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The p-Block Elements, The Boron Family & Its Physical and Chemical Properties, Elements of six groups, from 13 to 18 in the periodic table, represent the p-block elements., Valence shell electronic configuration is ns2np1−6 ., The given table lists the important oxidation states of the p-block elements., Non-metals and metalloids exist only in the p-block of the periodic table., Non-metallic character of the elements decreases down the group., Non-metals have higher ionisation enthalpies and higher electronegativities than the metals., Non-metals readily form ions., Group 13 Elements − The Boron Family, Boron (B) is a non-metal; aluminium (Al) is a metal (but shows many chemical similarities to boron). Gallium (Ga), indium (In) and thallium (Tl) are metallic in character., B occurs as (borax) and (kernite), Two isotopic forms of boron − and, Aluminium is the most abundant metal. Bauxite and cryolite (Na3AlF6) are the important minerals of aluminium., Atomic Properties of the Elements of Group 13, Outer electronic configuration of these elements is ns2np1., On moving down the group, atomic radius is expected to increase. However, the atomic radius of gallium (135 pm) is less than that of aluminium (143 pm)., The order of ionisation enthalpies for these elements is. The sum of these ionisation enthalpies for each of the elements is very high., Down the group, electronegativity first decreases from B to Al, and then increases marginally., Physical Properties of Group 13 Elements, Boron is a non-metal and is an extremely hard, black-coloured solid. It has a high melting point due to a very strong crystalline lattice., Other members are soft metals, with low melting points and high electrical conductivities., Density of these elements increases down the group., Chemical Properties of Group 13 Elements, Oxidation State and Trends in Chemical Reactivity:, The sum of the first three ionisation enthalpies of boron is very high. Hence, it does not form + 3 ions, and forms only covalent compounds., For aluminium, the sum of the first three ionisation enthalpies is less than that of Br. Hence, it forms Al+3 ions., In Ga, In and Tl, both +1 and + 3 oxidation states are observed., The relative stability of the +1 oxidation state for heavier elements is Al < Ga < In < Tl, In thallium, +1 oxidation state is predominant and + 3 oxidation state has a high oxidising character., In BF3, B is in +3 oxidation state and it contains only six electrons. Hence, it is an electron-deficient molecule., BF3 can accept a pair of electrons to attain a stable electronic configuration. Thus, it behaves as a Lewis acid., AlCl3 achieves stability by forming a dimer., Being covalent in trivalent state, most of the compounds are hydrolysed in water., Trichlorides on hydrolysis in water form tetrahedral species, (M is sp3 hybridised)., AlCl3 in acidified aqueous solution forms octahedral ion, Al is in sp3d2 hybridised state., To test your knowledge of this concept, solve the following puzzle., Reactivity Towards Air:, Boron is un-reactive in crystalline form., Aluminium metal and amorphous boron give Al2O3 and B2O3 respectively (on heating in air), (E = Element), B2O3 is acidic in nature; aluminium and gallium oxides are amphoteric; and the oxides of indium and thallium are basic in nature., Reactivity towards Acids and Alkalies:, Boron does not react with acids and alkalies, even at moderate temperature., Aluminium dissolves in mineral acids (HCl) to form hydrogen gas., Aluminium also dissolves in aqueous alkalies to liberate hydrogen gas., Concentrated HNO3 forms a protective oxide layer on the surface of aluminium., Reactivity towards Halogens:, They react with halogens to form trihalides (Exception − TlI3), Anomalous Properties of Boron and Some Important Compounds of Boron, Important trends and anomalous properties of boron, Tri-chlorides, bromides, and iodides of group 13 elements being covalent in nature are hydrolysed in water., Species such as (tetrahedral) and (octahedral) exist in aqueous medium. [Exception − Boron], Monomeric trihalides are strong Lewis acids (as they are electron deficient)., BF3 reacts with NH3 (Lewis base) to complete octet around boron., The maximum covalence of B is 4. This is due to the absence of d-orbitals. On the other hand, for aluminium and other elements, the maximum covalence can be expected beyond 4 (since d-orbitals are available)., Most of the other metal halides (such as AlCl3) are dimerised (example, Al2Cl6) through halogen bridging. Aluminium completes its octet by accepting electrons from halogen in the formation of Al2Cl6., Some Important Compounds of Boron, Borax, White crystalline solid of formula, It contains tetra-nuclear units of and therefore, its correct formula is Na2., It dissolves in water to form an alkaline solution., It loses water molecules and swells up on heating., On further heating, it turns into transparent liquid which solidifies into glass-like material (known as borax bead)., Meta-borates of many transition elements have characteristic colours. It is called borax bead test that can be used to identify metals in laboratory., Orthoboric Acid, A white crystalline solid with soapy touch, Sparingly soluble in water and highly soluble in hot water, Can be prepared by acidifying an aqueous solution of borax, Also formed by the acidic hydrolysis of most boron compounds such as halides, hydrides, etc., Structure − Layered structure with planar BO3 units which are joined by hydrogen bonds (shown in figure), A weak monobasic acid (not a protonic acid, but acts as a Lewis acid), On heating above 370 K, it gives metaboric acid (HBO2), which on further heating yields boric oxide (B2O3)., Diborane, Prepared by treating boron trifluoride (BF3) with LiAlH4 in diethyl ether, Laboratory preparation − Oxidation of sodium borohydride with iodine gives diborane., On industrial scale, it can be produced by the reaction of BF3 and NaH., Properties, Colourless and highly toxic gas with a b. p of 180 K, Catches fire spontaneously upon exposure to air, Burns in oxygen to give large amount of energy, Readily hydrolysed by water to give boric acid, With Lewis bases (L), they give borane adducts (BH3.L)., Ammonia reacts with diborane to form, which on further heating gives borazine (B3N3H6). Borazine is also called inorganic benzene., Structure, Four terminal B-H bonds are two centre - two electron bonds. Two bridge B − H − B bonds are three centre - two electron bonds (shown in figure)., Hydridoborates, Boron also forms a series of hydridoborates. [Example − ion], Lithium and sodium tetrahydridoborates are known as borohydrides., They can be prepared by the reaction of metal hydrides with B2H6 in diethyl ether., [M = Li or Na], LiBH4 and NaBH4 are used as reducing agents in organic synthesis and as starting material for the preparation of other metal borohydrides., Uses of Boron and Aluminium and their Compounds, Boron and Its Compounds, Boron fibres are used in making bullet-proof vest and light composite material for aircrafts., 10B isotope has high ability to absorb neutrons. Hence, it is used in nuclear industry as protective shields and control rods., Borax and boric acid are used in the manufacture of heat resistant glasses, glass wool, and fibre glass., Borax is used as flux for soldering metals and is used as a constituent of medicinal soaps., An aqueous solution of orthoboric acid is used as mild antiseptic., Aluminium and its Compounds, Used in making pipe, tubes, rods, utensils, foils used in packing, in construction, in aeroplane, and in transport industry, The Carbon Family & Its Physical and Chemical Properties, Group 14 elements (carbon family): C, Si, Ge, Sn, Pb, Carbon is the 17th most abundant element in the earth’s crust (by mass)., In elemental state, carbon is available as coal, graphite and diamond., In combined state, it exists as carbonates, hydrogen carbonates and carbon dioxide in air (0.03%), Two stable, naturally occurring isotopes: 12C and 13C, 14C is a radioactive isotope used for radiocarbon dating., Silicon is the second most abundant element in the earth’s crust (27% by mass)., Silicon is the important component of ceramics, glass and cement., Germanium is present only in traces., Tin exists as cassiterite (SnO2) and lead as galena (PbS)., Germanium and silicon (in ultra pure form) are used for making transistors and semiconductor devices., Atomic Properties of the Elements of Carbon Family, Valence shell electronic configuration is ns2 np2., Covalent radius increases from C to Si; after that there is a small increase from Si to Pb., First ionisation enthalpy of group 14 members is higher than that of group 13 elements., Electronegativity: The elements of this group are slightly more electronegative than the elements of group 13., Physical Properties of the Elements of Carbon Family, All the elements are solids., C and Si are non-metals; Ge is a metalloid; Sn and Pb are soft metals., Melting and boiling points of these elements are higher than those of group 13 elements., Chemical Properties, Oxidation States:, Common oxidation states are + 4 and + 2., Compounds in + 4 state are generally covalent. (Since sum of the first four ionisation enthalpies is very high), C and Si mostly show + 4 oxidation state., Ge in + 4 state, forms stable compounds, and in + 2 state, forms only a few compounds., Sn forms compounds in both + 2 and + 4 oxidation states., Compounds of lead in + 2 state are stable and in + 4 state are strong oxidising agents., Due to the presence of d-orbitals in Si, Ge, Sn and Pb, these elements can exceed covalence more than 4. Thus, the halides of these elements undergo hydrolysis and have tendency to form complexes by accepting electron pairs from donor species., Examples − (central atom is sp3d2), Carbon cannot exceed its covalence by more than 4 (due to the absence of d-orbitals), Reactivity towards Oxygen:, All members form two types of oxides, MO (monoxide) and MO2 (dioxide)., Oxides with higher oxidation states of elements (CO2, SiO2 and GeO2) are more acidic than those in lower oxidation states. SnO2 and PbO2 are amphoteric., Among monoxides, CO is neutral; GeO is acidic; SnO and PbO are amphoteric., Reactivity towards Water:, C, Si and Ge are not affected by water., Sn reacts with steam to form dioxide and dihydrogen gas., Due to the formation of a protective oxide film, lead is unaffected by water., Reactivity towards Halogens:, Form halides of formula MX2 and MX4 (X = F, Cl, Br, I), Most of the MX4 are covalent in nature. (Exceptions − SnF4 and PbF4 are ionic in nature), The central metal atom in the covalent halides of the form MX4 undergoes sp3 hybridisation, and the molecule is tetrahedral in shape., Most tetrachlorides are easily hydrolysed by water because of the presence of d-orbital in the central metal atom. d-orbital can accommodate the lone pair of electrons from the oxygen atom of a water molecule., For example −, Anomalous Behaviour of Carbon & Allotropes of Carbon, Due to its smaller size, higher electronegativity, higher ionisation enthalpy and unavailability of d-orbitals, a carbon atom differs from the rest of the members of its group., The maximum covalence of carbon is four because it contains only s and p orbitals. On the other hand, other members of the carbon family can expand their covalence due to the presence of d-orbitals., Ability to form pπ − pπ multiple bonds with itself; e.g., C=C, C≡C, C=O, C=S and C≡N. Other members do not form pπ − pπ bonds and their atomic orbitals are too large., Tendency to link with one another through covalent bonds to form chains and rings; this property is called catenation, The order of catenation is C >> Si > Ge ≈ Sn. Pb does not show catenation., Ability to show allotropic forms (due to the property of catenation and pπ − pπ bonding), Allotropes of Carbon, Diamond, Graphite, Fullerenes, Diamond, Each carbon atom undergoes sp3 hybridisation., Each carbon is linked to four other carbon atoms in a tetrahedral fashion., C−C bond length = 154 pm, Rigid three-dimensional network of carbon atoms (shown in the figure), In this structure, directional covalent bonds are present throughout the lattice., It is the hardest substance on earth (as it is difficult to break extended covalent bonding)., It has a very high melting point., It is used as an abrasive for sharpening hard tools; also used for making dyes and manufacturing tungsten filaments for electric light bulbs., Graphite, Layered structure (shown in the figure), Layers are held by van der Waals forces, C−C bond length within the layer = 141.5 pm, Each carbon atom is sp2 hybridised., Each layer is composed of hexagonal rings of C-atoms., Electrons are delocalised over the entire sheet., It conducts electricity (as electrons are mobile along the sheets)., It is used as a dry lubricant in machines running at high temperature as it very soft and slippery (this is because graphite cleaves easily between the layers)., Fullerenes, Made by heating graphite in an electric arc, in the presence of inert gases (He or Ar), C60, with smaller quantity of C70, and traces of fullerenes consisting of an even number of carbon atoms up to 350 or above are formed, They are cage-like structures., C60 molecule is called Buckminsterfullerene., C60 has a shape like a soccer ball (shown in the figure)., C60 contains 20 six-membered rings and 12 five-membered rings., All the carbon atoms are sp2 hybridised., Both single and double bonds are present., C−C bond length = 143.5 pm, C=C bond length = 138.3 pm, Spherical fullerenes are also called bucky balls., Other Forms of Carbon, Carbon black, Coke, Charcoal (amorphous form), Carbon black is obtained by burning hydrocarbons in a limited supply of air., By heating wood or coal at high temperatures in the absence of air, charcoal and coke are obtained respectively., Uses of Carbon, Graphite is used for making the electrodes present in batteries and in industrial electrolysis., Activated charcoal is used:, For absorbing poisonous gas, As water filters to remove organic contaminators, In air condition systems, to remove odour, Coke is used as:, Fuel, Reducing agent in metallurgy, Diamond is used for making jewellery., Carbon black is used as:, A black pigment ink, A filler in automobile tyres, Some Important Compounds of Carbon and Silicon, Compounds of Carbon, Carbon Monoxide, Preparation, Carbon in limited supply of air or oxygen yields carbon monoxide., Dehydration of formic acid with concentrated H2SO4 at 373 K gives pure CO., The passage of steam over hot coke gives carbon monoxide., Mixture of CO(g) and H2(g) → Water gas, A mixture of CO and N2 (called producer gas) is produced when air is used instead of steam., Properties and Structure, Colourless, odourless, and almost water insoluble gas, Powerful reducing agent − Can reduce almost all metal oxides (exceptions − alkali and alkaline earth metals, aluminium), That is why it is used in the extraction of many metals from their oxide ores., The structure of CO can be represented as (contains one sigma, and two π bonds; two lone pairs of electrons)., CO can donate lone pair to certain metal to form metal carbonyls., CO is a poisonous gas. It forms stable complex with haemoglobin (which is 300 times more stable than oxygen-haemoglobin complex)., It prevents haemoglobin from carrying blood and ultimately results in death., Carbon Dioxide, Preparation, Complete combustion of carbon and carbon containing fuels, Action of dilute hydrochloric acid on calcium carbonate (Laboratory preparation):, Properties, Structure, and Uses, Colourless and odourless gas, Low solubility in water, With water, CO forms carbonic acid (H2CO3)., H2CO3 is a weak dibasic acid. It dissociates as, buffer system maintains the pH of blood between 7.26 to 7.42., CO2 is acidic in nature. It combines with alkali to form metal carbonates., In the atmosphere, CO2 is present to the extent of ∼0.03% by volume., CO2 is used in the process of photosynthesis., Increase in CO2 content of the atmosphere leads to greenhouse effect because of which there is rise in the temperature of atmosphere., Carbon atom undergoes sp hybridisation in the CO2 molecule. It contains two sigma and two pi bonds., It has linear shape (with equal C − O bond lengths)., CO2 has zero dipole moment., Resonance structures of CO2 are, Dry ice is the solid form of CO2. It is used as refrigerant for ice-cream and frozen food., Gaseous CO2 is used in carbonated soft drinks., It can be used as fire extinguisher., Compounds of Silicon, Silicon Dioxide (SiO2), Commonly known as silica, Quartz, cristobalite, and tridymite are the crystalline forms of silica., Structure, It is a covalent and three-dimensional network solid., Each silicon atom is covalently bonded to four oxygen atoms in tetrahedral manner. (Shown in figure), Reactivity, Si − O bond has very high enthalpy. Hence, silica in normal form is almost non-reactive., Resists the attack by halogens, most acids, and metals even at elevated temperature, Reacts with HF and NaOH as, Uses, Quartz is used as piezoelectric material., Silica gel is used as drying agent., Silica gel is also used as support for chromatographic materials and catalyst., Amorphous form of silica (kieselguhrr) is used in filtration plants., Silicones, Group of organosilicon polymers, Repeating units is ., Preparation, Alkyl or aryl substituted silicon chlorides, RnSiCl(4 − n), are the starting materials for the manufacture of silicones. [R is alkyl or aryl group], The chain length of the polymer can be controlled by adding (CH3)3SiCl., Properties, Silicones are water repelling in nature., Have high thermal stability and dielectric strength, Resistant to oxidation and chemicals, Applications, Used as sealant, greases, electrical insulator, and for water proofing of fabrics, Also used in surgical and cosmetic plants, Silicates, Examples − Feldspar, zeolites, mica, asbestos, Structure, is the basic structural unit of silicates., In, Si atom is bonded to oxygen atoms in tetrahedral fashion., Silicate units are linked together to form chain, ring, sheet, or three-dimensional structures., Negative charge on oxygen is neutralized by positively charged metal ions., Glass and silicate are two man-made silicates., Zeolites, In the three-dimensional network of silicon dioxide, a few silicon atoms are replaced by aluminium atoms, then the overall structure is called aluminosilicate., Aluminosilicate acquires a negative charge., The negative charge can be balanced by cations such as Na+, K+, or Ca2+., Examples − Feldspar and zeolites, Uses of Zeolites, Used as catalyst in petrochemical industries for cracking of hydrocarbons and isomerisation, ZSM−5 (a zeolite) is used to convert alcohols to gasoline., Hydrated zeolites are used in softening of hard water as ion exchangers