Chapter 4
B.sc 1st year Book
(Page 9)
Group IV A (14) Elements: C, Si, Ge, Sn, Pb Carbon Family
Introduction: Element carbon, Silicon germanium, tin and lead constitute group 14 of the periodic table, Carbon is the important constituent of all living and non-living matters, Whereas silicon is the main constituent of inorganic matter. Carbon occurs in a free state as diamond, graphite, coal etc. and in combined form, it occurs in carbon dioxide, carbonates, and hydrocarbons. Silicon is the second most abundant element of the earth’s crust ( −27.2 wt%) occurs as silica, and silicates, Germanium is a rare element (1.5 ppm) and is mainly recovered from the flue specks of dust arising from the roasting of Zinc ores. Tin and lead occur mainly as cassiterite (SnO2) and galena ( PbS) respectively in nature.
Silicon and germanium are used in the production of semiconductors and integrated circuits. Silicon is a very important component of quartz, ceramics, glass and cement. Germanium is used in making prisms, lenses and infrared windows because of its transparent nature. Tin is used as a coating on metals to prevent their corrosion by rusting and also in making various alloys like solder and bronze. Lead is mostly used in storage batteries, alloys and pigments.
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Electronic configuration of Carbon Family :
All elements have four electrons in their outermost shell which can be represented in general by ns2np2. Where the value of n changes from 2 to 6 from carbon to lead. The electronic configuration of these elements is tabulated in table 4.7.
Table 4.7: Electronic configuration of Group 14 or IV A elements
| Element | Symbol | Atomic No. | Electronic configuration with an inert gas core |
| Carbon | C | 6 | [He]2 s2,2p2 |
| Silicon | Si | 14 | [Ne]3 s2,3p2 |
| Gemminium | Ge | 32 | [Ar]3 d2,4 s2.4p2 |
| Tin | Sn | 50 | [Kr]4 d10,5 s25p2 |
| Lead | Pb | 82 | [Xe]4 F14,5 d10,6 s2,6p2 |
The similar electronic configuration ns2np2 of the valence shell of these elements indicates that they should exhibit similar properties. However, the penultimate shell (n−1) of these elements have different configuration for example carbon and silicon have only s-and electrons but other elements have a completely filled d-subshell in the penultimate shell.
The penultimate shell of Ge and Sn contains 18 electrons whereas lead has 32 electrons in its penultimate shell.
2. Atomic and ionic size of Carbon Family :
As it is expanded atomic size and ionic size in the group increase from top to bottom. Thus, carbon is the smallest and lead has a larger size.
3. Ionization potential and electronegativity :
Since atomic size increases in a group, ionization potential and electronegativity decrease from top to bottom. This decrease is though considerable for silicon, electronegativity remains almost unchanged after silicon, The decrease in ionization potential and electronegativities decrease with the increase in their atomic size. Lead has greater ionization potential than that tin due to lanthanide contraction. It is clear from table 4.8.
4. Electron affinity :
The electron affinities of these elements are greater than that of the elements of group IIIA.
5. Metallic Character:
Carbon and silicon are nonmetals even though silicon is slightly more metallic than carbon. Germanium is a metalloid (Semi metal) while Tin and lead are metals. Thus, this property increases from top to bottom with increasing atomic numbers.
6. Oxidation states :
The valence shell electronic configuration ns2np2 of these elements indicates that these elements can lose or gain 4 electrons to attain noble gas configuration. If means, theoretically they can form M4+ and M4-ions. How not form stable M4+ ions. In addition, due to low electron affinity than higher group members, these elements do not exhibit much tendency to form M4 – ions by gaining 4 electrons. However, the +4 oxidation state is shown by all elements, it is more important in carbon and silicon and becomes increasingly less important in Ge, Sn and Pb in which the +2 oxidation state becomes increasingly important. The +4 oxidation state compounds are covalent while +2 oxidation state compounds are ionic. −4 oxidation stability decreases down the group.
7. Catenation :
The tendency of an element to form a long chain by its atoms is termed catenation, This tendency decreases from carbon to lead in this group. The actual decreasing order of catenation is:
C ≫ Si > Ge – Sn > >Pb
Carbon forms various types of branched straight and cyclic chains. For example :
This property is attributed to the steady decrease in the M – M bond strength. The greater the bond strength between the like atoms greater will be the tendency of an atom to form long chains.
| Bond | Bond energy in kJmol−1 | Remark |
| C−C | 346.9 | Forms many chains |
| Si-Si | 221.5 | Forms tew chain |
| Ge-Ge | 167.2 | Forms no chain |
Due to the smaller size of the carbon atom, the p-orbitals can come close enough to effectively take part in π-overlapping with the suitable orbitals of other elements. This gives rise to the presence of multiple bonds in most carbon compounds(Boran Family). Whereas silicon has no tendency for such bond formation, In some cases, its d-orbitals are involved in dπ=pπ overlapping which imparts only a partial double bond character. Catenation is the characteristic property of carbon but in silicon, masteron buy this tendency is less pronounced. Because of the greater C−C bond energy than Si−Si bond energy (dC−C=83kcal mol−1 and d Si−Si=42kcalmol−1 ). Hence, carbon-chain compounds are comparatively more stable. i.e. Hydro carbons are stable and do not react with air without heating while silanes react spontaneously with air.
8. Allotropy of Carbon Family :
When an element or compound is found in more than one physical form, then all these forms are known as allotropes and the phenomenon as allotropy. Except for Pb all elements of the 14th group show allotropy e.g. carbon exhibits mainly into two crystalline forms diamond and graphite. Si exists in crystalline and amorphous forms. Ge exists in two crystalline forms and Sn exists in three solid forms. α-Tin has a diamond structure that exists only in ccp metallic form.
This property arises either due to the different number of atoms in a molecule or due to the different arrangement of atoms in molecules. In these two allotropes: if the difference lies in chemical bonding then it is termed as ‘primary modification’ but if the difference lies in molecular aggregation, it is termed as ‘secondary modification’, Carbon exhibits primary modification. The allotropic forms of carbon obtained from natural and artificial sources are discussed as follows :
(i) Diamond :
The diamond is the densest and hardest variety of carbon (density 3.514 g cm−3). It is transparent to light and X-rays and possesses a high refractive index (2.45). Diamond has the highest thermal conductivity of any known substance (five times higher than that of copper) and one of the lowest known coefficients of thermal expansion. The lack of p bonds makes the diamond an insulator and leads to its strength and unusual hardness in all three spatial directions of its lattice.
If one heats diamond above 1500∘C in the absence of aï, it transforms exothermally into graphite; diamond is metastable at room temperature, and becomes unstable under these conditions. Diamond is a bad conductor of electricity. It burns on strong heating in oxygen when heated with K2Cr2O2 and H2SO2 it is oxidised into CO2(Carbon Family).
It is widely used in jewellery because diamonds glitter when light falls on them. This is because of the high refractive index of diamond that the incident light is totally reflected, in industries diamonds are used in cutting glass and other substances. The diamond structure is derived from methane in which the hydrogen atoms have been substituted by carbon atoms. Thus, in a crystal of diamond, each carbon atom is tetrahedrally surrounded by four other carbon atoms. All carbon atoms are sp3 hybridized and each hybrid orbitals make an s bond. In diamond, all the four valence electrons are used up in s bond formation, no electron is, therefore left for p-bonding. Thus, each carbon atom in a diamond is surrounded by four single bonds and has a pure carbon-carbon single bond distance of 1.54 A. (Carbon Family).
(ii) Graphite :
Graphite is a dark black-coloured soft substance which is used in pencils for writing on paper. The lead of pencils is actually graphite (formerly, graphite was thought to be a form of lead). Its density is 2.26gcm−3. Carbon atoms in graphite exist in the form of regular hexagon i.e. every carbon atom is bonded to three other carbons and the bond angle is always 120. Thus, several benzene rings fuse together to form a layer and several layers form a crystal of graphite. Thus, in graphite, every carbon atom is assumed to be in an sp² hybridized state. The carbon atoms utilize these orbitals in making three localized o-bonds to its neighbouring C-atoms/Carbon Family with three of its four valency electrons. The fourth valence electron of each carbon atom is accommodated in a delocalised s-molecular orbital formed from p2 atomic orbitals which lie perpendicular to the sp2 hybrid orbitals and do not participate in the hybridization. The carbon atoms in graphite are therefore linked by both s and p bonds as indicated schematically in figure 4.07.
The graphite rod sublimes producing a fluffy mass called “fullerene soot” This soot is then dissolved in organic solvents such as benzene and chromatographed when the solution of pure C60 (about 79% ) and C70 (about 20%) separate out as major fractions. A minor amount (about 1%) of higher fullerenes( C76, C78, C80 etc.) can also separate out in the process. The solutions of pure fullerenes when concentrated, yield crystals containing solvent molecules trapped in the interstices. The sublimation of these crystals in vacuum yields solvent-free fullerene crystals.
Since all p-orbitals are parallel to each other they overlap together to form a delocalised system of π-electrons. This enables graphite to conduct electricity. Thus, the delocalised π – Electrons are responsible for the metallic conductivity of graphite, in directions parallel to carbon planes (two-dimensional electron gas). The specific conductivity of graphite parallel to the layers is 2.6×104SR−1 cm−1, It decreases with increasing temperature. l.e, it has a negative temperature coefficient as is usual for metallic conductors perpendicular to the layers. The conductivity is four orders of magnitude smaller and increases with increasing temperature.
The hexagonal layers are bonded together by a very long bond (dC−C=3−5 A) formed by some p-orbitals. These bonds are so weak in nature that the upper hexagonal layer can easily slip away from the crystal. For this reason, graphite is very soft and is used as a dry lubricant.
Several forms of amorphous carbon θ.g. charcoal, lamp black, cake etc. are also different forms of graphite. These amorphous forms especially charcoal are characterized by the tendency of adsorption.
Several forms of amorphous carbon θ.g. charcoal, lamp black, cake etc. are also different forms of graphite. These amorphous forms especially charcoal are characterized by the tendency of adsorption.
(iii) Fullerenes :
Fullerenes are allotropes of carbon having the general formula C2n(n=30 to 48). These are prepared by passing high voltage current through graphite rads in the inert atmosphere of helium.
The graphite rod sublimes producing a fluffy mass called “fullerene soot” This soot is then dissolved in organic solvents such as benzene and chromatographed when the solution of pure C60 (about 79% ) and C70 (about 20%) separate out as major fractions. A minor amount (about 1%) of higher fullerenes( C76, C78, C80 etc.) can also separate out in the process. The solutions of pure fullerenes when concentrated, yield crystals containing solvent molecules trapped in the interstices. The sublimation of these crystals in vacuum yields solvent-free fullerene crystals.
C60 Fullerene:
The most common of these fullerenes, C60 contains 20 hexagons and 12 pentagons of carbon atoms fused with one another resulting in spherical football-like geometry as shown in figure…There are two types of C−C bonds in the structure of C60 fullerene one of which is 1.48 A0 and the other is 1.38 A0 in length. C60 fullerene is one of the most strained molecules known so far yet it shows high kinetic stability. Nevertheless, the thermal stability of fullerene is less than that of graphite and diamond. It starts decomposing at about 700∘C. C70 Fullerene: There are eight distinct types of C−C bonds in the structure of C70 fullerene with lengths varying between 1.39 A0 to 1.54 A0. This fullerene has also a spherical, football-like structure containing several hexagons and pentagons of carbon atoms fused together.
9. Halides of Carbon Family:
All these elements give tetrahalides of the type MX4 which are tetrahedral and covalent in character. The ionic character and thermal stability of tetrahalides decrease with an increasing atomic number of the central atom as we move down the group from carbon to lead. Except for SnF4 and PbF4 other halides are volatile. The tetrahalides of carbon CCl4 and CF4 are stable and are not hydrated by water. Whereas tetrahalides of other elements are readily hydrated by water. This is due to the fact that the maximum covalency of carbon is four and that of silicon is six on account of its vacant orbitals(Carbon Family). Thus, silicon tetrahalide coordinates with two molecules of water to attain its maximum covalency of six. However, the coordination compound SiCl4⋅2H2O breaks up with the elimination of two molecules of HCl and then further coordinates with two more molecules of water. The resulting compound eliminates further two molecules of HCl to give silicic acid as shown below :
HF so obtained may combine with SiF4 to form fluoro silicic acid; H2SiF6 which does not react with water
SiF4 + 2HF⟶ H2SiF6 (Fluoro silicic acid )
Germanium, tin and lead form dihalides of the type MX2 also in addition to tetrahalides. M(II) halides are ionic but M(IV) halides are covalent in character. The stability of the M(II) halides increases steadily in the sequence
CX2 ≪ S2 ≪ X2 ≪ X2 ≪ SnX2 ≪ PbX2
Thus, the divalent state i.e. M(II) state becomes more stable as we descend the group.
10. Hydrides of Carbon Family :
All of these elements form covalent hydrides of the typo MH4. The number of hydrides and the ease with which these are formed decreases from carbon to lead as we move first from top to bottom in the group. Thus, carbon forms a vast number of hydrides: alkanes, alkenes, alkynes, alicyclic and aromatic hydrocarbons. Silicon forms a very small number of saturated hydrides called ‘silanes’, Germanium forms only a limited number of hydrides called ‘germanes’ while tin and lead form only one type of hydride stannate (SiH4) and Plumbane (PbH2). The hydrides of the MH4 type are gaseous in nature and their stability decreases in the order of:
CH4– > SiH4– > GeH4 –> SnH4– > PbH4
because of very little electronegativity difference between the elements and hydrogen. In M1H4, the M – H bond is almost non-polar and central atom M is sp 3 hybridized. Unlike alkanes, silanes are strong/highly reducing agents, explode in chlorine/Cl and are readily hydrolysed by alkaline solutions. The difference is probably due to the difference in electronegativity between carbon and silicon resulting in a difference between C−H and Si−H linkage as shown below:
This polar form of CH4 explains inability. This polar form explains why SiH4 is towards hydrolysis. undergoes hydrolysis.
11. Inert pair effect of Carbon Family:
The inert pair effect is more prominent in the case of heavier members of this group. As we descend the group the stability of the M(II) halides increases and M(IV) halides decreases. The GeCl2 is a strong reducing agent while GeCl4 is stable. Sn(II) exists as a simple ion and is a strong reducing agent whereas Sn(IV) is covalent. PbCl2 is ionic, stable and more common than PbCl4.PbCl4 acts as a strong oxidising agent
Comparative Study of Carbon and Silicon :
Although carbon and silicon belong to the same group but differ from one another due to the following:
(i) Carbon is smaller than silicon.
(ii) Carbon has no empty d-orbitals in its valency shell but silicon does.
(iii) Carbon has a great tendency to form multiple bonds but silicon does nor
(iv) Carbon forms a number of different kinds of long-chain than silicon.
(v) Carbon has more electronegativity (2.5) than silicon (1.8)
(ii) Carbon has no empty d-orbitals in its valency shell but silicon does.
(iii) Carbon has a great tendency to form multiple bonds but silicon does nor
(iv) Carbon forms a number of different kinds of long-chain than silicon.
(v) Carbon has more electronegativity (2.5) than silicon (1.8)
(vi) Carbon-Carbon bond energy (83kcalmol−1) is greater than silicon-silicon bond energy (42kcalmal−1).
Because of the absence of d-orbitals in the valency shell, carbon can not use more than four orbitals for bonding. It is evident from the following electronic configuration:
Therefore, carbon shows a maximum covalency of fours whereas in silicon two empty d-orbitals can also participate in 5p3d2 – hybridization and hence it can show a maximum covalency of six.
This is the reason why? saturated carbon compounds are not reactive and do not hydrolyse or hydrated. Whereas the corresponding silicon compounds are very reactive and hydrolysed when treated with water.
Abnormal behaviour of Carbon:
Carbon is the first member of its group.: It differs in properties from the rest members of group IV A due to:
(i) it is a small size
(ii) its high electronegativity
(iii) a great tendency to show catenation
(iv) absence of d-orbitals:
The abnormal behaviour of carbon & Its family is also supported by the following properties.
(1) It is quite hard whereas other members of this group are comparatively soft.
(ii) its high electronegativity
(iii) a great tendency to show catenation
(iv) absence of d-orbitals:
The abnormal behaviour of carbon & Its family is also supported by the following properties.
(1) It is quite hard whereas other members of this group are comparatively soft.
(2)CF4 can’t form a complex ion while SiF4 forms the complex ion, SiF62.-
(3)CCl4 can not form additional compounds like CCl4−2H2O, and CCl4⋅2NH3 while SiCl4 forms SiCl4⋅2H2O and SiCl4⋅2NH3.
(4)CCl4 can not be easily hydrolysed.
(5) lots unique ability to form multiple bonds such as
(3)CCl4 can not form additional compounds like CCl4−2H2O, and CCl4⋅2NH3 while SiCl4 forms SiCl4⋅2H2O and SiCl4⋅2NH3.
(4)CCl4 can not be easily hydrolysed.
(5) lots unique ability to form multiple bonds such as