Silicon (Si) is the second most abundant element in the earth’s crust. It is the second element in group 4 of the periodic table. Its electron configuration is 2.8.4. (or at advanced level 1s22s22p63s23p2).
PHYSICAL PROPERTIES OF SILICON
Melting point (oC) 1410
Heat of fusion (kjmol-1) +46.4
Boiling point (oC) 2680
Heat of vaporisation (kjmol-1) +376.7
Density (gcm-3) 2.33
Molar volume (cm3mol-1) 12.1
Atomic number 14
Atomic radius (nm) 0.117
Conductivity semi-conductor
First ionisation energy (kjmol-1) 787
Silicon does not fit conveniently into the categories of metals and non-metals. It is therefore described as a metalloid or semi-metal. Here is some information about silicon.
Silicon is a solid.
APPEARANCE: grey, shiny crystals or a brown powder.
CONDUCTION OF ELECTRICITY: The crystalline form of silicon conducts electricity whereas the brown powder form does not.
MECHANICAL PROPERTIES: Hard but brittle.
From this information it is almost impossible to decide whether or not silicon is a metal, especially as it has two solid forms. This decision becomes somewhat easier on examination of some of the compounds of silicon.
SILICON (IV) OXIDE: this has the formula SiO2. It is a white solid which reacts with an alkali to form a salt.
SILICON HYDROXIDE: This is usually known as silic acid and as such will react with alkalis.
SILICON CHLORIDE: This is a colourless liquid which will react with water.
Silicon forms a family of useful compounds known as SILICONES. These are colourless, non-poisonous substances with molecules of varying size. Some of them are clean, oily liquids used in cosmetics. Others are rubbery solids, which can be shaped into replacement flesh. Silicones repel water, which explains why they may be used as waterproofing fabrics and building materials.
THE STRUCTURE OF SILICON
Silicon has a medium electronegativity and the ability to form four covalent bonds and therefore giant molecular structures of covalently bonded atoms. Silicon and silicon carbide exist in a similar crystal structure to diamond. In diamond every carbon atom can be imagined to be at the centre of a regular tetrahedron surrounded by four other carbon atoms. Their centres are at the corners of the tetrahedron. Every carbon atom forms four covalent bonds with its four nearest neighbours. This is achieved through the sharing of electrons.
Silicon (IV) oxide is another example of a molecule with a macromolecular structure. In each of these structures atoms are linked by localised electrons in strong covalent bonds throughout the whole three dimensional lattice.
It is therefore very difficult to distort a covalently bonded crystal, because this would involve breaking covalent bonds. Consequently these compounds are hard and brittle. They also have very high melting and boiling points. Such materials do not conduct electricity because electrons cannot move and cannot carry charge from one position to another.
THE MANUFACTURE OF PURE SILICON
Silicon can be found in sand and other clays as impure silicon (IV) oxide. This is reduced through heating with carbon and impure silicon is formed. The impure silicon is then reacted with chlorine, thus forming silicon tetrachloride by direct synthesis. However this silicon tetrachloride is impure and is therefore purified by fractional distillation. Once pure silicon tetrachloride is formed it is then reduced using hydrogen to form a reasonably pure form of the element silicon. So called ultra pure silicon can be obtained through the zone refining process. In order to do this the silicon is packed into a cylindrical tube and is suspended vertically. At the top of the tube, a short length of the cylinder is surrounded by an electrical heating coil. This melts a narrow band of material within the tube. During zone refining, the tube is raised through the heating element and the zone of molten material moves down the tube. As the sample moves above the heating element it begins to recrystallise. Impurities are left in the molten zone. In this way impurities collect in the molten phase and end up concentrated in one end of the tube.
CHEMICAL PROPERTIES OF SILICON
Silicon is a relatively unreactive element. For example it does not react with hydrogen, dilute acid or concentrated nitric acid, which acts as an oxidising agent. It reacts slowly when heated in dry chlorine to form SiCl4 and also slowly with oxygen to form SiO2.
Silicon burns when heated in excess oxygen to form silicon (IV) oxide.
Si + O2 = SiO2
It also reacts with sulphur when heated as shown below:
Si + 2S = SiS2
Silicon combines directly with all halogens when heated. Halogens are the most reactive non-metallic elements and are found in group seven of the periodic table. In the equation below X represents the halogen.
Si + 2X2 = SiX4
Silicon does not react with cold water but does react with steam.
Si + 2H2O = SiO2 + 2H2
Although silicon is unaffected by both dilute and concentrated acids, it does react with alkalis to form solutions of silicates. An example of this is shown below.
Si + 2OH- + H2O = SiO32- + 2H2
SAND AND SILICATES
After water, sand, which consists of particles with a diameter of between 0.05mm and 2mm, is probably the most common material on earth. Sand is mainly silicon (IV) oxide, SiO2, sometimes known as silica. (The (IV) is the oxidation number of silicon in the compound.) There are several forms of SiO2 with different crystal structures. These different solid forms of the same compound are called polymorphs and can be compared to allotropy in elements. Allotropes are different forms of the same element in the same state.
The most common form of silicon (IV) oxide is quartz, of which sand is an impure form. It is brown in colour due to the impurities of iron (III) compounds which it contains.
Sand's structure is based on tetrahedra of silicon atoms covalently bonded to four oxygen atoms. In silicon (IV) oxide each SiO4 tetrahedron shares its corners with four other SiO4 tetrahedra. This means that each silicon atom half shares four oxygen atoms and explains the formula SiO2. SiO2 is therefore a giant covalent or giant molecular compound. The strength and high bond energy of the Si-O bond accounts for the hardness, very high melting point, electrical insulating and thermal insulating properties of silica. Its structure and physical properties are similar to those of diamond.
Anions derived from silica are called silicates. All silicates have structures based on SiO4 tetrahedra.
In silica itself, each SiO4 tetrahedron shares its corner with four others. If none of the oxygen atoms are shared, then the silicate ion SiO44- results. Between these extremes various ring, chain, and sheet structures are possible for silicates.
In such structures the negative charges of the silicate sheets and chains are balanced by cations (i.e. positive ions). Cations are formed when metallic elements lose negatively charged electrons. They therefore have more protons than electrons and hence the overall positive charge. In such structures the charges must always balance. For example, a charge of 2+ must be balanced by an equal and opposite charge, 2-.
In chain silicates, the charge on each tetrahedron is 2-, because two of the four oxygen atoms are not bonded to other tetrahedra. Silicates have bonding characteristic of both giant covalent and giant ionic structures.
USES OF SILICON AND ITS COMPOUNDS
Silicon is the second most abundant element on the earth next to oxygen. It does not occur as a free element, but occurs widely in the form of silica and silicates. Sand, sandstone, quartz and flint are all examples of silica.
More than 1000 silicate elements are known. They make up about 75% of the earth's crust. Included amongst the natural silicates are asbestos, talc, clays, quartz and micas.
Uses of silicates include:
Building materials
Cements
Pigments
Detergents
Chemistry Department: Loreto, Coleraine.