Class 11 Chemistry Chapter 11 The p-Block Elements
Carbon family include elements Carbon(C), Silicon (Si), Germanium (Ge), Tin (Sn), Lead(Pd). The general electronic configuration of this family is ns2 np2 that means these elements have valency 4. In nature Carbon is seventeenth most abundant element by mass in earth’s crust. However, carbon is found in both free and combined state. In elemental form it occurs in form of coal, graphite and diamond. In combined state it occurs as metal carbonates, hydrocarbons and carbon dioxide gas. Carbon combined with dihydrogen, dioxygen, chlorine and sulphur forms important compounds. Moreover, carbon has three isotopes 12C, 13C, 14C. Silicon is the second most abundant element in the earth’s crust. It is found in the form of silica and silicates. Silicon is important component of ceramics, glass and cement. Germanium exist only in traces. Tin occurs mainly as cassiterite, SnO2. Ultrapure form of germanium and silicon are used to make transistors and semiconductor devices.
Physical Properties of carbon family
- Electronic configuration: The general electronic configuration of carbon family elements is ns2np2. The inner core electronic configuration differs due to presence of d and f orbitals.
- Radius: Radius increases from C to Si. After Si, till Pb increase in the radius is extremely small.
- Ionization enthalpy: the first ionisation enthalpy of group 14 element is higher than corresponding group 13 elements. However, ionization enthalpy decreases down the group. there is small decrease in ionization enthalpy from Si to Ge to Sn and then there is slight increase from Sn to Pb
- Electronegativity: Due to small size, the elements of carbon are slightly more electronegative. From Si to Pb, the electronegativity values of elements are almost same.
- Other physical properties: All group 14 members are solids. Carbon and silicon are nonmetals. Germanium is metalloid. Tin and lead are soft metals having low melting point. In comparison with group 13 elements, group 14 elements have higher melting and boiling point.
Chemical properties of carbon family
- Oxidation: Group 14 elements have four electrons in their valence shell, hence, the oxidation state is +4. As we move down the group, larger size and poor shielding effect due to d and f orbitals, the outer electrons are tightly held by the nucleus. Thus, it restrict ns2 electrons to take part in bonding and therefore only np2 electron take part in bonding. Thus stability of +2 oxidation state increases. In +4 oxidation state it acts as strong oxidising agent. Carbon can also show negative oxidation state (-4).
- Reactivity towards oxygen: Carbon family members when heated in oxygen undergoes oxidation to form oxides. These form two types of oxide, monoxide and dioxide. In monoxides CO is neutral, GeO is acidic, whereas SnO and PbO are amphoteric. Among dioxides CO2, SiO2 and GeO2 are acidic, SnO2 and PbO2 are amphoteric.
- Reactivity towards water: In general, Carbon, Silicon, Germanium donot react with water. However, tin react with steam and form dioxide and hydrogen gas. Lead form protective oxide layer on its surface, which prevents lead from reacting with water.
Sn + 2H2O → SnO2 + 2H2
Anomalous behaviour of Carbon
Carbon is odd from other family members due to its smaller size, higher electronegativity, higher ionisation enthalpy and unavailability of d orbitals in comparison to other group elements. Carbon have only s and p orbitals available for bonding, therefore, maximum covalency of carbon is four. In comparison to carbon, other group members have additional d orbitals in valence shell. Hence they have the ability to expand covalency beyond four. Carbon has the ability to form pπ – pπ bonds with itself or with other atoms having comparable size and high electronegativity. For example: C=C, C ≡ C, C=O, etc. carbon shows the property of catenation. That is the carbon has the ability of carbon atom to form chains by linking to other carbon atoms. Thus carbon forms long chain structures and ring structures. As we move down the group catenation properties of elements decreases. Due to property of catenation, carbon exist in different allotropic forms like graphite, diamond and fullerene.
Allotropes of carbon
In diamond each carbon atom is sp3 hybridised and it linked to other four carbon atoms in tetrahedral shape. C-C single bond length is 154pm. Carbon atom is strongly held together by covalent bond that is very difficult to break them. This make diamond hardest natural substance. This is also the reason why diamond have high melting point. Moreover, diamond do not have free electron, thus it is a bad conductor of electricity. Diamond is used as an abrasive for sharpening hard tools, in making dyes and in manufacture of tungsten filament.
Each carbon atom in graphite is sp2 hybridised and it is linked to three carbon atoms in same plane, forming C-C sigma bond. The C-C bond length is 141.5 pm. Carbon atoms are linked with each other forming a planar hexagonal rings of carbon atom. The different layers are held by van der waals forces. The distance between two layers is 340 pm. The fourth electron is free. Thus it allows graphite to conduct electricity. Physically graphite is soft and slippery. Thus it is also used as lubricants rather than oil lubricants.
Fullerenes are cage like molecules, produced by heating graphite in an electric arc in the presence of inert gas. During this process Cn small molecules get produced which on condensation produce sooty material. It consist of mainly C60 and in small quantity C70 and traces of fullerenes, having even number of carbon atoms up to 350 or more. Fullerenes are the only pure form of carbon because they smooth structure without having dangling bonds. The C60 molecule have shape of soccer ball. It is known as Buckminsterfullerene.
Uses of Carbon
- Diamond is precious stone used in jewellery.
- Graphite fibres are used to form high strength, lightweight composite materials. Such composites are used in many products like tennis rackets, fishing rods, aircrafts and canoes. Since graphite is good conductor of electricity, it is used as electrode in batteries and industrial electrolysis. Crucibles made from graphite are inert to dilute acids and alkalis.
- Being highly porous, activated charcoal is used in adsorbing poisonous gases. It is also used in water filters for removal of organic contaminators and in air conditioning systems to control odour. Activated carbon, is also called activated charcoal. It is a form of carbon that is processed to have small, low-volume pores which are used to increase the increase the surface area available for adsorption or chemical reaction.
- Carbon black is used as black colored pigment in black ink. It is also used as filler in automobile tyres.
- Coke is used as a fuel and largely as a reducing agent in metallurgy. It is measured in carats (1 carat = 200 mg)
Keywords: Allotropes, Graphite, Fullerenes, Diamond, Electronegativity, Isotope, Ionization Enthalpy.