## The p - Block Elements

Category : UPSC

The p - Block Elements

1.           The Boron Family

• Boron is a fairly rare element, mainly occurs as orthoboric acid, (${{H}_{3}}B{{O}_{3}}$), borax, $N{{a}_{2}}{{B}_{4}}{{O}_{7.}}10{{H}_{2}}O$, and kemite, $N{{a}_{2}}{{B}_{4}}{{O}_{7}}.4{{H}_{2}}O$. In India borax occurs in Puga Valley (Ladakh) and Sambhar Lake (Rajasthan).
• Aluminium is the most abundant metal and the third most abundant element in the earth's crust ($8.3$per cent by mass) after oxygen ($45.5$per cent) and Si ($27.7$per cent). Bauxite, and cryolite, are the important minerals of aluminium. In India it is found as mica in Madhya Pradesh, Kamataka, Orissa and Jammu.
• Gallium, indium and thallium are less abundant elements in nature.
• Boron is non-metallic in nature. It is extremely hard and black coloured solid. It exists in many allotropic forms. Due to very strong crystalline lattice, boron has unusually high melting point. Rest of the members are soft metals with low melting point and high electrical conductivity.
• It is worthwhile to note that gallium with unusually low melting point ($303$K), could exist in liquid state during summer. Its high boiling point ($2676$K) makes it a useful material for measuring high temperatures. Density of the elements increases down the group from boron to thallium.
• Boron is unreactive in crystalline form. Aluminium forms a very thin oxide layer on the surface which protects the metal from further attack.
• White fumes appear around the bottle of anhydrous aluminium chloride. Reason is that Anhydrous aluminium chloride is partially hydrolysed with atmospheric moisture to liberate $HC1$gas. Moist $HC1$ appears white in colour.
• Borax is the most important compound of boron. It is a white crystalline solid. On heating, borax first loses water molecules and swells up. On further heating it turns into a transparent liquid, which solidifies into glass like material known as borax bead.
• The metaborates of many transition metals have characteristic colours and, therefore, borax bead test can be used to identify them in the laboratory. For example, when borax is heated in a Bunsen burner flame with $CoO$on a loop of platinum wire, a blue coloured $Co{{(B{{O}_{2}})}_{2}}$ bead is formed.
• Boric acid is considered as a weak acid because it is not able to release proton $({{H}^{+}})$ ions on its own. It receives $O{{H}^{-}}$ions from water molecule to complete its octet and in turn releases ${{H}^{+}}$ ions.

2.           Uses of Boron and Aluminium and their Compounds

• Boron being extremely hard refractory solid of high melting point, low density and very low electrical conductivity, finds many applications. Boron fibres are used in making bullet-proof vest and light composite material for aircraft.
• The boron-10 ($^{10}B$) isotope has high ability to absorb neutrons and, therefore, metal borides are used in nuclear industry as protective shields and control rods.
• The main industrial application of borax and boric acid is in the manufacture of heat resistant glasses (e.g., Pyrex), glass-wool and fibreglass.
• Borax is also used as a flux for soldering metals, for heat, scratch and stain resistant glazed coating to earthenwares and as constituent of medicinal soaps. An aqueous solution of orthoboric acid is generally used as a mild antiseptic.
• Aluminium is a bright silvery-white metal, with high tensile strength. It has a high electrical and thermal conductivity. On a weight-to-weight basis, the electrical conductivity of aluminium is twice that of copper.
• Aluminium is used extensively in industry and everyday life. It forms alloys with Cu, Mn, Mg, Si and Zn. Aluminium and its alloys can be given shapes of pipe, tubes, rods, wires, plates or foils and, therefore, find uses in packing, utensil making, construction, aeroplane and transportation industry.
• The use of aluminium and its compounds for domestic purposes is now reduced considerably because of their toxic nature.

3.           Group 14 Elements : The Carbon Family

• Carbon (C), silicon (Si), germanium (Ge), tin (Sn) and lead (Pb) are the members of group 14. Carbon is the seventeenth most abundant element by mass in the earth's crust.
• It is widely distributed in nature in free as well as in the combined state. In elemental state it is available as coal, graphite and diamond; however, in combined state it is present as metal carbonates, hydrocarbons and carbon dioxide gas ($0.03$per cent) in air.
• One can emphatically say that carbon is the most versatile element in the world. Its combination with other elements such as dihydrogen, dioxygen, chlorine and sulphur provides an astonishing array of materials ranging from living tissues to drugs and plastics.
• Organic chemistry is devoted to carbon containing compounds. It is an essential constituent of all living organisms. Naturally occurring carbon contains two stable isotopes: $^{12}C$ and $^{13}C$. In addition to these, third isotope, $^{14}C$ is also present. It is a radioactive isotope with halflife $5770$years and used for radiocarbon dating.
• Silicon is the second ($27.7$per cent by mass) most abundant element on the earth's crust and is present in nature in the form of silica and silicates. Silicon is a very important component of ceramics, glass and cement.
• Germanium exists only in traces. Tin occurs mainly as cassiterite, $Sn{{O}_{2}}$ and lead as galena, PbS. Ultrapure form of germanium and silicon are used to make transistors and semiconductor devices.
• Lead is unaffected by water, probably because of a protective oxide film formation.

4.           Allotropes of Carbon

• Carbon exhibits many allotropic forms; both crystalline as well as amorphous. Diamond and graphite are two well-known crystalline forms of carbon. In 1985, third form of carbon known as fullerenes was discovered by H.W. Kroto, E. Smalley and R.F. Curl. For this discovery they were awarded the Nobel Prize in 1996.

• Diamond
• It has a crystalline lattice. In diamond each carbon atom undergoes sp3 hybridisation and linked to four other carbon atoms by using hybridised orbitals in tetrahedral fashion. The C-C bond length is 154 pico meter (pm).
• The structure extends in space and produces a rigid threedimensional network of carbon atoms. In this structure directional covalent bonds are present throughout the lattice. It is very difficult to break extended covalent bonding and, therefore, diamond is a hardest substance on the earth. It is used as an abrasive for sharpening hard tools, in making dies and in the manufacture of tungsten filaments for electric light bulbs.
• Diamond is covalent, yet it has high melting point. Diamond has a three-dimensional network involving strong C-C bonds, which are very difficult to break and, in turn has high melting point.

• Graphite
• Graphite has layered structure. Layers are held by van der Waals forces and distance between two layers is 340 pm. Each layer is composed of planar hexagonal rings of carbon atoms. C-C bond length within the layer is 141 J pm.
• Each carbon atom in hexagonal ring undergoes sp2 hybridisation and makes three sigma bonds with three neighbouring carbon atoms. Fourth electron forms a n bond. The electrons are delocalised over the whole sheet. Electrons are mobile and, therefore, graphite conducts electricity along the sheet.
• Graphite cleaves easily between the layers and, therefore, it is very soft and slippery For this reason graphite is used as a dry lubricant in machines running at high temperature, where oil cannot be used as a lubricant.

• Fullerenes
• Fullerenes are made by the heating of graphite in an electric arc in the presence of inert gases such as helium or argon. The sooty material formed by condensation of vapourised ${{C}^{n}}$ small molecules consists of mainly ${{C}_{60}}$ with smaller quantity of ${{C}_{70}}$ and traces of fullerenes consisting of even number of carbon atoms up to 350 or above.
• Fullerenes are the only pure form of carbon because they have smooth structure without having 'dangling' bonds. Fullerenes are cage like molecules. ${{C}_{60}}$ molecule has a shape like soccer ball and called Buckminsterfullerene.
• It contains twenty six-membered rings and twelve five membered rings. A six membered ring is fused with six or five membered rings but a five membered ring can only fuse with six membered rings.
• All the carbon atoms are equal and they undergo sp2 Each carbon atom forms three sigma bonds with other three carbon atoms. The remaining electron at each carbon is delocalised in molecular orbitals, which in turn give aromatic character to molecule. This ball shaped molecule has 60 vertices and each one is occupied by one carbon atom and it also contains both single and double bonds with C—C distances of $143.5$pm and $138.3$pm respectively. Spherical fullerenes are also called bucky balls in short.
• It is very important to know that graphite is thermodynamically most stable allotrope of carbon. Other forms of elemental carbon like carbon black, coke, and charcoal are all impure forms of graphite or fullerenes. Carbon black is obtained by burning hydrocarbons in a limited supply of air. Charcoal and coke are obtained by heating wood or coal respectively at high temperatures in the absence of air.

5.           So Uses of Carbon

• Graphite fibres embedded in plastic material form high strength, lightweight composites. The composites are used in products such as tennis rackets, fishing rods, aircrafts and canoes.
• Being good conductor, graphite is used for electrodes in batteries and industrial electrolysis. Crucibles made from graphite are inert to dilute acids and alkalies.
• Being highly porous, activated charcoal is used in adsorbing poisonous gases; also used in water filters to remove organic contaminators and in airconditioning system to control odour
• Carbon black is used as black pigment in black ink and as filler in automobile tyres. Coke is used as a fuel and largely as a reducing agent in metallurgy.
• Diamond is a precious stone and used in jewellery. It is measured in carats (1 carat =200 mg).

6.           Carbon Monoxide

• Direct oxidation of C in limited supply of oxygen or air yields carbon monoxide.
• On commercial scale it is prepared by the passage of steam over hot coke. The mixture of $CO$and ${{H}_{2}}$, thus produced is known as water gas or synthesis gas.
• When air is used instead of steam, a mixture of $CO$ and ${{N}_{2}}$ is produced, which is called producer gas.
• Water gas and producer gas are very important industrial fuels. Carbon monoxide in water gas or producer gas can undergo further combustion forming carbon dioxide with the liberation of heat.
• Carbon monoxide is a colourless, odourless and almost water insoluble gas. It is a powerful reducing agent and reduces almost all metal oxides other than those of the alkali and alkaline earth metals, aluminium and a few transition metals. This property of $CO$is used in the extraction of many metals from their oxides ores.
• The highly poisonous nature of $CO$ arises because of its ability to form a complex with haemoglobin, which is about 300 times more stable than the oxygen-haemoglobin complex. This prevents haemoglobin in the red blood corpuscles from carrying oxygen round the body and ultimately resulting in death.

7.           Carbon Dioxide

• Unlike $CO$, it is not poisonous. But the increase in combustion of fossil fuels and decomposition of limestone for cement manufacture in recent years seem to increase the $C{{O}_{2}}$ content of the atmosphere. This may lead to increase in greenhouse effect and thus, raise the temperature of the atmosphere which might have serious consequences.
• Carbon dioxide can be obtained as a solid in the form of dry ice by allowing the liquefied $C{{O}_{2}}$, to expand rapidly. Dry ice is used as a refrigerant for ice-cream and frozen food. Gaseous $C{{O}_{2}}$ is extensively used to carbonate soft drinks. Being heavy and non-supporter of combustion it is used as fire extinguisher. A substantial amount of $C{{O}_{2}}$, is used to manufacture urea.

8.           Silicon Dioxide., $\mathbf{Si}{{\mathbf{O}}_{\mathbf{2}}}$

• 95 per cent of the earth's crust is made up of silica and silicates. Silicon dioxide, commonly known as silica, occurs in several crystallographic forms.
• Quartz, cristobalite and tridymite are some of the crystalline forms of silica, and they are interconvertable at suitable temperature. Silicon dioxide is a covalent, three-dimensional network solid in which each silicon atom is covalently bonded in a tetrahedral manner to four oxygen atoms. Each oxygen atom in turn covalently bonded to another silicon atoms. Each comer is shared with another tetrahedron. The entire crystal may be considered as giant molecule in which eight membered rings are formed with alternate silicon and oxygen atoms.
• Silica in its normal form is almost nonreactive because of very high Si-O bond enthalpy. It resists the attack by halogens, dihydrogen and most of the acids and metals even at elevated temperatures. However, it is attacked by $HF$and $NaOH.$
• Quartz is extensively used as a piezoelectric material; it has made possible to develop extremely accurate clocks, modem radio and television broadcasting and mobile radio communications. Silica gel is used as a drying agent and as a support for chromatographic materials and catalysts. Kieselghur, an amorphous form of silica is used in filtration plants.

9.           Nitrogen

• Molecular nitrogen comprises 78 per cent by volume of the atmosphere. In the earth's crust, it occurs as sodium nitrate, $NaN{{O}_{3}}$ (called Chile saltpetre) and potassium nitrate (Indian saltpetre).
• It is found in the form of proteins in plants and animals. Phosphorus occurs in minerals of the apatite family, which are the main components of phosphate rocks.
• Phosphorus is an essential constituent of animal and plant matter. It is present in bones as well as in living cells. Phosphoproteins are present in milk and eggs. Arsenic, antimony and bismuth are found mainly as sulphide minerals.
• Except nitrogen, all the elements show allotropy.
• The major use of nitric acid is in the manufacture of ammonium nitrate for fertilisers and other nitrates for use in explosives and pyrotechnics. It is also used for the preparation of nitroglycerin, trinitrotoluene and other organic nitro compounds. Other major uses are in the pickling of stainless steel, etching of metals and as an oxidiser in rocket fuels.

10.        Phosphorus Allotropic Forms

• White phosphorus is a translucent white waxy solid. It is poisonous, insoluble in water but soluble in carbon disulphide and glows in dark (chemiluminescence). White phosphorus is less stable and therefore, more reactive than the other solid phases under normal conditions. It readily catches fire in air to give dense white fumes of ${{P}_{4}}{{O}_{10}}$. It consists of discrete tetrahedral ${{P}_{4}}$ molecule.
• Red phosphorus is obtained by heating white phosphorus at 573 K in an inert atmosphere for several days. When red phosphorus is heated under high pressure, a series of phases of black phosphorus is formed. Red phosphorus possesses iron grey lustre. It is odourless, nonpoisonous aud insoluble in water as well as in carbon disulphide. Chemically, red phosphorus is much less reactive than white phosphorus. It does not glow in the dark, It is polymeric, consisting of chains of ${{P}_{4}}$ tetrahedra linked together.
• Black phosphorus has two forms a-black phosphorus and $\alpha$-black phosphorus. $\beta$-Black phosphorus is formed when red phosphorus is heated in a sealed tube at 803 K. It can be sublimed in air and has opaque monoclinic or rhombohedral crystals. It does not oxidise in air. $\beta$-Black phosphorus is prepared by heating white phosphorus at 473 K. under high pressure. It does not bum in air upto 673 K.

11.        Group 16 Elements

• Oxygen, sulphur, selenium, tellurium and polonium constitute Group 16 of the periodic table. This is sometimes known as group of chalcogens. The name is derived from the Greek word for brass and points to the association of sulphur and its congeners with copper. Most copper minerals contain either oxygen or sulphur and frequently the other members of the group.
• Oxygen is the most abundant of all the elements on earth. Oxygen forms about $46.6$per cent by mass of earth's crust. Dry air contains $20.946$per cent oxygen by volume.
• However, the abundance of sulphur in the earth's crust is only $0.03-0.1$per cent. Combined sulphur exists primarily as sulphates such as gypsum $CaS{{O}_{4}}.2{{H}_{2}}O$, epsom salt $MgS{{O}_{4}}.7{{H}_{2}}O$, baryte $BaS{{O}_{4}}$and sulphides such as galena PbS, zinc blende ZnS, copper pyrites$CuFe{{S}_{2}}$. Traces of sulphur occur as hydrogen sulphide in volcanoes. Organic materials such as eggs, proteins, garlic, onion, mustard, hair and wool contain sulphur.
• Selenium and tellurium are also found as metal selenides and tellurides in sulphide ores. Polonium occurs in nature as a decay product of thorium and uranium minerals.
• Next to fluorine, oxygen has the highest electronegativity value amongst the elements.

12.        Oxygen (${{\mathbf{O}}_{\mathbf{2}}}$)

• Dioxygen is a colourless and odourless gas. Oxygen atom has three stable isotopes: $^{16}O$, $^{17}O$ and $^{18}O$.
• Dioxygen directly reacts with nearly all metals and non-metals except some metals (e.g., Au, Pt) and some noble gases. Its combination with other elements is often strongly exothermic which helps in sustaining the reaction. However, to initiate the reaction, some external heating is required.
• In addition to its importance in normal respiration and combustion processes, oxygen is used in oxyacetylene welding, in the manufacture of many metals, particularly steel. Oxygen cylinders are widely used in hospitals, high altitude flying and in mountaineering. The combustion of fuels, e.g., hydrazines in liquid oxygen, provides tremendous thrust in rockets.

13.        Ozone

• Ozone is an allotropic form of oxygen. It is too reactive to remain for long in the atmosphere at sea level. At a height of about 20 kilometres, it is formed from atmospheric oxygen in the presence of sunlight.
• The ozone layer protects the earth's surface from an excessive concentration of ultraviolet (UV) radiations.
• Pure ozone is a pale blue gas, dark blue liquid and violet-black solid. Ozone has a characteristic smell and in small concentrations it is harmless. However, if the concentration rises above about 100 parts per million, breathing becomes uncomfortable resulting in headache and nausea.
• Ozone is thermodynamically unstable with respect to oxygen since its decomposition into oxygen results in the liberation of heat ($\Delta$H is negative) and an increase in entropy ($\Delta$S is positive).
• High concentrations of ozone can be dangerously explosive.
• Due to the ease with which it liberates atoms of nascent oxygen, it acts as a powerful oxidising agent.
• Experiments have shown that nitrogen oxides (particularly nitric oxide) combine very rapidly with ozone and there is, thus, the possibility that nitrogen oxides emitted from the exhaust system of supersonic jet aeroplanes might be slowly depleting the concentration of the ozone layer in the upper atmosphere.
• Another threat to this ozone layer is probably posed by the use of freons which are used in aerosol sprays and as refrigerants.
• The two oxygen-oxygen bond lengths in the ozone molecule are identical (128 pm) and the molecule is angular as expected with a bond angle of about $117{}^\circ .$
• It is used as a germicide, disinfectant and for sterilising water. It is also used for bleaching oils, ivory, flour, starch, etc. It acts as an oxidising agent in the manufacture of potassium permanganate.

14.        Sulphur Allotropic Forms

• Sulphur dioxide is used: in refining petroleum and sugar, in bleaching wool and silk and as an anti-chlor, disinfectant and preservative. Sulphuric acid, sodium hydrogen sulphite and calcium hydrogen sulphite (industrial chemicals) are manufactured from sulphur dioxide. Liquid $S{{O}_{2}}$ is used as a solvent to dissolve a number of organic and inorganic chemicals.
• Sulphuric acid is a very important industrial chemical. A nation's industrial strength can be judged by the quantity of sulphuric acid it produces and consumes. It is needed for the manufacture of hundreds of other compounds and also in many industrial processes. The bulk of sulphuric acid produced is used in the manufacture of fertilisers (e.g., ammonium sulphate, superphosphate).
• Other uses are in : petroleum refining, manufacture of pigments, paints and dyestuff intermediates, detergent industry, metallurgical applications (e.g., cleansing metals before enameling, electroplating and galvanising, storage batteries, in the manufacture of nitro- cellulose products and as a laboratory reagent.

15.        Halogens

• Fluorine, chlorine, bromine, iodine and astatine are members of Group 17. These are collectively known as the halogens (Greek halo means salt and genes means born i.e., salt producers). The halogens are highly reactive non-metallic elements. Astatine is a radioactive element.
• Fluorine and chlorine are fairly abundant while bromine and iodine less so. Fluorine is present mainly as insoluble fluorides (fluorspar $Ca{{F}_{2}},\,$ cryolite $N{{a}_{3}}A1{{F}_{6}}$ and fluoroapatite $3Ca{{(P{{O}_{4}})}_{2}}.Ca{{F}_{2}}$) and small quantities are present in soil, river water plants and bones and teeth of animals.
• Sea water contains chlorides, bromides and iodides of sodium, potassium, magnesium and calcium, but is mainly sodium chloride solution ($2.5$per cent by mass). The deposits of dried up seas contain these compounds, e.g., sodium chloride and camallite. Certain forms of marine life contain iodine in their systems; various seaweeds, for example, contain up to $0.5$per cent of iodine and Chile saltpetre contains upto 0.2 per cent of sodium iodate.
• They have very high electronegativity. The electronegativity decreases down the group. Fluorine is the most electronegative element in the periodic table.
• Fluorine and chlorine are gases, bromine is a liquid and iodine is a solid. Their melting and boiling points steadily increase with atomic number. All halogens are coloured. This is due to absorption of radiations in visible region which results in the excitation of outer electrons to higher energy level. By absorbing different quanta of radiation, they display different colours. For example, Fluorine (${{F}_{2}}$), has yellow, Chlorine ($C{{I}_{2}}$), greenish yellow, Bromine ($B{{r}_{2}}$), red and Iodine (${{I}_{2}}$), violet colour. Fluorine and chlorine react with water. Bromine and iodine are only sparingly soluble in water but are soluble in various organic solvents such as chloroform, carbon tetrachloride, carbon disulphide and hydrocarbons to give coloured solutions.
• All the halogens are highly reactive. Halogens form many oxides with oxygen but most of them are unstable. Fluorine forms two oxides $O{{F}_{2}}$ and ${{O}_{2}}{{F}_{2}}$. However, only $O{{F}_{2}}$ is thermally stable at 298 K. These oxides are essentially oxygen fluorides because of the higher electronegativity of fluorine than oxygen. Both are strong fluorinating agents. ${{O}_{2}}{{F}_{2}}$ oxidises plutonium to $Pu{{F}_{6}}$and the reaction is used in removing plutonium as $Pu{{F}_{6}}$ from spent nuclear fuel.
• $C1{{O}_{2}}$is used as a bleaching agent for paper pulp and textiles and in water treatment.
• The iodine oxides, ${{I}_{2}}{{O}_{4}},\,\,{{I}_{2}}{{O}_{5}},\,\,{{I}_{2}}{{O}_{7}}$ are insoluble solids and decompose on heating. ${{I}_{2}}{{O}_{5}}$ is a very good oxidising agent and is used in the estimation of carbon monoxide.
• Fluorine exhibits only -1 oxidation state whereas other halogens exhibit $+1,+3,+5$and $+7$oxidation states also. Fluorine is the most electronegative element and cannot exhibit any positive oxidation state. Other halogens have d orbitals and therefore, can expand their octets and show $+1,+3,+5$and $+7$oxidation states also.
• Chlorine was discovered in 1774 by Scheele by the action of $HCI$on $Mn{{O}_{2}}$. In 1810 Davy established its elementary nature and suggested the name chlorine on account of its colour. It is a greenish yellow gas with pungent and suffocating odour It is abcut $2-5$times heavier than air.
• Chlorine water on standing loses its yellow colour due to the formation of $HCI$ and $HOCI$Hypochlorous acid $\left( HOC1 \right)$so formed, gives nascent oxygen which is responsible for oxidising and bleaching properties of chlorine.
• It is used for bleaching woodpulp (required for the manufacture of paper and rayon), bleaching cotton and textiles, in the extraction of gold and platinum, in the manufacture of dyes, drugs and organic compounds such as $CC{{I}_{4}},\,CHC{{I}_{3}},\,DDT$, refrigerants;, etc , in sterilising drinking water and preparation of poisonous gases such as phosgene $(COC{{I}_{2}})$, tear gas ($CC{{I}_{3}}N{{O}_{2}}$), mustard gas $(CIC{{H}_{2}}C{{H}_{2}}SC{{H}_{2}}C{{H}_{2}}CI)$.
• When three parts of concentrated $HCI$and one part of concentrated$HN{{O}_{3}}$, are mixed, aqua regia is formed which is used for dissolving noble metals, e.g., gold, platinum.
• Used of$HCI$:
• in the manufacture of chlorine, $N{{H}_{4}}CI$and glucose (from corn starch),
• for extracting glue from bones and purifying bone black,
• in medicine and as a laboratory reagent.

16.        Group 18 Elements

• Group 18 consists of six elements: helium, neon, argon, krypton, xenon and radon. All these are gases and chemically unreactive. They form very few compounds. Because of this they are termed noble gases.
• All the noble gases except radon occur in the atmosphere. Their atmospheric abundance in dry air is $\tilde{\ }1$per cent by volume of which argon is the major constituent. Helium and sometimes neon are found in minerals of radioactive origin e.g., pitchblende, monazite, cleveite. The main commercial source of helium is natural gas. Xenon and radon are the rarest elements of the group. Radon is obtained as a decay product of$^{226}Ra$.
• All the noble gases are monoatomic. They are colourless, odourless and tasteless. They are sparingly soluble in water. They have very low melting and boiling points because the only type of interatomic interaction in these elements is weak dispersion forces Helium has the lowest boiling point $\left( 4.2\text{ }K \right)$of any known substance. It has an unusual property of diffusing through most commonly used laboratory materials such as robber, glass or plastics.

• Helium is a non-inflammable and light gas. Hence, it is used in filling balloons for meteorological observations. It is also used in gas-cooled nuclear reactors. Liquid helium $\left( b.p.\text{ }4.2\text{ }K \right)$finds use as cryogenic agent for carrying out various experiments at low temperatures. It is used to produce and sustain powerful superconducting magnets which form an essential part of modem NMR spectrometers and Magnetic Resonance Imaging (MRI) systems for clinical diagnosis. It is used as a diluent for oxygen in modem diving apparatus because of its very low solubility in blood.
• Neon is used in discharge tubes and fluorescent bulbs for advertisement display purposes. Neon bulbs are used in botanical gardens and in green houses.
• Argon is used mainly to provide an inert atmosphere in high temperature metallurgical processes (arc welding of metals or alloys) and for filling electric bulbs. It is also used in the laboratory for handling substances that are air-sensitive.
• There are no significant uses of Xenon and Krypton. They are used in light bulbs designed for special purposes.

17.        Important Facts

• Silicones being surrounded by non-polar alkyi groups are water repelling in nature. They have in general high thermal stability, high dielectric strength and resistance to oxidation and chemicals. They have wide applications. They are used as sealant, greases, electrical insulators and for water proofing of fabrics. Being biocompatible they are also used in surgical and cosmetic plants.
• A large number of silicates minerals exist in nature. Some of the examples are feldspar, zeolites, mica and asbestos. The basic structural unit of silicates in which silicon atom is bonded to four oxygen atoms in tetrahedron fashion. Two important man-made silicates are glass and cement.
• Zeolites are widely used as a catalyst in petrochemical industries for cracking of hydro- carbons and isomerisation, e.g., $ZSM-5$(A type of zeolite) used to convert alcohols directly into gasoline. Hydrated zeolites are used as ion exchangers in softening of "hard" water.
• It is a colourless gas with rotten fish smell and is highly poisonous. The spontaneous combustion of phosphine is technically used in Holme's signals. Containers containing calcium carbide and calcium phosphide are pierced and thrown in the sea when the gases evolved bum and serve as a signal. It is also used in smoke screens.

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