Metals and Non-Metals - Part XVII

10. Study of some non-metals
Non-metals occur in the earth’s crust mostly combined with metal elements. Oxides are most commonly found compounds, hence oxygen is the most abundant of all non-metals within the earth’s crust. The second most abundant non-metal found on the earth and within its crust is silicon. Phosphorus and sulphur are also found in large quantities inside the earth’s surface, but their occurrence is less than that of oxygen and silicon.

We will now study a little more about three important non-metals : Silicon (Si), phosphorus  (P) and sulphur (S). These three non-metals are indispensable in modern life. Silicon is used in making electronic components like transistors, integrated circuits, etc. Phosphorus in the form compounds is used as fertilizers, etc. Sulphur finds many uses especially in rubber industry and medicines.

Silicon
Silicon has chemical symbol Si. It has 14 protons and 14 neutrons. An atom of Si is represented as ²⁸₁₄Si. Si is largely separated from sand or SiO₂ by oxidation method. The Si obtained is then purified by zone refining. Si is characterized as having very useful properties. It is a semiconductor and when doped (with B or P in small quantities), it has many applications in electronics industries.  

Since Si has 14 protons, it also has 14 electrons. The electronic configuration of Si is K-shell – 2 electrons, L-shell – 8 electrons and M-shell has 4 electrons. Si is placed in Group IVA of the periodic table, just after Aluminium, and below carbon. The reaction of Si is similar to carbon, and it forms covalent bonds.

Mg powder can be used instead of coke as a reducing agent. The reaction then is:

If any further purification is required, then zone refining is done on the silicon obtained from above processes.  

Physical properties of silicon : Si  is a gray, hard, shiny solid. Its density is low (~2.0gm/cc). Si has a high melting point of about 1410°C. Si is a semiconductor (mid way between a conductor and an insulator). Si forms 4 covalent bonds with 4 other Si atoms. The structure is therefore similar to the diamond structure. Si is not as hard as diamond, as the Si atom is larger than the C atoms forming a diamond crystal, the inter-atomic distances in Si is larger than in diamond which makes the Si-Si bonding weaker. 

Chemical properties of silicon :
1. Valence : Since Si has 4 electrons in its M-shell, the M-shell is half filled. Hence Si does not give off its electrons easily. It readily forms covalent bonds to complete its M-shell.

2. Action of oxygen : Si is an unreactive nonmetal and it reacts with air very slowly and at high temperatures only. It forms silicon dioxide. 

3. Action of water : Si heated to a red hot temperature reacts with steam. Silicon dioxide and hydrogen is formed. 

4. Reaction with acids : Si reacts with HCl to form silicon tetra chloride and releases hydrogen.  
Because of the need to form tetra valent covalent bonds with Si, other acids do not react with Si.

5. Action of alkalis : Si reacts with NaOH and KOH to form sodium and potassium silicates, releasing hydrogen. 

6. Reaction with chlorine : Si tetra chloride is formed when Si is heated in chlorine atmosphere at 450°C. The reaction is shown below. 

7. Reaction with metals : When Si is heated strongly with a metal, metal-silicides are formed. The reaction is as follows :

Next        Main        Previous