Comprehensive Study Guide for the Chemistry of Nonmetals

Nanotechnology and Boron Nitride Nanotubes

• Discovery of Nanostructures

  • In $1991$, scientists discovered carbon nanotubes, which are long, thin, hollow cylinders composed of carbon atoms.

  • In the late $1990\text{s}$, research led to the discovery of similar tubes made of boron nitride ($BN$).

• Boron Nitride ($BN$) Properties

  • $BN$ units are isoelectronic with carbon units, meaning they possess the same number of electrons.

  • Each $BN$ unit contains eight valence electrons ($4$ per atom), mirroring carbon.

  • The atomic radius, ionization energy, and electronegativity of carbon are nearly identical to the average of the properties of boron and nitrogen.     

• Comparison of Atomic Properties (Table 24.1)

  • Atomic Radius ($\text{pm}$): $B = 85$, $N = 70$ (Average = $77.5$); $C = 77$.

  • Ionization Energy ($\text{kJ/mol}$): $B = 800$, $N = 1402$ (Average = $1101$); $C = 1086$.

  • Electronegativity: $B = 2.0$, $N = 3.0$ (Average = $2.5$); $C = 2.5$.

• Carbon vs. Boron Nitride Nanotubes

  • Carbon nanotubes are electrical conductors.

  • Boron nitride nanotubes act as insulators.

  • In $2003$, scientists successfully combined these materials to create a conducting carbon nanotube encased in an insulating boron nitride sheath.

  • The resulting insulated wire is $100,000$ times thinner than a human hair and may be utilized in future high-efficiency, miniaturized electronic devices.

Principles of Main-Group Elements (Groups 3A to 7A)

• Electronic Configuration and the p-block

  • Main-group elements in these groups comprise the majority of the p-block.

  • The p orbitals fill incrementally across a row, containing from one electron (Group $3A$) to five electrons (Group $7A$).

• Periodic Trends across the p-block

  • As effective nuclear charge increases to the right:

    • Atomic radii decrease.

    • Electronegativity increases.

    • Ionization energy increases.

  • Elements on the far right (nonmetals) are easily reduced, acting as strong oxidizing agents to attain noble gas configurations.

  • The smallest halogens and oxygen group elements are the strongest oxidizing agents in the p-block.

• Bonding Transitions

  • Center of the p-block: Elements typically share electrons rather than forming ions, resulting in a vast array of covalent molecular compounds.

  • Far left of the p-block: Elements have only one p electron and tend to form cations or electron-deficient covalent species (incomplete octets).

  • Metallic character increases down each column. A diagonal group of metalloids stretching from boron to astatine separates metals (left) from nonmetals (right).

Silicates: The Building Blocks of Earth's Crust

• Elemental Abundance in the Crust (Figure 24.2)

  • Oxygen: $46.4\%$

  • Silicon: $28.2\%$

  • Other elements > 1%: Aluminium ($8.32\%$), Iron ($5.63\%$), Calcium ($4.15\%$), Magnesium ($2.33\%$), Sodium ($2.36\%$), and Potassium ($2.09\%$).

• Quartz and Silica

  • Quartz (silica) has the formula unit $SiO_2$.

  • It consists of a network covalent structure where one Silicon atom is tetrahedrally bonded to four Oxygen atoms via sigma bonds.

  • To satisfy the octet rule for Oxygen, each O atom acts as a bridge between two Si tetrahedra.

• Aluminosilicates

  • Aluminum atoms ($3$ valence electrons) substitute for silicon atoms ($4$ valence electrons) in the silica lattice.

  • A $SiO_2$ unit becomes $AlO_2^{-}$ upon substitution, requiring a counter-cation (e.g., $Na^{+}$, $Ca^{2+}$, $K^{+}$).

  • Feldspars: A common group of aluminosilicates.

    • Albite: $Na(AlSi_3O_8)$, where $1/4$ of Si is replaced by Al.

    • Anorthite: $Ca(Al_2Si_2O_8)$, where $1/2$ of Si is replaced by Al.

    • Orthoclase: $K(AlSi_3O_8)$, where $1/4$ of Si is replaced by Al.

• Classification of Silicate Structures (Table 24.2)

  • Orthosilicates (Nesosilicates): Individual $SiO_4^{4-}$ tetrahedra (e.g., Willemite $Zn_2SiO_4$, Olivines $(Mg, Fe)_2SiO_4$).

  • Pyrosilicates (Sorosilicates): Two tetrahedra share one corner, forming $Si_2O_7^{6-}$ (e.g., Hardystonite $Ca_2ZnSi_2O_7$, Thortveitite $Sc_2Si_2O_7$).

  • Pyroxenes (Inosilicates): Single chains of tetrahedra where each tetrahedron shares two corners. Formula unit: $SiO_3^{2-}$ (e.g., Jadeite $NaAl(SiO_3)_2$, Diopside $CaMg(Si_2O_6)$, Spodumene $LiAlSi_2O_6$).

  • Amphiboles: Double chains of tetrahedra. Repeating unit: $Si_4O_{11}^{6-}$ (e.g., Tremolite $Ca_2(OH)_2Mg_5(Si_4O_{11})_2$, Asbestos).

  • Phyllosilicates (Sheet Silicates): Three corners of each tetrahedron are shared, forming sheets with the formula unit $Si_2O_5^{2-}$ (e.g., Talc $Mg_3(Si_2O_5)_2(OH)_2$, Mica).

  • Tectosilicates (Network Covalent): All four corners are shared (e.g., Quartz $SiO_2$).

Structures and Applications of Boron

• Physical and Chemical Nature

  • Boron is a semimetal in Group $3A$. Due to its small size and electronegativity, it forms complex structures and is usually found bonded to oxygen in nature (e.g., Borax, Kernite, Colemanite).

  • It comprises less than $0.001\%$ of the Earth's crust by mass.

• Elemental Allotropes

  • Boron has at least five allotropes based on the $B_{12}$ icosahedron (a shape with $20$ triangular faces and $12$ vertices).

• Boron-Halogen Compounds

  • General formula: $BX_3$. They have a trigonal planar structure with $120^{\circ}$ bond angles.

  • The boron atom is $sp^2$ hybridized. An empty p orbital perpendicular to the plane can overlap with filled halogen p orbitals, creating coordinate covalent character.

  • They act as strong Lewis acids.

• Boron-Hydrogen Compounds: Boranes

  • Closo-boranes: Fully closed polyhedrons, formula $B_nH_n^{2-}$.

  • Nido-boranes: "Nest-like" cages missing one corner, formula $B_nH_{n+4}$.

  • Arachno-boranes: "Web-like" cages missing two or three corners, formula $B_nH_{n+6}$.

  • Boranes are used as catalysts in organic reactions, such as the hydrogenation of alkenes.

• Industrial Applications

  • Glass Manufacture: Boron oxide ($B_2O_3$) is added to silica glass to reduce thermal expansion, creating borosilicate glass (Pyrex).

  • Nuclear Energy: Boron readily absorbs neutrons and is used in nuclear reactor control rods.

Carbon, Carbides, and Carbon Oxides

• Amorphous Carbon and Coal

  • Carbonization: Ancient plant material buried under high pressure without air loses hydrogen and oxygen as methane and water, leaving carbon-rich coal.

  • Anthracite: Highest carbon content ($88-93\%$), highest energy yield.

  • Bituminous: High carbon ($60-80\%$) but high sulfur content ($5\%$), contributing to acid rain.

  • Coke: Solid residue from heating coal without air; used in steel production to reduce iron ore.

  • Charcoal / Activated Carbon: High surface area (> 10^3\,m^2/g) due to many voids; used for filtration and decolorizing agents.

  • Soot / Carbon Black: Formed by incomplete combustion. Carbon black strengthens rubber tires ($25\%$ of tire mass).

• Carbides

  • Ionic Carbides: Formed with low-electronegativity metals. Contain the dicarbide/acetylide ion ($C_2^{2-}$).

    • $CaC_2 + 2H_2O \rightarrow Ca(OH)_2 + C_2H_2(g)$ (acetylene production).

  • Covalent Carbides: Silicon carbide ($SiC$) is nearly as hard as diamond. Used as an abrasive and sold as the gemstone substitute "moissanite."

  • Metallic Carbides: Carbon atoms fit into metal lattice holes. Examples include cementite ($Fe_3C$) in steel and tungsten carbide ($WC$).

• Oxides and Carbonates

  • Carbon Monoxide ($CO$): Poisonous, colorless, odorless. Binds to hemoglobin. Used as a reducing agent in industry.

    • Reduction of iron(III) oxide: $Fe_2O_3(s) + 3CO(g) \rightarrow 2Fe(s) + 3CO_2(g)$.

  • Carbon Dioxide ($CO_2$): Soluble in water, stays stable in atmosphere ($0.04\%$ by volume). Sublimes at atmospheric pressure ("dry ice").

  • Carbonates:

    • Washing Soda: $Na_2CO_3 \cdot 10H_2O$.

    • Baking Soda: $NaHCO_3$. When heated: $2NaHCO_3(s) \rightarrow Na_2CO_3(s) + H_2O(l) + CO_2(g)$.

    • Alka-Seltzer: Mixture of $NaHCO_3$, citric acid, and aspirin.

Nitrogen and Phosphorus: Life-Essential Elements

• Nitrogen ($N_2$)

  • $78\%$ of the atmosphere. Obtained by liquifying air and boiling off $N_2$ at $-196\,^{\circ}\text{C}$.

  • Triple bond ($N \equiv N$) makes it exceptionally stable and unreactive.

  • Ammonia ($NH_3$): Produced via the Haber-Bosch process ($N_2 + 3H_2 \rightleftharpoons 2NH_3$) at high pressure and lower temperatures with a catalyst.

  • Hydrazine ($N_2H_4$): Powerful reducing agent; nitrogen analog of hydrogen peroxide.

  • Sodium Azide ($NaN_3$): Decomposes rapidly into sodium and nitrogen gas for automotive airbags.

• Nitrogen Oxides

  • Nitric Oxide ($NO$): Biological messenger for blood pressure, memory, and erections. Formed in atmosphere by lightning: $N_2(g) + O_2(g) \rightarrow 2NO(g)$.

  • Nitrous Oxide ($N_2O$): "Laughing gas," used as an anesthetic and whipped-cream propellant.

  • Nitric Acid ($HNO_3$): Commercially produced by the Ostwald process (Step 1: Catalyst oxidation of ammonia to $NO$; Step 2: Oxidation to $NO_2$; Step 3: Dissolving in water).

• Phosphorus

  • White Phosphorus ($P_4$): Tetrahedral, waxy, highly toxic, spontaneously burns in air due to $60^{\circ}$ strained bond angles.

  • Red Phosphorus: Amorphous chain structure; less reactive and toxic than white phosphorus. Used in match heads.

  • Black Phosphorus: Most stable allotrope; layered like graphite.

  • Phosphoric Acid ($H_3PO_4$): Used in rust removal, soft drinks (tart taste), and fertilizers.

Oxygen and Ozone

• Elemental Oxygen ($O_2$)

  • Discovered by Joseph Priestley in $1774$. Composes $21\%$ of the atmosphere. Produced industrially by air fractionation or electrolysis of water ($2H_2O \rightarrow 2H_2 + O_2$).

• Classification of Oxides (Table 24.6)

  • Regular Oxide ($O^{2-}$): Oxidation state $-2$. Examples: $Li_2O$, $MgO$.

  • Peroxide ($O_2^{2-}$): Oxidation state $-1$. Examples: $Na_2O_2$, $BaO_2$.

  • Superoxide ($O_2^{-}$): Oxidation state $-1/2$. Examples: $RbO_2$, $CsO_2$, $KO_2$.

• Ozone ($O_3$)

  • Toxic, blue, diamagnetic gas. Found in upper atmosphere (UV absorber) and lower atmosphere (pollutant/oxidizer).

  • Safe replacement for chlorine in water purification because its only by-product is $O_2$.

Sulfur: Properties, Sourcing, and Sulfuric Acid

• Elemental Sulfur

  • Occurs naturally as cyclooctasulfur ($S_8$).

  • Frasch Process: Extracts molten sulfur from underground using superheated water and compressed air.

  • Claus Process: Recovers sulfur from hydrogen sulfide ($H_2S$) in natural gas using oxidation.

• Sulfuric Acid ($H_2SO_4$)

  • Most abundantly produced chemical globally. Used as a strong acid, oxidizer, and dehydrating agent ($C_{12}H_{22}O_{11} \xrightarrow{H_2SO_4} 12C + 11H_2O$).

  • Contact Process: Sulfur is burned to $SO_2$, catalyst-oxidized to $SO_3$, and absorbed into acid to form oleum ($H_2S_2O_7$), which then reacts with water to yield $H_2SO_4$.

Halogens: High Electronegativity and Reactivity

• Elemental Trends (Table 24.8)

  • Fluorine: Most electronegative ($4.0$). Small size leads to high lattice energy and weak $F-F$ bond ($159\,\text{kJ/mol}$), making it exceptionally reactive.

  • Chlorine: Used in water disinfection and bleaching ($ClO_2$).

• Interhalogen Compounds ($AB_n$)

  • Compounds between two different halogens.

  • Heptahalide: Only $IF_7$ is known because seven small F atoms can fit around the large I atom.

  • Geometry: Can be predicted using VSEPR (e.g., $IBr_2^{-}$ is linear; $IF_7$ is pentagonal bipyramidal).

  • Applications: $ClF_3$ is used to produce $UF_6$ for uranium isotope separation.

• Hydrofluoric Acid ($HF$)

  • Weak acid in solution but strong enough to etch glass ($SiO_2 + 6HF \rightarrow SiF_6^{2-} + 2H^{+} + 2H_2O$).

  • Extremely dangerous; penetrates tissues and damages bone directly.

Questions & Discussion

• Identification of p-block Elements

  • Question: Which element is a p-block element? (a) Ca (b) S (c) Ag (d) U

  • Response: (b) Sulfur ($S$) is a member of Group $6A$ and is located in the p-block.

• Determining Borane Class

  • Question: Which species is most likely to be an example of a nido-borane?

  • Response: $B_{10}H_{14}$ follows the $B_nH_{n+4}$ formula, identifying it as a nido-borane.

• Carbide Formulas

  • Question: What is the correct formula for potassium carbide?

  • Response: (b) $K_2C_2$ (Potassium acetylide).

• Carbonate Solubility

  • Question: Carbonates of metal ions other than Group $1A$ are insoluble. Which action increases their solubility?

  • Response: (a) Adding acid to the solution; the acid reacts with the carbonate ion to form bicarbonate or $CO_2$.