Silicates are classified into various types based on the way in which the tetrahedral units, [SiO₄]^4- are linked together.
(i) Ortho silicates (Neso silicates):
The simplest silicates which contain discrete [SiO₄]^4- tetrahedral units are called ortho silicates or nesosilicates.
Examples: Phenacite - Be₂SiO₄ (Be²⁺ ions are tetrahedrally surrounded by O^2- ions)
(ii) pyro silicate (or) Soro silicates: Silicates:
Which contain [Si₂O₇]^6- ions are called pyro silicates (or) Soro silicates.
Example: Thortveitite - Sc₂Si₂O₇
(iii) Cyclic silicates (or Ring silicates): 
Silicates which contain (SiO₃)₃^2n- ions which are formed by linking three or more tetrahedral SiO₄^4- units cyclically are called cyclic silicates.
Example: Beryl [Be₃Al₂ (SiO₃)₆] (an aluminosilicate with each aluminium is surrounded by 6 oxygen atoms octahedrally)
(iv) Inosilicates: Silicates which contain 'n':
number of silicate units liked by sharing two or more oxygen atoms are called inosilicates.
Example: They are further classified as chain silicates and double chain silicates.
(v) Chain silicates (or pyroxenes):
These silicates contain [(SiO₃)_n]^2n- ions formed: by linking 'n' number of tetrahedral [SiO₄]^4- units linearly. Each silicate unit shares two of its oxygen atoms with other units.
Example: Spodumene - LiAl(SiO₃)₂·
(vi) Double chain silicates (or amphiboles): 
These silicates contains \(\left[ { Si }_{ 4 }{ O }_{ 11 } \right] _{ n }^{ 6n- }\) ions. In these silicates there are two different types of tetrahedra:
(a) Those sharing 3 vertices
(b) those sharing only 2 vertices.
Example:
Asbestos: These are fibrous and non-combustible silicates.
(vii) Sheet or phyllo silicates: 
Silicates which contain \(({ Si }_{ 2 }{ O }_{ 5 })_{ n }^{ 2n- }\) are called sheet or phyllo silicates. In these, Each [SiO₄]^4- tetrahedron unit shares three oxygen atoms with others and thus by forming two dimensional sheets.
Example: Talc, Mica etc.
(viii) Three dimensional silicates (or tectosilicates): 
Silicates in which all the oxygen atoms of [SiO₄]^4- tetrahedra are shared with other tetrahedra to form three dimensional network are called three dimensional or tectosilicates.
Example: Quartz.

Allotropism:
1. Some elements exist in more than one crystalline or molecular forms in the same physical state
(a) In Greek "allos" means ⇒ Another
(b) "trope" means ⇒ Change
2. The different forms of an element are called allotropes.
Allotropy of carbon:
Carbon exists as diamond, graphite, fullerenes, carbon nanotubes and graphene.
Graphite:
1. Graphite is the most stable allotropic form of carbon at normal temperature and pressure.
2. It is soft and conducts electricity.
3. It is composed of flat two dimensional sheets of carbon atoms.
4. Each sheet is a hexagonal.
5. It is "sp²" hybridised.
6. C-C bond length is 1.41 Å
7. Each C-atom forms three σ bonds with three neighbouring carbon atoms using three of its valence electrons and the fourth electron present in the unhybridised p-orbital form a π-bond.
8. The successive C-sheets are held together by weak Vander Waals forces.
9 The distance successive sheet is 3.40 Å.
10. It is used as a lubricant either on its own or as a graphited oil.

Diamond:
1. It is very hard.
2. It is "sp³" hybridised.
3. C-C bond length is 1.54 Å
4. It is used for sharpening hard tools, cutting glasses, making bores and rock drilling.

Fullerenes:
1. These allotropes are discrete molecules such as \(C_{32}, C_{50}, C_{60}, C_{70}, C_{76}\) etc.
2. It has cage like structure
3. The C₆₀ molecules have a "soccer" ball like structure and is called buckminster fullerene or buckyballs.
4. It has a fused ring structure consists of 20 six membered rings and 12 five membered ring.
5. Each carbon atom is "sp²" hybridised.
6. It has three σ bonds and a delocalised π bond giving aromatic character to these molecules.
7. The C-C bond distance is 1.44 Å
8. The C=C bond distance is 1.38 Å.

Carbon nanotubes:
1. Carbon nanotubes, another recently discovered allotropes, have graphite like tubes with fullerene ends.
2. Along the axis, these nanotubes are stronger than steel and conduct electricity.
3. These have many applications in nanoscale electronics, catalysis, polymers and medicine.
Graphene:
 It has a single planar sheet of "sp²" hybridised carbon atoms that are densely packed in a "honeycomb crystal" lattice.

(i) Boron trifluoride is used for preparing HBF₄, a catalyst in organic chemistry
(ii) It is also used as a fluorinating reagent.

(i) Boranes are colourless diamagnetic compounds with low thermal stability.
(ii) Diborane is a gas at room temperature with sweet smell and it is extremely toxic.
(iii) It is also highly reactive At high temperatures it forms higher boranes liberating hydrogen.

(i) Boric acid reacts with calcium fluoride in presence of conc. sulphuric acid and gives boron trifluoride.
3CaF₂ + 3H₂SO₄ + 2B(OH)₃ ⟶ 3CaSO₄ + 2BF₃ + 6H₂O
(ii) Borax when heated with soda ash it gives borax
Na₂CO₃ + 4B(OH)₃ ⟶ Na₂B₄O₇ +.CO₂ + 6H₂O

(a) B(OH)₃ + NH₃\(\overset { \Delta }{ \longrightarrow } \) BN + 3H₂O
                                (Boron nitride)
(b) Na₂B₄O₇ + H₂SO₄ + 5H₂O \(\longrightarrow \) 4H₃BO₃ + Na₂SO₄
                                                           (Boric acid)
(c) B₂H₆ + 2NaOH + 2H₂O \(\longrightarrow \)2NaBO₂ + 6H₂
                                              (Sodium metaborate)
(d) B₂H₆ + 6CH₃OH  \(\longrightarrow \)2B(OCH₃)₃ + 6H₂O
                                              (Trimethyl borate)
(e) 4BF₃ + 3H₂O \(\longrightarrow \) H₃BO₃ + 3H⁺ + 3[BF₄]^-
                                      (Boric acid)
(f) HCOOH + H₂SO₄ \(\longrightarrow \)CO + H₂SO₄. H₂O
                                        (Carbon monoxide)   
(g) 2SiCl₄ + NH₃ \(\overset { 330K }{ \underset { ether }{ \longrightarrow } } \)Cl₃Si- NH - SiCl₃ + 2HCl
                                        (Chlorosilazane)
(h) SiCl₄ + 4C₂H₅OH\(\longrightarrow \)Si(OC₂H₅)₄ + 4HCI
                                        (Tetraethoxysilane)
(i) 2B + 6NaOH\(\longrightarrow \) 2Na₃BO₃ + 3H₂
(j) H₂B₄O₇ \(\xrightarrow[]{Redhot}\) 2B₂O₃ + H₂O

(i) On industrial scale it is produced by burning coke in excess of air
2CO + O₂ ⟶ 2CO₂ [H = 394 kJ mol^-1]
(ii) Calcination of lime produces carbon dioxide as by product.
CaCO₃ ⟶ CaO + CO₂

(i) Graphite is the most stable allotropic form of carbon at normal temperature and pressure.
(ii) It is soft and conducts electricity. It is composed of flat two dimensional sheets of carbon atoms.
(iii) Each sheet is a hexagonal net of Sp² hybridised carbon atoms with a C-C bond length of 1.41 Å which is close to the C-C bond distance in benzene (1.40 Å).
(iv) Each carbon atom forms three a bonds with three neighbouring carbon atoms using three of its valence electrons and the fourth electron present in the unhybridised p orbital forms a π-bond.
(v) These π electrons are delocalised over the entire sheet which is responsible for its electrical conductivity.
(vi) The successive carbon sheets are held together by weak van der Waals forces. The distance between successive sheet is 3.40 Å.
(vii) It is used as a lubricant either on its own or as a graphited oil.

(i) Anhydrous aluminium chloride is used as a catalyst in Friedel Crafts reactions.
(ii) It is used for the manufacture of petrol by cracking the mineral oils.
(iii) It is used as a catalyst in the manufacture on dyes, drugs and perfumes.

(i) When aluminium metal or aluminium hydroxide is treated with hydrochloric acid, aluminium trichloride is formed.
(ii) The reaction mixture is evaporated to obtain hydrated aluminium chloride.
2Al + 6HCl ⟶ 2AICl₃ + 3H₂
Al(OH)₃ + 3HCl ⟶ AlCl₃ + 3H₂O

1. A double salt which contains fourth-period alkali metal (A) is potash alum
K₂SO₄ Al₂(SO₄)₃ - 24 H₂O
2. On heating potash alum (A) 500 k give anhydrous potash alum (or) burnt alum (B).
\(\mathrm{K}_{2} \mathrm{SO}_{4} \cdot \mathrm{Al}_{2}\left(\mathrm{SO}_{4}\right)_{3} \cdot 24 \mathrm{H}_{2} \mathrm{O} \stackrel{500 \mathrm{~K}}{\longrightarrow} \mathrm{K}_{2} \mathrm{SO}_{4} \cdot \mathrm{Al}_{2}\left(\mathrm{SO}_{4}\right)_{3}+24 \mathrm{H}_{2} \mathrm{O}\)
[Potash alum (A)]                         [Burnt alum (B)]
3. Aqueous solution of burnt alum, has sulphates ion, potassium ion and aluminium ion. Sulphate ion reacts with BaCl₂ to form a white precipitate of Barium Sulphate
(SO₄)₂ + BaCl₂ → BaSO₄ + 2 Cl¯
Aluminium ion reacts with alizarin solution to give a red colour compound.

Fusion of urea with B(OH)_3' in an atmosphere of ammonia at 800 - 1200 K gives boron nitride.
B(OH)₃ + NH₃ \(\overset { \Delta }{ \longrightarrow } \) BN+ 3H₂O