Comprehensive Study Notes on p-Block Elements: Groups 13 and 14

Group 13 Elements: Physical and Chemical Properties

• Density Trends: On moving down the group, the density of Group 13 elements increases. This occurs because the extent of increase in atomic mass is greater than the extent of increase in atomic volume.

• Atomic Mass and Effective Nuclear Charge ($Z_{eff}$): Moving from $B$ to $Tl$, both atomic mass and the number of inner $d$- and $f$-subshell electrons increase. Due to the poor shielding effect of $d$- and $f$-electrons, the effective nuclear charge increases from $B$ to $Tl$. Consequently, the atomic size from $B$ to $Tl$ does not increase significantly.

• Melting and Boiling Points:

  • Group 13 elements do not show a regular trend in melting points.

  • Melting points first decrease from $B$ to $Ga$ and then increase from $Ga$ to $Tl$.

  • Boron ($B$): Boron has a very high melting point ($2453\,K$ ) because it possesses an unusual covalent polymeric crystal structure consisting of icosahedral units. In these units, $B$-atoms are at all $12$ vertices, and each atom is bonded to five equidistant neighbors by strong attractive forces.

  • Gallium ($Ga$): Gallium has an exceptionally low melting point ($303\,K$) because its crystal structure consists of discrete $Ga_2$ molecules.

  • Boiling Points: The boiling points of these elements decrease regularly down the group.

  • Gallium's Liquid Range: Gallium remains in the liquid state over a vast range of temperatures (from $303\,K$ up to $2676\,K$). No other metal can compare with this range. Additionally, molten gallium expands upon solidification. Due to this high liquid range and low vapor pressure even when heated high, gallium is used in high-temperature thermometers.

• Electropositivity and Electronegativity:

  • Group 13 elements are more electronegative than Group 1 (alkali metals) and Group 2 (alkaline earth metals).

  • Electropositivity first decreases from $B$ to $Al$ and then increases gradually down the group.

  • Electronegativity decreases considerably from $B$ ($2.0$) to $Al$ ($1.5$) due to a significant increase in atomic size and lower attractive force on valence electrons. From $Al$ to $Tl$, the electronegativity increases slightly because the effective nuclear charge increases due to the poor shielding of inner $d$- and $f$-orbitals.

Stability and Oxidation States of Group 13

• Oxidation States: The elements exhibit $+1$ and $+3$ oxidation states.

• Inert Pair Effect: As one moves down the group, the stability of the $+3$ oxidation state decreases, while the stability of the $+1$ oxidation state increases. This is due to the "inert pair effect," where the inner $s$-electrons ($ns^2$) become increasingly reluctant to participate in bonding.

  • Stability of $+3$ state: $Al > Ga > In > Tl$.

  • Stability of $+1$ state: $Ga < In < Tl$.

  • Oxidizing Nature: Because $Tl^{3+}$ is less stable than $Tl^+$, the $Tl^{3+}$ ion acts as a strong oxidizing agent in solution.

Compounds of Boron and Aluminum

• Boron Halides and Back Bonding:

  • In $BF_3$, p\text{\pi}-p\text{\pi} back bonding occurs. The vacant $2p$-orbital of Boron overlaps with the filled $2p$-orbital of Fluorine. This increases electron density on Boron and reduces its Lewis acid strength.

  • The extent of back bonding depends on the size of the halogen's orbital. As the size increases ($3p$ for $Cl$, $4p$ for $Br$, $5p$ for $I$), the overlap with Boron's $2p$ orbital becomes less effective.

  • Lewis Acid Strength Order: $BF_3 < BCl_3 < BBr_3 < BI_3$.

• Aluminum Halides:

  • In the vapor state or inert solvents (like benzene), aluminum halides exist as dimers, such as $Al_2Cl_6$.

  • In the monomer $AlCl_3$, Aluminum has only six electrons. In the dimer, it completes its octet by accepting a lone pair from a Chlorine atom of another $AlCl_3$ molecule.

  • At very high temperatures, $Al_2Cl_6$ dissociates back into $AlCl_3$ monomers.

  • In polar solvents like water, the dimer dissociates due to high hydration energy, forming hydrated $[Al(H_2O)_6]^{3+}$ ions and $Cl^-$ ions. Anhydrous $AlCl_3$ is covalent, while hydrated aluminum chloride is ionic.

• Boron Anomalies: Boron halides exist only as monomers because the Boron atom is too small to accommodate four large halogen atoms (except in specific complex ions).

• Boron Carbide ($B_4C$): Known as one of the hardest compounds of boron, used as an abrasive for polishing and grinding.

• Specific Uses:

  • Boron: Rocket fuels (high energy/mass ratio), mild antiseptic (orthoboric acid $H_3BO_3$), heat-resistant glass (Pyrex), and semiconductors.

  • Alumina ($Al_2O_3$): Extraction of aluminum, catalyst, preparation of potash alum [$K_2SO_4 \times Al_2(SO_4)_3 \times 24H_2O$], and making precious stones like sapphire and ruby.

  • Potash Alum: Used to stop bleeding from small cuts.

Group 14 Elements: Electronic Configuration and Catenation

• General Electronic Configuration: The outermost shell configuration is $ns^2 np^2$.

• Ionization Enthalpy: Correct order of first ionization enthalpy is $C > Si > Ge > Sn < Pb$. Carbon has the highest value.

• Catenation: This is the property of an element to form long chains or rings by bonding with itself. Carbon exhibits the highest tendency for catenation due to its small size and high bond enthalpy.

  • Catenation Order: $C \text{ >> } Si > Ge \thicksim Sn$.

• Oxidation States: Common oxidation states are $+2$ and $+4$. For Carbon and Silicon, $+4$ is most stable. Due to the inert pair effect, the stability of the $+2$ state increases down the group, making it the most stable state for Lead ($Pb$).

Allotropic Forms of Carbon

• Crystalline Allotropes:

  1. Diamond: $sp^3$ hybridized, extremely hard, non-conductor, highest thermal conductivity of any known material.

  2. Graphite: $sp^2$ hybridized, planar layers held by van der Waals forces, contains mobile electrons making it a good electrical conductor. Also known as "Black Lead."

  3. Fullerene ($C_{60}$): Spherical molecule (Buckminsterfullerene) containing $20$ hexagons and $12$ pentagons. It is considered zero-dimensional.

  4. Carbon Nanotubes: One-dimensional structures; the building block is graphene.

• Amorphous Allotropes: Charcoal, soot (lamp black), coke, and gas carbon.

  • Lamp Black: The most pure amorphous form, used in printing ink and shoe polish.

  • Charcoal: Used for decolourising sugar (adsorbs impurities) and in gas masks.

  • Coke: Residue left after destructive distillation of coal, used as fuel and a reducing agent.

Oxides and Compounds of Group 14

• Carbon Monoxide ($CO$):

  • Neutral oxide, burns with a blue flame.

  • Strong reducing agent (used in metallurgy).

  • Highly poisonous because it forms a complex with hemoglobin that is much stronger than the oxygen-hemoglobin complex, preventing oxygen transport.

• Carbon Dioxide ($CO_2$):

  • Linear molecule, $C$ is $sp$-hybridized, $O$ is $sp^2$-hybridized.

  • Non-polar due to symmetry.

  • Soluble in water to form carbonic acid ($H_2CO_3$).

  • Supercritical $CO_2$: Used as a solvent for extracting organic compounds.

  • Dry Ice: Solid $CO_2$, also known as "dry kold."

• Silicon Compounds:

  • Silicates: Basic structural unit is the $SiO_4^{4-}$ tetrahedron.

    • Chain Silicates: Share two oxygen atoms per tetrahedron; formula $(SiO_3)_{2n}^{n-}$.

    • Amphiboles: Double chain silicates where two chains are linked by oxygen atoms (e.g., asbestos like crocidolite and tremolite).

    • Sheet Silicates: Share three oxygen atoms per tetrahedron; formula $(Si_2O_5)_{2n}^{n-}$ (e.g., kaolinite, talc).

    • Three-dimensional Silicates: All four oxygen atoms are shared (e.g., quartz, feldspar).

  • Zeolites: Three-dimensional aluminosilicates with open channels used as molecular sieves. ZSM-5 is a zeolite used to convert alcohol directly into gasoline.

  • Silicones: Synthetic organosilicon polymers containing repeated $R_2SiO$ units. They are hydrophobic (water-repellent) and used as electrical insulators.

Questions & Discussion

• Question: Why does $CCl_4$ not undergo hydrolysis while $SiCl_4$ does?

  • Response: Carbon has no vacant $d$-orbitals in its valence shell and cannot expand its coordination number beyond $4$. Silicon has vacant $d$-orbitals and can accept a lone pair of electrons from water to initiate hydrolysis.

• Question: What is the correct order of stability for the $+3$ oxidation state in Group 13?

  • Response: $Al > Ga > In > Tl$.

• Question: Which element expands on solidification?

  • Response: Gallium ($Ga$).

• Question: What is "Inorganic Benzene"?

  • Response: Borazine ($B_3N_3H_6$). It has a structure and number of electrons similar to benzene.

• Question: Which gas is used in fire extinguishers?

  • Response: $CO_2$. It is produced in extinguishers by the reaction of sodium bicarbonate ($NaHCO_3$) with dilute sulphuric acid ($H_2SO_4$).

• Question: Why does anhydrous $AlCl_3$ fume in air?

  • Response: It undergoes partial hydrolysis with moisture in the air to release hydrogen chloride ($HCl$) gas.

• Question: What is an alloy of copper, zinc, and nickel?

  • Response: German silver (Note: It contains no silver).

• Question: What is the result of Napoleon's 1812 campaign regarding tin?

  • Response: In extremely cold temperatures, silvery white tin buttons underwent a change in crystal structure (tin pest), turning into a brittle grey powder.

• Question: What is used to make high-temperature thermometers?

  • Response: Gallium, because of its very high boiling point and extremely low melting point.

• Question: Which compound is used in cosmetic surgery?

  • Response: Silicones.

• Question: What happens to lime water when $CO_2$ is passed through it?

  • Response: It turns milky due to the formation of insoluble calcium carbonate ($CaCO_3$). On passing excess $CO_2$, the milkiness disappears as soluble calcium bicarbonate [$Ca(HCO_3)_2$] forms.

• Question: What is "Sugar of Lead"?

  • Response: Lead acetate, $Pb(CH_3COO)_2$.