14 Si 28.09

The chemical element silicon is classed as a metalloid. It was discovered in 1824 by Jacob Berzelius.

Data Zone Classification: Silicon is a metalloid Color: silvery Atomic weight: 28.0855 State: solid Melting point: 1414 oC, 1687 K Boiling point: 3265 oC, 3538 K Electrons: 14 Protons: 14 Neutrons in most abundant isotope: 14 Electron shells: 2,8,4 Electron configuration: 1s2 2s2 2p6 3s2 3p2 Density @ 20oC: 2.33 g/cm3 Show more, including: Heats, Energies, Oxidation,

Reactions, Compounds, Radii, Conductivities Atomic volume: 12.1 cm3/mol Structure: diamond structure Hardness: 7 mohs Specific heat capacity 0.71 J g-1 K-1 Heat of fusion 50.21 kJ mol-1 Heat of atomization 456 kJ mol-1 Heat of vaporization 359 kJ mol-1 1st ionization energy 786.4 kJ mol-1 2nd ionization energy 1577 kJ mol-1 3rd ionization energy 3231.4 kJ mol-1 Electron affinity 133.6 kJ mol-1 Minimum oxidation number -4 Min. common oxidation no. -4 Maximum oxidation number 4 Max. common oxidation no. 4 Electronegativity (Pauling Scale) 1.9 Polarizability volume 5.4 Å3 Reaction with air none Reaction with 15 M HNO 3 none Reaction with 6 M HCl none Reaction with 6 M NaOH mild, ⇒ silicates Oxide(s) SiO 2 Hydride(s) SiH 4 (silane), Si 2 H 6 + more Chloride(s) SiCl 4 , Si 2 Cl 6 + more Atomic radius 110 pm Ionic radius (1+ ion) – Ionic radius (2+ ion) – Ionic radius (3+ ion) – Ionic radius (1- ion) – Ionic radius (2- ion) – Ionic radius (3- ion) – Thermal conductivity 149 W m-1 K-1 Electrical conductivity 4 x 102 S m-1 Freezing/Melting point: 1414 oC, 1687 K

Discovery of Silicon

Dr. Doug Stewart

Quartz (crystalline silicon dioxide) has been known to people for many thousands of years. Flint is a form of quartz, and tools made from flint were in everyday use in the stone age.

In 1789, the French chemist Antoine Lavoisier proposed that a new chemical element could be found in quartz. This new element, he said, must be very abundant. (1) He was right, of course. Silicon accounts for 28% of the weight of Earth’s crust.

It is possible that in England, in 1808, Humphry Davy isolated partly pure silicon for the first time, but he did not realize it. (2)

In 1811, French chemists Joseph L. Gay-Lussac and Louis Jacques Thénard may also have made impure silicon by reacting potassium with silicon tetrafluoride to produce a reddish brown solid which was probably amorphous silicon. They did not, however, attempt to purify this new substance. (3), (4)

In 1824, Swedish chemist Jacob Berzelius produced a sample of amorphous silicon, a brown solid, by reacting potassium fluorosilicate with potassium and purifying the product with repeated washing. He named the new element silicium. (3), (4)

At that time the concept of semiconductors lay a century in the future. Unaware that such materials existed and that silicon was an excellent example of a semiconductor, scientists debated whether the new element should be classed as a metal or a nonmetal.

Berzelius believed it was a metal, while Humphry Davy thought it was a nonmetal. (5) The problem was that the new element was a better conductor of electricity than nonmetals, but not as good a conductor as a metal.

Silicon was given its name in 1831 by Scottish chemist Thomas Thomson. He retained part of Berzelius’s name, from ‘silicis,’ meaning flint. He changed the element’s ending to on because the element was more similar to nonmetals boron and carbon than it was to metals such as calcium and magnesium. (Silicis, or flint, was probably our first use of silicon dioxide. (4), (6))

In 1854, Henri Deville produced crystalline silicon for the first time. He did this by electrolyzing an impure melt of sodium aluminum chloride to produce aluminum silicide. The aluminum was removed with water, leaving silicon crystals. (4)

Interesting Facts about Silicon

The lowest acceptable purity for electronic grade silicon is 99.9999999%. This means that for every billion atoms, only one non-silicon atom is allowed.

Silicon is the second most abundant element in our planet’s crust. Oxygen (47.3%) and silicon (27.7%) together make up 75% of the weight of Earth’s crust. Most of the crust’s silicon exists as silicon dioxide; we are familiar with this as sand or quartz.

Silicon is the eighth most abundant element in the Universe; it is made in stars with a mass of eight or more Earth suns. Near the end of their lives these stars enter the carbon burning phase, adding helium nuclei to carbon to produce oxygen, neon, magnesium and silicon.

Silicon burning is the last phase of a heavy star’s life before a supernova and lasts only about one day. During this phase, helium nuclei add to silicon to make sulfur, argon, calcium, titanium, chromium, iron and nickel.

How silicon is made into wafers and computer chips. Not only about silicon: how the next generation of electronic devices may work.

Appearance and Characteristics

Harmful effects:

Silicon is not known to be toxic, but if breathed in as a fine silica/silicate dust it may cause chronic respiratory problems. Silicates such as asbestos are carcinogenic.

Characteristics:

Silicon is a hard, relatively inert metalloid and in crystalline form is very brittle with a marked metallic luster.

Silicon occurs mainly in nature as the oxide and as silicates.

The solid form of silicon does not react with oxygen, water and most acids.

Silicon reacts with halogens or dilute alkalis.

Silicon also has the unusual property that (like water) it expands as it freezes.

Four other elements expand when they freeze; gallium, bismuth, antimony and germanium

Uses of Silicon

Silicon chips are the basis of modern electronic and computing. The silicon must be ultrapure, although depending on final use it may be doped with part per million levels of arsenic, boron, gallium, germanium, or phosphorus.

Silicon is alloyed with aluminum for use in engines as the presence of silicon improves the metal’s castability. Silicon can enhance iron’s magnetic properties; it is also an important component of steel, which it toughens.

Silicon carbide, more commonly called carborundum, is extremely hard and is used in abrasives.

Silica (SiO 2 ) in sand and minerals in clay is used to make concrete and bricks. Silica, as sand, is also the main constituent of glass.

Pure, crystalline silicon dioxide (quartz) resonates at a very precise frequency and is used in high-precision watches and clocks.

Silicones are important silicon based polymers. Having heat-resistant, nonstick, and rubber-like properties, silicones are often used in cookware, medicine (implants), and as sealants, adhesives, lubricants, and for insulation.

Abundance and Isotopes

Abundance earth’s crust: 28 % by weight, 21 % ppm by moles

Abundance solar system: 900 parts per million by weight, 40 parts per million by moles

Cost, pure: $5.4 per 100g

Cost, bulk: $0.14 per 100g

Source: Silicon is the second most abundant element in Earth’s crust, after oxygen and the eighth most abundant in the Universe. It is most commonly found as silicon dioxide (silica). Two elements, silicon and oxygen, make up almost three-quarters of our planet’s crust.

Commercial quantities of silicon are obtained by the reaction of silicon dioxide and carbon in an electric furnace using carbon electrodes. The carbon reduces the silicon dioxide to silicon. Silicon produced in this way is about 98% pure.

Very high purity silicon for semiconductors is obtained using the Siemens process; the silicon is reacted to produce trichlorosilane, which is first purified by distillation, then reacted with purified hydrogen on high purity silicon rods at 1150 oC to yield high purity, polycrystalline silicon with hydrochloric acid byproduct. Impurities in the silicon are about 1 part per billion or less.

Isotopes: Silicon has 14 isotopes whose half-lives are known, with mass numbers 22 to 36. Naturally occurring silicon is a mixture of its three stable isotopes and they are found in the percentages shown: 28Si (92.2%), 29Si (4.7%) and 30Si (3.1%).

References

R.W. Cahn, Silicon: Child and Progenitor of Revolution., Into The Nano Era, Springer Series in Materials Science, Volume 106. (2009) p3. Thomas Edward Thorpe, Humphry Davy, Poet and Philosopher., (1896). Mary Elvira Weeks, Discovery of the Elements., (2003) p162, Kessinger Publishing Reprints. John Emsley, Nature’s Building Blocks: An A-Z Guide to the Elements., (2002) p387. Oxford University Press. H. R. Huff, U. Gösele, H. Tsuya, Semiconductor Silicon., (1998) p70, The Electrochemical Society. Keith B. Hutton, Chemistry. (2001) p224. Routledge.

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