The Carbon Family and Group 14 Elements

The Carbon Family

• The valence configuration ns²np² indicates that the +4 oxidation state is most common for these elements.
• Lead is a major exception, with +2 being its most common oxidation state, two less than the group maximum.
• The inert-pair effect explains the relative stability of the lower oxidation state, a key feature in heavier p-block elements.

Group 14 Elements

• Group 14 includes arguably the most important elements, with carbon forming the basis for life and silicon being crucial for the Earth's physical structure.
• The elements in this group vary greatly in their properties, from non-metallic carbon to metallic tin and lead.
• Carbon and silicon are non-metals, germanium is a metalloid, while tin and lead are metals.
• Descending Group 14, metallic properties increase due to increasing atomic radius and decreasing ionization energy.
• The electronegativities of carbon and silicon are similar to hydrogen and they form many covalent hydrogen and alkyl compounds.
• Carbon and silicon are strong oxophiles and fluorophiles, demonstrating high affinities for hard anions O²⁻ and F⁻.
• Pb²⁺ forms more stable compounds with soft anions like I⁻ and S²⁻, classifying it as chemically soft.

Occurrence of Carbon

• Carbon exists as diamond and graphite, as well as forms with low crystallinity.
• Richard Smalley, Robert Curl, and Harold Kroto won the 1996 Nobel Prize in Chemistry for discovering buckminsterfullerene (C60).
• Less pure carbon forms include coke, created through coal pyrolysis, and lamp black, which is from incomplete hydrocarbon combustion.
• Carbon is present as carbon dioxide in the atmosphere and dissolved in natural waters, also as calcium and magnesium carbonates.

Occurrence of Silicon

• Silicon makes up 26% of the Earth's crust by mass.
• Silicon occurs as sand, quartz, amethyst, agate, and opal and is also found in asbestos, feldspar, clays, and micas.

Diamond vs Graphite

• Diamond and graphite are common crystalline allotropes of elemental carbon with different properties.
• Diamond is an electrical insulator, whereas graphite is a good conductor.
• Diamond is the hardest known natural substance, while graphite is slippery and used as a lubricant.
• These property differences arise from structural and bonding variations.
• Diamond has a rigid, covalent, three-dimensional framework, where each carbon atom forms single bonds of 154 pm with four neighbors in a tetrahedral arrangement.
• Graphite is made of stacked planar graphene layers, and each carbon atom has three neighbors at 142 pm.
• Graphite's slipperiness comes from easy cleavage parallel to atomic planes due to impurities.
• Within graphite sheets, sp² hybrid orbitals form bonds between neighbors, with remaining p orbitals forming delocalized π bonds.

Carbon Clusters

• Striking an electric arc between carbon electrodes in an inert atmosphere forms soot, containing significant C60 quantities and related fullerenes like C70, C76, and C84.
• The structure of C60 has been determined through X-ray crystallography on solids at low temperatures and electron diffraction in the gas phase.
• The C60 molecule has five- and six-membered carbon rings with an icosahedral symmetry.
• Carbon nanotubes have risen as an interesting result of fullerene research.
• Carbon nanotubes consist of one or more concentric cylindrical tubes made of graphene sheets, potentially closed by fullerene-like caps with six five-membered rings.

Applications of the Elements

• Elemental carbon, as coal or coke, is a fuel and reducing agent in metal recovery from ores.
• Graphite is a lubricant in pencils, and diamond is used in industrial cutting tools.
• Silicon's semiconductivity is responsible for applications in integrated circuits, computer chips, solar cells, and electronic devices and Silica (SiO2) is the primary material for glass production.
• Germanium was the first material widely used for making transistors, and it's a better intrinsic semiconductor than silicon (0.72 eV band gap for Ge, 1.11 eV for Si).
• Tin resists corrosion, so it plates steel for tin cans.
• Bronze is an alloy of tin and copper, typically containing less than 12% tin by mass, and bronze with higher tin content makes bells.
• Solder is an alloy of tin and lead, used since Roman times.
• Window or float glass is made by floating molten glass on molten tin, and the 'tin side' has a tin(IV) oxide haze under ultraviolet radiation.
• Trialkyl and triaryltin compounds are widely used as fungicides and biocides.
• The softness and malleability of lead led to its use in plumbing, but plumbing use is now illegal in many countries due to lead poisoning concerns.
• Lead's low melting point helps its use in solder, and its high density (11.34 g/cm³) leads to ammunition and shielding applications.
• Lead oxide is added to glass to increase its refractive index for 'lead' or 'crystal' glass.

Simple Compounds of Carbon and Silicon

• Group 14 elements form tetravalent hydrides, EH4.
• Carbon and silicon form series of catenated molecular hydrides.
• Carbon forms many hydrocarbon compounds e.g. alkanes (CnH2n+2).
• Long-chain, catenated hydrocarbons are stable due to high C-C and C-H bond enthalpies.
• Carbon forms strong multiple bonds in unsaturated alkenes and alkynes.
• Carbon's strong C-C bonds and ability to form multiple bonds is largely responsible for the diversity and stability of its compounds.
• Silicon forms silanes, similar to alkanes, but the longest chain has seven Si atoms, such as hepta-silane, Si7H16.
• Silanes are less volatile than hydrocarbon analogues due to more electrons and intermolecular forces.
• Group 14 elements form simple binary compounds with hydrogen, oxygen, halogens, and nitrogen, while carbon and silicon form carbides and silicides with metals.
• Silanes are less volatile than hydrocarbons due to stronger intermolecular forces.
• Propane (C3H8) is a gas and trisilane (Si3H8) is a liquid that boils at 53°C.
• Tetrahalomethanes vary from stable CF4 to thermally unstable CI4.
• The full range of tetrahalides is known for silicon and germanium; all are volatile molecular compounds.

Oxides of Carbon and Silicon

• The familiar oxides of carbon are CO and CO₂.
• An uncommon oxide of carbon is carbon sub-oxide, O=C=C=C=O.
• CO has a short, strong bond (1076 kJ mol⁻¹ bond enthalpy) and a high force constant, indicating a triple bond (C≡O).
• Carbon dioxide (CO₂) has longer bonds and smaller stretching force constants than CO, equivalent to double bonds.
• Silicon has a high affinity for oxygen, forming many silicate minerals and synthetic silicon-oxygen compounds used in mineralogy, industrial processing, and labs.
• Structures of silicates are based on four-coordinate Si tetrahedra, except for rare high-temperature phases.
• Orthosilicate is [SiO₄]⁴⁻, and disilicate is [O₃SiOSiO₃]⁶⁻.
• Silica and many silicates crystallize slowly.
• Amorphous solids, glasses, can be obtained by cooling the melt at a specific rate.
• Glasses resemble liquids.
• Glasses are ordered over short distances (e.g., within SiO₄ tetrahedra).

Compounds with Nitrogen

• Carbon forms hydrogen cyanide (HCN), ionic cyanides with the CN⁻ ion, and gas cyanogen ((CN)₂).
• All compounds are extremely toxic.
• HCN is highly volatile (b.p. 26 °C) and like CN⁻, highly poisonous.
• Cyanide's toxicity is similar to CO as both form complexes with iron porphyrin molecules.
• CO binds to Fe in hemoglobin, causing oxygen starvation.
• CN targets Fe in cytochrome c oxidase, causing rapid energy collapse as the enzyme in mitochondria that aids the reduction of oxygen to water is halted.
• Cyanogen ((CN)₂) is a toxic, flammable pseudo-halogen, due to its similarities to halogens.
• Silicon reacts directly with nitrogen gas at high temperatures to produce silicon nitride (Si₃N₄).
• Silicon nitride is hard, inert, and used in high-temperature ceramic materials.

Carbides and Silicides

• Binary compounds of carbon with metals and metalloids are called carbides.
• Saline carbides are largely ionic solids formed by Group 1, Group 2 elements, and aluminium.
• Metallic carbides have metallic conductivity and lustre and are formed by d-block elements.
• Metalloid carbides are hard covalent solids formed by boron and silicon.
• Silicon, forms silicides (binary compounds with metals).
• Some silicides have isolated Si atoms.
• Silicon carbide (SiC) is widely used as the abrasive carborundum.

Extended Silicon-Oxygen Compounds

• Silicon forms extended network solids that have applications in industry
• Aluminosilicates and Zeolite Aluminosilicates are formed when Al atoms replace some of the Si atoms in a silicate and occur naturally as clays, minerals, and rocks.
• Zeolite aluminosilicates are used as molecular sieves, microporous catalysts, and catalyst support materials.
• The presence of Al(III) in place of Si(IV) creates a negative charge and an additional cation, such as H⁺, Na, or ½Ca²⁺ is required, affecting the material's properties.
• Important minerals are layered aluminosilicates containing metals (lithium, magnesium, iron): clays, talc, and various micas.
• Kaolinite (Al₂(OH)₄Si₂O₅) is a layered aluminosilicate used as china clay and diarrhea remedy.
• Framework minerals include feldspars.
• Molecular sieves are crystalline microporous aluminosilicates with apertures.
• 'Molecular sieve' describes materials that absorb small molecules used to separate different sizes.
• Zeolites are a subclass of molecular sieves with an aluminosilicate framework with cations trapped inside tunnels or cages.
• Zeolites are used as ion-exchange resins as they can exchange ions.
• Zeolites are used for shape-selective heterogeneous catalysis.

Oxides of Germanium, Tin, and Lead

• Germanium(II) oxide (GeO) is a reducing agent and disproportionates to Ge and GeO₂.
• Germanium(IV) oxide (GeO₂) is based on tetrahedral four-coordinate GeO₄ units.
• Tin (II) oxide (SnO) exists in two polymorphs: blue-black and red.
• The red form of SnO has a similar structure and is converted to the blue-black form by treatment with heat, pressure, and alkali.
• SnO heated in the absence of air disproportionates into Sn and SnO₂.
• SnO₂ is found naturally as the mineral cassiterite and has a rutile structure.
• Red lead(II) oxide (PbO) has the same structure as blue-black SnO with a stereochemically active lone pair.
• Lead forms mixed oxidation state oxides such as red lead, Pb₃O₄, which has Pb(IV) in an octahedral environment and Pb(II) in an irregular six-coordinate environment.

Halides and Hydrides of Germanium, Tin, and Lead

• Tin dihalides and tetrahalides are both known.
• Tetrachloride, tetrabromide, and tetraiodide are molecular compounds, but tetrafluoride is an ionic solid.
• Due to the inert-pair effect, PbCl₄ is unstable and decomposes into PbCl₂ and Cl₂ at room temperature.
• Lead tetrabromide and tetraiodide are unknown, so dihalides dominate.
• The hydrides of Ge, Sn, and Pb are called germane (GeH₄), stannane (SnH₄), and plumbane (PbH₄).
• Germane (GeH₄) and stannane (SnH₄) can be synthesized by reacting the tetrachloride with LiAlH₄ in tetrahydrofuran.
• Plumbane (PbH₄) has been synthesized in trace amounts by protolysis of magnesium/lead alloy but is extremely unstable.
• SiH₄ < GeH₄ > SnH₄ > PbH₄ demonstrates the stability order of the tetrahydrides.
• Thermal stability decreases from germane to stannane and plumbane.