Comprehensive Study Guide: p-Block Elements (Group 13 and 14)

Kenneth Wade and Borane Clusters

• Kenneth Wade (1932–2014): A prominent British chemist and professor emeritus at Durham University.

• Key Contribution: Developed a method for predicting the shapes of borane clusters.

• Wade’s Rules: These rules are used to rationalize the shape of borane clusters. The logic involves calculating the total number of skeletal electron pairs ($SEP$) available for cluster bonding.

• Honors and Awards:

  • Granted FRS award from the Royal Society, London in $1989$.

  • Received the Tilden prize award in $1999$ from the Royal Society of Chemistry for advances in chemistry.

General Introduction to p-Block Elements

• Definition: Elements in which the last electron enters the 'p' orbital constitute the p-block elements.

• Position: They occupy groups $13$ to $18$ in the modern periodic table.

• Diversity: This block contains metals, nonmetals, and metalloids. Nonmetallic elements within this group exhibit more varied properties compared to the metals.

• Primary Group Members:

  • Group $13$: Boron ($B$)

  • Group $14$: Carbon ($C$)

  • Group $15$: Nitrogen ($N$)

  • Group $16$: Oxygen ($O$)

  • Group $17$: Fluorine ($F$)

  • Group $18$: Helium ($He$) / Inert gases

• Biological and Technological Importance:

  • Molecular oxygen is essential for the living system.

  • Aluminium is the most abundant metal, used from household utensils to aircraft parts.

  • Silicon and germanium are semiconductors central to modern electronics.

General Trends in Properties

• Electronic Configuration:

  • General outer shell configuration: $ns^2, np^{1-6}$.

  • Group $18$ (Inert gases) have completely filled p-orbitals ($ns^2 np^6$), making them stable and least reactive.

• Oxidation States:

  • The highest oxidation state (group oxidation state) equals the total number of valence electrons ($s + p$).

  • Unlike s-block elements, p-block elements show both positive and negative oxidation states.

  • Halogens commonly show $-1$ oxidation state to achieve a stable halide ion.

  • Pnictogens (Group $15$) and Chalcogens (Group $16$) also display negative oxidation states.

• Metallic Nature:

  • Tied to the ability to form cations (electropositivity), which depends on ionization energy.

  • Trend: As you descend a group, ionization energy decreases, and metallic character increases.

  • Distribution: Lower left part of the p-block consists of metals; upper right part consists of nonmetals.

• Ionization Enthalpy:

  • Generally decreases down a group due to increased atomic radius.

  • Exceptions (Group 13): There is only a marginal difference between Aluminium and Thallium due to the poor shielding effect of inner d and f-electrons. This increases the effective nuclear charge on valence electrons.

  • Successive Groups: Ionization enthalpy increases as you move from one group to the next (e.g., Group 14 > 13).

• Electronegativity:

  • Trend in Group 13: Decreases from Boron to Aluminium, then increases marginally for Gallium, after which there is no appreciable change.

  • Other Groups: Generally decreases down the group, correlating with atomic radius.

Anomalous Properties of First Elements

The first member of each group (B, C, N, O, F) differs significantly from subsequent members due to:

1. Small size.

2. High ionization enthalpy and high electronegativity.

3. Absence of d orbitals in their valence shell.

• Boron Specifics: Boron is a metalloid, while others in Group $13$ are reactive metals. It exhibits a diagonal relationship with Silicon.

  • Both Boron and Silicon form acidic oxides.

  • Both form covalent hydrides that hydrolyze easily.

  • Halides (except $BF_3$) are readily hydrolyzed.

• Carbon Specifics: Only carbon is strictly nonmetallic in its group. It can form multiple bonds ($C=C$, $C=O$) and exhibits high catenation.

• Nitrogen and Oxygen: Both exist as diatomic gases ($N_2$, $O_2$) and can form hydrogen bonds due to high electronegativity.

• Fluorine: The most electronegative element. It shows only $-1$ oxidation state and is the strongest oxidizing agent among halogens.

Inert Pair Effect and Allotropism

• Inert Pair Effect: In heavier post-transition elements (Groups $13$ to $16$), the outer s-electrons ($ns^2$) show reluctance to bond. This results in stable oxidation states that are two units less than the group oxidation state.

  • Example: In Group $13$, $Al^{+3}$ is more stable than $Al^{+1}$, but $Tl^{+1}$ is more stable than $Tl^{+3}$. $TlCl_3$ is unstable and disproportionates to $TlCl$ and $Cl_2$.

• Allotropism: The existence of an element in more than one crystalline or molecular form in the same physical state (Greek: allos = another, trope = change).

Group 13: The Boron Group

• Occurrence:

  • Boron: Found as Borax ($Na_2[B_4O_5(OH)_4] · 8H_2O$) and Kernite ($Na_2[B_4O_5(OH)_4] · 2H_2O$).

  • Aluminium: Most abundant metal. Chief ore is Bauxite ($Al_2O_3 · 2H_2O$).

• Chemical Properties of Boron:

  • Borides: Forming $M_x B_y$ type compounds. Example: $Cr + nB \rightarrow CrB_n$ ($1500\,K$).

  • Hydrides: Does not react directly with $H_2$. Forms boranes. Diborane ($B_2H_6$) is prepared via $2BF_3 + 6NaH \rightarrow B_2H_6 + 6NaF$ at $450\,K$.

  • Boron Nitride ($BN$): Prepared by burning Boron with dinitrogen: $2B + N_2 \xrightarrow{\Delta} 2BN$.

  • Oxides: $4B + 3O_2 \rightarrow 2B_2O_3$ ($900\,K$).

  • Acids/Alkali: Reacts with oxidizing acids ($H_2SO_4$, $HNO_3$) to form Boric acid ($H_3BO_3$). Reacts with fused $NaOH$ to form Sodium borate ($2Na_3BO_3$).

• Uses of Boron:

  • $^{10}B_5$ isotope is a neutron absorber used as a moderator in nuclear reactors.

  • Amorphous boron is a rocket fuel igniter.

  • Boric oxide is used in Pyrex glass manufacture.

Boron Compounds

Borax ($Na_2B_4O_7 · 10H_2O$)

• Forms: Prismatic ($10H_2O$), Octahedral/Jeweller’s borax ($5H_2O$), and Borax glass ($Na_2B_4O_7$).

• Reaction with water: Basic solution. $Na_2B_4O_7 + 7H_2O \rightarrow 4H_3BO_3 + 2NaOH$.

• Borax Bead Test: Heating borax results in a transparent bead ($NaBO_2 + B_2O_3$).

• Uses: Identification of colored metal ions, manufacture of optical glass, flux in metallurgy.

Boric Acid ($H_3BO_3$ or $B(OH)_3$)

• Nature: Monobasic acid; it is a Lewis acid that accepts $OH^-$ rather than donating $H^+$.

• Action of Heat:

  • $373\,K \rightarrow HBO_2$ (Metaboric acid).

  • $413\,K \rightarrow H_2B_4O_7$ (Tetraboric acid).

  • Red hot $\rightarrow B_2O_3$ (Boric anhydride).

• Ethyl Alcohol Test: Forms trialkylborate ester which burns with a green edged flame. This is the standard test for borates.

• Structure: Two-dimensional layered structure with $[BO_3]^{3-}$ units linked by hydrogen bonds.

Diborane ($B_2H_6$)

• Structure: Has eight $B-H$ bonds but only $12$ valence electrons.

  • Four terminal bonds: normal $2c-2e$ bonds.

  • Two bridge bonds ($B-H-B$): three-centre two-electron bonds (3c-2e), also called "banana bonds."

  • Boron is $sp^3$ hybridized.

• Reactions:

  • With Water: $B_2H_6 + 6H_2O \rightarrow 2H_3BO_3 + 6H_2$.

  • Hydroboration: Addition to alkenes/alkynes in ether at room temperature.

  • With Ammonia: Low temp gives diboranediammonate; high temp in closed vessel gives Borazole ($B_3N_3H_6$), known as "Inorganic Benzene."

Aluminium Chloride and Alums

• AlCl3 Prep: McAfee process involves heating Alumina and Coke in Chlorine: $Al_2O_3 + 3C + 3Cl_2 \rightarrow 2AlCl_3 + 3CO_2$.

• AlCl3 Nature: Lewis Acid; used as a catalyst in Friedel-Crafts reactions.

• Alums: Double salts with formula $M'_2SO_4 · M''_{2}(SO_4)_3 · 24H_2O$.

  • Potash Alum: $K_2SO_4 · Al_2(SO_4)_3 · 24H_2O$.

  • Burnt Alum: Potash alum loses water of hydration at $475\,K$ and swells.

  • Uses: Water purification, styptic agent to stop bleeding, leather tanning.

Group 14: The Carbon Group

• Catenation: Ability to form chains of atoms. Requires valence $\ge 2$, ability to bond with self, strong self-bonds, and kinetic inertness. Carbon is the most effective element at catenation (C >> Si > Ge ≈ Sn > Pb).

• Allotropes of Carbon:

  • Graphite: Stable at normal temp/pressure. $sp^2$ hybridized, hexagonal sheets, distance between sheets is $3.40\,Å$. Conducts electricity due to delocalized $\pi$ electrons.

  • Diamond: Hardest element. $sp^3$ hybridized, tetrahedral arrangement ($C-C$ bond length $1.54\,Å$). Electrical insulator.

  • Fullerenes (C60): Soccer ball structure ("Buckyballs"). Contains $20$ six-membered and $12$ five-membered rings. $sp^2$ hybridized.

  • Carbon Nanotubes: Graphite-like tubes with fullerene ends. Stronger than steel.

  • Graphene: Single planar sheet of $sp^2$ carbon atoms in honeycomb lattice.

Carbon Oxides

Carbon Monoxide ($CO$)

• Prep: Limited oxygen supply or methanoic acid + conc. $H_2SO_4$.

• Industrial: Produced as "Producer Gas" ($CO + N_2$) or "Water Gas" ($CO + H_2$).

• Reactions:

  • With Chlorine: Forms Phosgene ($COCl_2$), a poisonous gas.

  • Fischer-Tropsch Synthesis: Reacts with $H_2$ (< 50\text{ atm}, metal catalyst, $500-700\,K$) to yield various hydrocarbons.

• Structure: Linear, bond distance $1.128\,Å$.

Carbon Dioxide ($CO_2$)

• Prep: Burning coke or calcination of lime ($CaCO_3 \rightarrow CaO + CO_2$).

• Properties: Very stable. At $3100\,K$, only $76\%$ decomposes. Acts as a weak acid in water forming carbonic acid ($H_2CO_3$).

• Structure: Linear, involves $3c-4e$ bonding.

Silicon Compounds

• Silicon Tetrachloride ($SiCl_4$): Colorless fuming liquid ($freezes at -70\,^∘C$). Used to produce semiconducting silicon.

• Silicones (Polysiloxanes): Organosilicon polymers $(R_2SiO)_n$.

  • Prepared from dialkyldichlorosilanes ($R_2SiCl_2$).

  • Properties: Water-repellent, thermal and electrical insulators, stable viscosity.

• Silicates: Minerals containing $[SiO_4]^{4-}$ tetrahedral units.

  • Ortho silicates: Discrete units (e.g., Phenacite $Be_2SiO_4$, Olivine $(Fe/Mg)_2SiO_4$).

  • Pyro silicates: Two units share one corner oxygen ($[Si_2O_7]^{6-}$).

  • Cyclic silicates: Three or more units in a ring (e.g., Beryl).

  • Inosilicates: Chain (Pyroxenes) or Double Chain (Amphiboles like Asbestos).

  • Sheet silicates: Units share three oxygen atoms (e.g., Talc, Mica).

  • Three-dimensional silicates: All four oxygen atoms shared (e.g., Quartz).

• Zeolites: Microporous three-dimensional crystalline solids. Hydrated sodium alumino silicates. Act as molecular sieves and are used to remove permanent hardness from water.

Medical Application: Boron Neutron Capture Therapy (BNCT)

• Principle: Uses the high affinity of the $^{10}B$ isotope for thermal neutrons.

• Process: Boron compounds are injected into patients with brain tumors. Upon irradiation with thermal neutrons, the boron captures a neutron and releases a high-energy alpha particle ($\alpha$) and a lithium ($Li$) particle.

• Targeting: The alpha particle causes localized damage to tumor cells while sparing healthy tissue. Used for tumors of the brain, head, neck, breast, prostate, bladder, and liver.