Silicon

Silicon,  ₁₄Si

2 samples of elemental silicon, image provided by woelen.
General properties
Name, symbol	Silicon, Si
Appearance	Lustrous black solid, with bluish cast
Silicon in the periodic table
C ↑ Si ↓ Ge Aluminium ← Silicon → Phosphorus
Atomic number	14
Standard atomic weight (Ar)	28.085
Group, block	, p-block
Period	period 3
Electron configuration	[Ne] 3s2 3p2
per shell	2, 8, 4
Physical properties
Lustrous black
Phase	Solid
Melting point	1687 K ​(1414 °C, ​​2577 °F)
Boiling point	3538 K ​(3265 °C, ​5909 °F)
Density near r.t.	2.329 g/cm3
when liquid, at	2.57 g/cm3
Heat of fusion	50.21 kJ/mol
Heat of	383 kJ/mol
Molar heat capacity	19.789 J/(mol·K)
pressure
Atomic properties
Oxidation states	4, 3, 2, 1 −1, −2, −3, −4 ​(an amphoteric oxide)
Electronegativity	Pauling scale: 1.90
energies	1st: 786.5 kJ/mol 2nd: 1577.1 kJ/mol 3rd: 3231.6 kJ/mol
Atomic radius	empirical: 111 pm
Covalent radius	111 pm
Van der Waals radius	210 pm
Miscellanea
Crystal structure	​Face-centered diamond-cubic
Speed of sound thin rod	8433 m/s (at 20 °C)
Thermal expansion	2.6 µm/(m·K) (at 25 °C)
Thermal conductivity	149 W/(m·K)
Electrical resistivity	2.3×103 Ω·m (at 20 °C)
Magnetic ordering	Diamagnetic
Young's modulus	130–188 GPa
Shear modulus	51–80 GPa
Bulk modulus	97.6 GPa
Poisson ratio	0.064–0.28
Mohs hardness	6.5
CAS Registry Number	7440-21-3
History
Naming	After Latin 'silex' or 'silicis', meaning flint
Prediction	Antoine Lavoisier (1787)
Discovery and first isolation	Jöns Jacob Berzelius (1823)
Named by	Thomas Thomson (1817)

Silicon is a metalloid element with the atomic number 14 and the chemical symbol Si. It is an extremely common element, but it is difficult to get ahold of it. However, due to its numerous semiconductor applications, it is relatively easy to get silicon at extremely high purities, such as 99.9999%.

Properties

Physical properties

Silicon is a grayish metalloid with a slight blue tint. It is exceptionally light, to the point that it is sometimes described as feeling hollow. It is a semiconductor, so electricity will pass through it, albeit with great resistance. It crystallizes in the cubic crystal system, though amorphous forms exist. It is brittle and prone to chipping.

Chemical properties

Silicon is highly resistant to all acids, but dissolves readily in strong bases. Sodium hydroxide produces sodium silicate and hydrogen gas when reacted with silicon. A large part of silicon's chemical resistance comes from the formation of a passivation layer of silicon dioxide.

Availability

Silicon can be easily obtained from the dies found on many electronic devices, such as microprocessors and transistors (the late models, earlier and first generation usually had either germanium or germanium-silicon). Sometimes it can also be found on other devices, such as hard drive readers, and alone on electronic boards. Silicon dies are extremely fragile, and will shatter if you try to cut the, also being prone to chipping. A better way to separate the silicon die is to heat the device it's glued to, either with a flame or a heat gun, which will cause the die to fall off. CPU dies are generally covered in a multicolored protection layer, which requires removal, to increase the purity of the silicon.

Another source for elemental silicon are discarded silicon solar panels.

Larger quantities of very pure silicon can be bought from websites such as GalliumSource, 40$/30-35 g.

Preparation

An easy means of obtaining relatively pure elemental silicon in a home setting is by composing a thermite mixture of finely powdered silicon dioxide and aluminium, which can be initiated with a high temperature ignition source, often in the form of magnesium ribbon.

x Al + y SiO₂ → Al₂O₃ + Si + Al_xSi_y

Because this reaction is not particularly vigorous and therefore not self-sustaining, those that use this method of silicon extraction often add an additional component to the mixture to raise the temperature and prevent the reaction from stopping. For example, rather than a stoichiometric ratio of aluminum powder and silicon dioxide, a mixture of 9 parts silicon dioxide, 10 parts aluminum, and 12 parts sulfur by mass, can be used. Another method to sustain the reaction is through the addition of an oxidizer (e.g. sodium nitrate), though this will likely lower the yield by removing available aluminum. The resultant slag mixture from the thermite reaction can be broken up to reveal pieces of elemental silicon, which can be cleaned by a short immersion in hydrochloric acid. Caution must be taken in this step, as any aluminium sulfide produced earlier will react to form hydrogen sulfide, a dangerous and very potent-smelling gas which has effects similar to that of cyanide.

Projects

• Make silicides
• Make halosilanes
• Silane synthesis (dangerous!)
• Silicon alkoxides

Handling

Safety

Silicon is nontoxic, as are most of its compounds. Inhaled silicon and silicon dioxide may cause silicosis, however, if inhaled in large quantities.

Storage

No special storage is required. Silicon will slowly form an extremely thin protective oxide layer in open air, however this does not affect its purity significantly.

Disposal

Silicon is non-toxic to the environment. It will slowly oxidize to silicon dioxide in air.

References

Relevant Sciencemadness threads

• How to make Silicon
• Silicon electrode for electrolysis
• Silicon (and Boron)