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1. A covalent bond is formed by mutual sharing of electrons ‘This mutual sharing, , our electrons occurs in such a way that each of the combining atoms acquires the gy !, , Tips electronic configuration of the nearest noble gas ; hydrogen completes its duplet While, all other elements complete their respective octets,, , 2. A single covalent bond is formed when cach of the two combining atoms share One, electron cach, a double bond is formed when cach atom shares two electrons and q, triple bond is formed when each atom shares three electrons. The number Of electron,, contributed by cach atom for formation of a covalent bond is called its covalency,, , oe, , - Hydrogen and chlorine molecules contain a single covalent bond between the tw, atoms, oxygen molecule contains a double bond between the (wo oxygen atoms while, a nitrogen molecule contains a triple bond between the two nitrogen atoms,, , A molecule of ammonia (NH3) contains three nitrogen-hydrogen single covalent bonds, while a molecule of methane (CH,) contains four carbon-hydrogen single covalent, bonds. Methane is widely used as a fuel, It is the main component of biogas and, , >, , compressed natural gas (CNG),, , Covalently bonded molecules have strong bonds within the molecules but their, intermolecular forces of attraction are weak. Therefore, melting and boiling points of, , covalent compounds are quite low as compared to those of ionic compounds,, , t, , During the formation of covalent bonds only electrons are shared between atoms and, no charged particles are formed. Therefore, covalent compounds, in general, are bad, , conductors or non-conductors of electricity., , a, , 4.3..ALLOTROPIC FORMS OF CARBON, , 1e phenomenon of existence of an element in two or more. forms which have different, pie, roperties but identical chemical | properties. is called allotropy and the dj ifferent forms are, called allotropic forms or forms or simply allotropes. f 4, , Carbon occurs in three crystalline allotropic forms. These are :, , , , , , , , , , 1. Diamond, 2. Graphite, 3. Fullerenes, , All these three allotropes are the carbon. They have the same chemical pro ‘Properties,, For example, when diamond, graphite o1 or fullerene is burnt in air, a colourless and odourless gas calld, , carbon dioxide (CO,) is formed Which turns lime water-milky. ~, , yreeneronqrecr me pring, , ee, ‘The physical properties of these thre allotropes of c carbon | on are, he however, dif wever, differ ferent due to differert, arrangement. of.carbon.atoms in them: as discussed below :, , , , 1, Diamond., Occurrence. Although diamonds occur in nature (South Africa, Zaire, Australia, Russia, etc.), they, , have also been synthesized by subjecting pure carbon to very high pressure (50,000 — 60,000 atmospheres), and high temperature (1873 K). The synthetic diamonds are small in size but are otherwise indistinguishable, , from natural diamonds., The size of diamonds is measured in carats (1 carat = 200 mg). The famous Kohinoor diamond , _ decorates the.crown of England’s queen at present weighs 108-93 carats. However, when it was taken from
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UNDS)., , fn, ais ae, , , , , , , , Bis Comm, , on AN, “oe it weighed 181 ca sahil :, ¢ gnglinds it welgne’ Carats, To restore its brilliance and lustre about 39-8% of its weight was, , rl . ;, , Pe gt ee ie ee atom is attached to four other carbon atoms by strong single, yet onds w i Taeienen pm) long. The four surrounding carbon atoms lie at the four vertices, oes of ov Sates as shown in Fig, 4.7 (a), The angle between any two adjacent carbon, a 5 109° ar SO called the tetrahedral angle. This tetrahedral arrangement of carbon, yom ves diamon a rigid three-dimensional network striicture-as shown in Fig, 4.7 (b). Thus, a crystal, , if!, , , , , , , , Each carbon Strong bonds exist, e_opt Is joined to between carbon, 10g =2g four other. atoms, carbon atoms, , , , , a) Tetrahedral arrangement of carbon ‘ato, 2e-dimensional network structure ‘of diam, , Physical Properties. (i) Hardness. The three-dimensional network structure of diamond makes it the, pasdest natural substance known., , (ii) High density. Due to network structure, the carbon atoms in diamond are closely packed and, hence diamond has a high density (3-51 g cm),, , (iii) High melting point. A large amount of energy is needed to break the network structure of diamond., therefore, the melting point of diamond is quite high (3930°C or 4203 K)., , (iv) Electrical and Thermal conductivity. Since all the four valence electrons are firmly held in, carbon-carbon single bonds, there are no free electrons in a diamond crystal. Therefore, diamond is a bad, conductor of electricity. However, it has the highest thermal conductivity of any known substance (about 5, times that of copper). It is because of this reason that the diamond tipped tools do not overheat and hence, ae extensively used for drilling and cutting purposes., , (v) Transparency. Because of high refractive index (2.5), diamonds can reflect and refractive light., Asa result, diamonds are transparent substances., , Uses. (i) Because of hardness, diamonds are used for cutting glass, making borers for drilling rocks, and for making abrasives. ,, , (ii) Diamonds are used for making ‘dies’ for drawing thin wires from metals,, , (iii) When a diamond is cut and polished, brilliant light is refracted from its surfaces, That is why, diamonds are used for making precious gems and jewellery., , (iv) Sharp edged diamonds are used as a tool by eye surgeons to remove cataract from eyes with great, , Precision,, , 2. Graphite ;, Occurrence. Graphite is 4 greyish black substance. It occurs in nature (China, South Korea, Russia,, , India, etc) mixed with mica, quartz and silica. It is also prepared artificially from carbon in an electric, furance at 2273-2773 K.
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Re, , , , ~, , , , a, , , , 2273 K 2273 K, 3C + SiO, —— SiC > C + Si, 2 4, 2.60 Silicon carbide Graphite Silicon, , Carbon Silica, Structure. The structure of graphite is altogether different from that of diamond. A Braphite, , actually consists of sheets or layers of carbon atoms as shown in Fig. 4.8 (a). Sst, , Each carbon atom in a graphite layer is bonded to three other carbons in the same plane form:, hexagonal ings. To satisfy the fourth valency of carbon, each hexagonal ring has three alternate sin ate, , double bonds, Thus, unlike diamond, graphite has a two-dimensional sheet like (layered) structure, mi i ‘, of a number of benzene* rings fused together (Fig. 4.8 b). The various sheets or layers are held test, Weak van der Waals forces (or surface forces) of attraction. The distance between any two SUCCESsive lie, i, , is 340 pm., Each carbon in the layer of hexagonal, , tings Is attached to three other carbon, atoms by strong covalent bonds, , 340 pm, , eak van der Waals, forces of attraction, , , , , , , , , , Structure of graphite (a) consisting of tw:, (b) each fayer consisting of fused benze:, , , , , , Physical properties. (i) Low density. Due to wide spacing (340 pm) between the two layers, the, carbon atoms in graphite are less closély packed and hence the density of graphite (2:22 g cm™) is much, lower than that of diamond (3-51 g cm~). Saat, , (ii) Softness. The various layers of carbon atoms in graphite are held together by weak van der Waals, forces of attraction. Therefore, one layer can easily slide over the other. This makes graphite soft and hence, a useful dry (solid) lubricant for heavy machinery. ,, , (iii) Electrical and thermal-conductivity. We know that carbon has four valence electrons, But in a, graphite crystal, each catbon atom is joined to three other carbon atoms by covalent bonds to form hexagonal, rings. Thus, only three valence electrons are used for bond formation and hence the fourth valence electron, is free to move. As a result, graphite is a-good conductor of heat and electricity: ~~., , Uses. (i) Due to its softness, powdered graphite is used either as a solid or dry lubricant or mixed with, petroleum jelly as graphite grease. Since graphite is non-volatile, it can also be used as a lubricant for heavy, , machinery operating at very high temperatures., *Benzene is a six-membered cyclic unsaturated carbon compound’ containing three alternate single and double, , bonds, refer to Fig. 4.17,.page 338.
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49°, ? att tl for making the aks paper black Mixed wi, e iS Cores of lead pencils, » Mixed with desired quantity of wax or, , i, , 5,, , , , afearbon atoms Fr., , jdentl, , ‘ ace ;, yo comparison of Properti, , s, , , , WAND. rs; COMPOUNDS)", ' ite is soft and black in colour and m, (Hag, pit, gf, , g, pelle”., 47 assaties., , good conductor of electricit, The carbon brushes of ie, , $0!, , , , graphite j i, , = e ite is used for making electrodes for dry cells and, ae Otors are also made up of graphite., lamond and Graphite, , pai points of difference bet, at : ween the properties of diamond and graphite are given below :, , , , Diamond, , "Graphite, , , , , , jamond has a three-dimensional network, , <irvcture, gris the hardest natural substance known,, , tis a bad conductor of electricity but is a ver, ood conductor of heat. Because of hardness ie, , pigh thermal conductivity, diamond tipped toots, , o not overheat and hence até extensively used, for cutting and drilling purposes” = tCSCS, It is 4 transparent substance with high refractive, index. T herefore, it is used for making gemstones, and jewellery., , Chemically diamond is extremely unreactive. , , , 1, Graphite has a two-dimensional sheet like, structure consisting of a number of benzene rings, fused together., , 2, Graphite is soft and greasy and is used as solid, , lubricant for heavy machinery operating at high, _ temperatures., , 3. It is a good conductor of both heat and electricity., , Because of high’electrical conductivity, graphite, is used for making electrodes of battery and arcs., , 4. It is an opaque greyish black substance., , 5. Graphite is reactive, , , , 3, Fullerenes., , Fullerenes are a new class of carbon allotropes. They are spheroidal in shape, anging from 60-350 or above. The Ce, ified. It contains 60 carbon atoms which are arrange, , ral (Fig. 4.9 5), therefor, , membet, but five-membered rings, , are fused together., , Further, because these allotrop, ster fullerenes or S!, , mngs, mings, , Fullel, , 1, they are called Buckmi, , es look like geodesic domes designed by, imply fullerenes., , fullerene is the most stable and was the first to be, din the shape of a football (Fig. 4.9 a) or a soccer, , ¢, it is also called buck all. It contains 20 six-memb, red rings. Please note that six-mem ered rings are fused to both six-membered and five-membered, words, no two five-membered, , are fused only to six-membered rings. In other, , the US architect-Buckminster, , , , , , , , , , and contain even number, , ered rings and 12 five
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—, , a ail, , , , , , “met RY ES |, solid at room temperature. Unlike fal, , fullerene is a dark, , Physical properties. Buckminster P, aqosbie whlch we giant molecules containing thousands and thousands of carbon atoms, Ce fa iy |, 1 molecule containing only 60 carbon atoms. met, differ from one another is their, , a very sma, , Another physical property, Whereas diamond is extremely h, very hard nor very soft., , 4.4, VERSATILE NATURE OF CARBON ; ;, g of this chapter that many of the things we use in our day to day i, ourselves are made up of carbon compounds. The nypy Ce, hree million. This number far exceeds the tota] meet, , ually start the study of carbon comp,, , before we act y, mpounds. The four main reasons for tpi, *, &, , in which the three allotropes ‘, ard, graphite is soft. Buckminster fullerene, on the other hang, i thn,, in, , We have seen in the beginnin|, made up of carbon compounds. In fact, we, carbon compounds known today is approximately tl, of compounds formed by all other elements. Therefore,, , we must understand as to why carbon forms so many CO, e number of or; anic com} s, , a larg! g pounds jg ty, , &, , discussed below :, 1. Catenation. The main reason as to why carbon forms ) y, ‘bon atoms to form straight or branched chains of varying lengip,, , , , carbon can combine with other car!, rings of different sizes as shown below : V/ <, Cc ee., rr ie \/ | apf NA Sea, —C-¢-0-C— —C-C-C- Bs a al at A al is, rT ri sf tle Or ee, I ol f N Ke {, , =—C|, , . Branched chain, , Straight chain, , Three membered ring Four membered ring Five membered ring Six membered ig |, , “This unique property of self-linking of carbon atoms through covalent bonds to form lon,, d rings of different sizes is called catenation. :, , - straight or branched chains an, ably due to (i) small size (ii) unique elect, , , , es, onic configuration ang, , The property of catenation is prob:, (iii) great strength of carbon-carbon bonds., , Both carbon and silicon have similar vi, , , , alence electronic configuration i.e. have four electrons in the, valence shell) but carbon shows catenation to a much greater ‘extent than silicon. For example, carbon, forms compounds with hydrogen jn which hundreds of carboratoms can- be joined together. These compounds, of carbon and hydrogen called hydrocarbons are very stable. On the other hand, silicon also forms compounds, with hydrogen which contain chains only upto seven or eight silicon atoms. These compounds of silicon, and hydrogen called silanes are, however, very reactive. The main reason for much greater extent of catenation, ~of carbon over silicon is that carbon-earbofi bonds are much stronger (355 kJ mol) than silicon-silican, (200 kJ mol") bonds. ;, From the above discussion,, which is responsible for existence, 2. Tetracovalency of carbon., other atoms of carbon or atoms of some, of carbon atom can hold its shared pairs 0:, , most of the other elements such as hydrogen,, thereby making these compounds exceptionally stable. This further inc, le (double ax, , compounds. ., 3, Tendency to form multiple bonds. Due to small size, carbon also forms multipl, triple) bonds with other carbon atoms, oxygen, sulphur, and nitrogen. This multiplicity of carbon-catbor, carbon-oxygen and carbon-nitrogen bonds further increases the number of carbon compounds., 4, Isomerism. Another reason for huge number of carbon compounds is the phenomenon of isomeris™, , Isomerism is defined as follows : ', , we conclude that it is the maximum tendency of carbon for catenation, , ofa large number of carbon compounds., Carbon has a valency of four. Therefore,, other monovalent elements. Further,, f electrons strongly. As a result, the, , oxygen, nitrogen, sulphur, chlorine,, reases th, , it is capable of bonding four, due to small size, the nucleus, bonds that carbon forms with, etc. are very strong, e number of carbot
