reported formation of various other oxyacid salts, such as a basic selenate 2TeO2·SeO3 and an analogous perchlorate and periodate 2TeO2·HXO4.

Steele[433] notes the paradoxical chemical behaviour of aluminium: "It resembles a weak metal in its amphoteric oxide and in the covalent character of many of its compounds ...

[455] Polonium is a reluctant oxidizing agent, unlike its lightest congener oxygen: highly reducing conditions are required for the formation of the Po2− anion in aqueous solution.

It has been extrapolated to have the narrow liquid range ordinarily associated with nonmetals (mp 302 °C; bp 337 °C),[481] although experimental indications suggest a lower boiling point of about 230±3 °C.

As of 5 April 2014, prices for small samples (up to 100 g) of silicon, antimony and tellurium, and graphite, aluminium and selenium, average around one third the cost of silver (US$1.5 per gram or about $45 an ounce).

Several dozen metallic pellets, reddish-brown. They have a highly polished appearance, as if they had a cellophane coating.
Copper-germanium alloy pellets, likely ~84% Cu; 16% Ge. [ 107 ] When combined with silver the result is a tarnish resistant sterling silver . Also shown are two silver pellets.
A clear glass dish on which is a small mound of a white crystalline powder.
Arsenic trioxide or white arsenic , one of the most toxic and prevalent forms of arsenic . The antileukaemic properties of white arsenic were first reported in 1878. [ 119 ]
A bunch of pale yellow semi-transparent thin strands, with bright points of white light at their tips.
Optical fibers , usually made of pure silicon dioxide glass, with additives such as boron trioxide or germanium dioxide for increased sensitivity
A man is standing in the dark. He is holding out a short stick at mid-chest level. The end of the stick is alight, burning very brightly, and emitting smoke.
Archaic blue light signal , fuelled by a mixture of sodium nitrate , sulfur , and (red) arsenic trisulfide [ 183 ]
A small square plastic piece with three parallel wire protrusions on one side; a larger rectangular plastic chip with multiple plastic and metal pin-like legs; and a small red light globe with two long wires coming out of its base.
Semiconductor -based electronic components. From left to right: a transistor , an integrated circuit , and an LED . The elements commonly recognised as metalloids find widespread use in such devices, as elemental or compound semiconductor constituents ( Si , Ge or GaAs , for example) or as doping agents ( B , Sb , Te , for example).
Several dozen small angular stone like shapes, grey with scattered silver flecks and highlights.
Boron, shown here in the form of its β- rhombohedral phase (its most thermodynamically stable allotrope ) [ 240 ]
A lustrous blue grey potato-shaped lump with an irregular corrugated surface.
Silicon has a blue-grey metallic lustre .
Greyish lustrous block with uneven cleaved surface.
Germanium is sometimes described as a metal
Two dull silver clusters of crystalline shards.
Arsenic , sealed in a container to prevent tarnishing
A glistening silver rock-like chunk, with a blue tint, and roughly parallel furrows.
Antimony , showing its brilliant lustre
A shiny silver-white medallion with a striated surface, irregular around the outside, with a square spiral-like pattern in the middle.
Tellurium , described by Dmitri Mendeleev as forming a transition between metals and nonmetals [ 379 ]
A shiny grey-black cuboid nugget with a rough surface.
Carbon (as graphite ). Delocalized valence electrons within the layers of graphite give it a metallic appearance. [ 400 ]
A silvery white steam-iron shaped lump with semi-circular striations along the width of its top surface and rough furrows in the middle portion of its left edge.
High purity aluminium is much softer than its familiar alloys . People who handle it for the first time often ask if it is the real thing. [ 420 ]
A small glass jar filled with small dull grey concave buttons. The pieces of selenium look like tiny mushrooms without their stems.
Grey selenium , being a photoconductor , conducts electricity around 1,000 times better when light falls on it, a property used since the mid-1870s in various light-sensing applications [ 435 ]
Shiny violet-black coloured crystalline shards.
Iodine crystals, showing a metallic lustre . Iodine is a semiconductor in the direction of its planes, with a band gap of ~1.3 eV. It has an electrical conductivity of 1.7 × 10 −8 S•cm −1 at room temperature . [ 489 ] This is higher than selenium but lower than boron, the least electrically conducting of the recognised metalloids. [ n 51 ]
Many small, shiny, silver-coloured spheres on the left; many of the same sized spheres on the right are duller and darker than the ones of the left and have a subdued metallic shininess.
White tin (left) and grey tin (right). Both forms have a metallic appearance.