Physical properties of Carbon Family

(1)     Non-metallic nature : The non-metallic nature decreases along the group.

C          Si         Ge         Sn         Pb        

Non-metal metalloid metal metal or semi metal

(2)     Abundance : Carbon and silicon are most abundant elements in earth’s crust whereas germanium occurs only as traces. Tin and lead also occur in small amounts. Only carbon occurs in free state as coal, diamond and graphite and in combined state as carbonates, CO₂ petroleum and natural gas Silicon is the second most abundant element after oxygen in earth’s crust in form of silicates and silica. Germanium found in traces in coal and in certain deposits. It important constituent for making conductors and transistors the important ore of tin is tin stone (SnO₂) or cassiterite. Lead is found is form of galena (PbS) anglesite (PbSO₄) and cerussite (PbCO₃) The abundance ratio in earth’s crust is given below,

          Element                                                             C            Si          Gs     Sn     Pb

          Abundance in earth’s crust (ppm)      320   277200        7      40     16

(3)     Density : The density of these elements increases down the group as reported below

          Element              C                Si                Ge               Sn               Pb

          Density               3.51            2.34           5.32            7.26       11.34

           (g/ml)         (for diamond)               2.22 (for graphite)

 

(4)     Melting point and boiling points

(i)      The m.pt and b.pt. of this group members decrease down the group.

          Element       C                Si                 Ge               Sn                Pb

          m.pt(K)      4373        1693          1218          505             600

          b.pt.(K)         –             3550          3123          2896           2024

(ii)     The m.pt and b.pt of group 14 elements are however, higher than their corresponding group 13 elements. This is due to the formation of four covalent bonds on account of four electrons in their valence shells which results in strong binding forces in between their atoms in solid as well as in liquid state.

 

(5)     Atomic radii and atomic volume

(i)      Both atomic radii and atomic volume increases gradually on moving down the group due to the effect of extra shell being added from member to member.

                                                                  C       Si      Ge     Sn     Pb

          Atomic radius (pm)              0.77  111   122   141   144

          Atomic volume (ml)              3.4    11.4  13.6  16.3  18.27

(ii)     The atomic radii of group 14 elements are than their corresponding group 13 elements due to increase in nuclear charge in the same period.  

(iii)    Some of the ionic radii involving six co-ordination of these group elements are given below,

                                                              C       Si      Ge     Sn     Pb

Ionic radius (M²⁺) in pm               –       –       73     118   119

Ionic radius (M⁺⁺) in pm               –       40     53     69     78

 

(6)     Electronegativity : The electronegativity decreases from C to Si and then becomes constant.

          C       Si      Ge     Sn     Pb

Electronegativity on pauling scale    2.5     1.8    1.8    1.7    1.6

The electronegativity from silicon onwards is almost is almost constant or shows a comparatively smaller decreases due to screening effects of d¹⁰ electrons in elements from Ge onwards.

 (7)     Ionisation energy

(i)      The ionisation energy decreases regularly down the group; Pb however shows a higher value than Sn due to poor shielding of inner f-orbitals as a result of which effective nuclear charge experienced by outer shell electrons becomes more in Pb.

Ionisation energy (kJ mol^-1)         C       Si      Ge     Sn     Pb

                                            IE₁            1086 786   761   708   715

                                           IE₂             2352 1577 1537 1411 1450

                                           IE₃             4620 3284 3300 2942 3081

                                           IE₄             6220 4354 4409 3929 4082

(ii)     The first ionisation energies of group 14 elements are higher than their corresponding group 13 elements because of smaller size.

(iii)    The electropositive character of these elements increases down the group because of decreases in ionisation energy.

 

(8)     Oxidation state

(i)      Presence of four electrons in outermost shell of these elements reveals that the members of this family can gain four electrons forming M⁴⁺ or M^4- ions to show ionic nature or exhibit tetravalent covalent nature by sharing of four electron pairs in order to attain stable configuration.

(ii)     The formation of M⁴⁺ or M^4- ions require huge amount of energy which is normally not available during normal course of reactions, therefore, these elements usually do not form M⁴⁺ or M^4- ions, but they usually form compounds with covalence of four.

(iii)    Ge, Sn and Pb also exhibit +2+ oxidation state due to inert pair effect.

(iv)    Sn²⁺ and Pb²⁺ show ionic nature.

(v)     The tendency to form +2 ionic state increases on moving down the group due to inert pair effect.

 

(9)     Catenation

(i)      The tendency of formation of long open or closed atom chains by the combination of same atoms in themselves is known as catenation.

(ii)     The catenation is maximum in carbon and decreases down the group.

(iii)    This is due to high bond energy of catenation.

Bond	Bond energy in kJ mol-1
C-C Si-Si Ge-Ge Sn-Sn Pb-Pb	348 180 167 155 No catenation

(iv)    Only carbon atoms also form double or triple bonds involving pπ-pπ multiple bond within itself. > C = C<; – C º C –

(v)     Carbon also possesses the tendency to form closed chain compounds with O,S and N atoms as well as forming pp-pp multiple bonds with other elements particularly nitrogen  and oxygen e.g. C =O; C=N; C ≡ N; C=S are the functional groups present in numerous  molecules due to this reason.

(vi)    Carbon can form chain containing any number of carbon atoms Si and Ge cannot extend the chain beyond 6 atoms, while Sn and Pb do not form chains containing more than one or two atoms.

(vii)   The reason for greater tendency of carbon for catenation than other elements in the group may further be explained by the fact that the C-C bond energy is approximately of the same magnitude as the energies of the bond between C and other elements. On the other hand, the Si-Si bond is weaker than the bond between silicon and other elements.

 

Bond	Bond energy (k J/mol)	Bond	Bond energy (kJ/mol)
C-C C-O C-H C-Cl C-F	348 315 414 326 439	Si-Si Si-O Si-H Si-Cl Si-F	180 372 339 360 536

 

(10)  Allotropy 

(i)      The phenomenon of existence of a chemical element in two or more forms differing in physical properties but having almost same chemical nature is known as allotropy. If an element or compound exists in two or more forms, it is also known as polymorphism e.g. zinc blende and wurtzite are polymorphs of ZnS. This phenomenon is due to the difference either in the number of atoms in the molecules [as in the case of oxygen (O₂) and ozone (O₃)] or arrangement of atoms in the molecules in crystal structure (as in the case of various forms of carbon).

(ii)     All the elements of group 14 except lead exhibit allotropy.

(iii)    Crystalline carbon occurs mainly into two allotropic forms (i) graphite and (ii) diamond (a third allotropic form called fullerenes e.g. C₆₀ and C₇₀ were recently discovered by Prof. Richard E. Smalley and his coworkes), amorphous carbon exists in different forms viz coal, coke, carbon black, lamp black, bone charcoal. Amorphous carbon is usually considered to contain microcrystals of graphite.

                               

(iv)    Diamond and Graphite : The two allotropic forms of crystalline carbon. Diamond is the purest and hardest form of carbon. Its structure involves a giant molecular form where each carbon atom is surrounded by four other carbon atoms (sp³ hybridization) In doing so, each carbon atom is located in the centre of a regular tetrahedron with its four valencies directed towards the four corners which are linked with four other carbon atoms (C – C – C angle = 109⁰ 28’C-C=154 pm = 1.54 Å). The hardness of diamond result due to the uniformity of the C-C covalent bonds. Since the C-C bond length is very small, it has very high density (3.51 g cm^-3) and has more compact structure than graphite (density, 2.25 g cm^-3) It does not melt (vapourises at 3773K) has very high refractive index (2.45) and is insoluble in all ordinary solvents. It does not conduct electricity as all the four valence electrons are used up in forming covalent bonds with other carbon atoms Diamond, because of its hardness  is used in cutting, grinding instruments such as and drilling equipments Its ability to reflect and refract light makes diamond an important jewelry material.

 

Difference between diamond and graphite

Diamond	Graphite
Crystalline, transparent with extra brilliance. Hardest natural form Bad conductor of electricity High Density (3.51 g /cm3) heavy Colourless Tetrahedral shaped  sp3 hybridisation Less stable, more energy CD → CG ; DH= – 0.5 k.cal Used in cutting glass and jewellery; an abrasive	Crystalline, opaque and shiny substance Soft having soapy touch Good conductor of electricity Low Density (2.25 g/cm3), lighter than diamond Greyish white Two dhnensional layer structure having regular hexagonal sheets. sp2 hybridization More stable, less energy  CG → CD at high temperature and high Pressure Used as lubricating agent, electrodes, in pencils, crucibles (due to high  m.pt)

 

Carbon also exists in three common microcrystalline or amorphous forms (charcoal, carbon black and cocke) Carbon black is formed when hydrocarbons, petroleum, turpentine oil or substances rich in carbon contents are heated in limited supply of oxygen, CH_4(g) +O_{2 (g)} → C_(s) + 2H₂O_(g)

These substances yield a large amount of smoke which is passed into chambers having wet blankets. The soot collected on these blankets is lamp black or carbon black or soot. It is almost pure carbon having as high as 98%to 99% carbon content with small amount of impurities It is a soft black power and is used as a pigment in black inks; large amounts are also used in making automobile tyres.

 

Charcoal is formed when wood cellulose or other substances containing carbonaceous matter are heated strongly in the absence of air Charcoal has highly open structure, giving it an enormous surface area per unit mass. Charcoal is of various forms such as wood charcoal, sugar charcoal, coconut charcoal, animal charcoal etc. These forms contain varying amount of carbon content. A very pure form of carbon is obtained from sugar. Activated charcoal, a pulverised form whose surface is cleaned. by heating with steam. is widely used to adsorb molecules. It is used in filters to remove offensive odours from air and coloured, foul smelling, bad tasting and toxic chemical as impurities from water.

 

Coke is an impure form of carbon and is produced when coal is heated strongly in the absence of air (as residue in the destructive distillation of coal) It is widely used as a reducing agent in metallurgical operations.

(v) Silicon also exists in crystalline and amorphous allotropic forms Germanium exists in two crystalline allotropic forms Tin has three allotropic forms as grey tin, white tin and rhombic tin.

 

Graphite occurs in Nature and can also obtained from coke, In graphite, out of four valence electrons, only three form covalent bonds (sp² hybridization) with three other carbon atoms. This forms hexagonal rings as sheets of on atom thickness. These sheets are held together by weak attractive forces one electron of each carbon atom is free and this enables these thin sheets slide over one another. For this reason graphite is a soft material with lubricating properties.

Graphite is a dark, opaque and soft material (density = 2250 kg/m³) Although graphite is non-metallic still it possesses a metallic lustre. It is insoluble in ordinary solvents. Graphite is a good conductor of heat and electricity because of the present of one free electron on each carbon atom. Graphite is used as a dry lubricant in making electrodes in electric furnaces. It is chiefly used in lead pencils.