Group 13 Elements: The Boron Family — Revision Notes

⚡ 30-Second Revision

Elements: — B, Al, Ga, In, Tl
Electronic Configuration: — $ns^2np^1$
Oxidation States: — +3 common; +1 stability increases down group (In, Tl) due to Inert Pair Effect.
Boron: — Non-metal, anomalous behavior, max covalency 4, electron-deficient, forms covalent compounds.
Aluminium: — Metal, amphoteric ($Al_2O_3$, $Al(OH)_3$).
Diborane ($B_2H_6$): — Electron-deficient, 2 terminal $BH_2$ groups, 2 bridging H atoms, 4 (2c-2e) B-H bonds, 2 (3c-2e) B-H-B 'banana bonds'. Boron is $sp^3$ hybridized.
Boric Acid ($H_3BO_3$): — Weak monobasic Lewis acid (accepts $OH^-$ from water), layered structure via H-bonding.
Lewis Acid Strength of $BX_3$: — $BI_3 > BBr_3 > BCl_3 > BF_3$ (due to decreasing back-bonding from F to I).
Atomic Radii Trend: — B < Al > Ga < In < Tl (Ga < Al due to poor d-shielding).
Ionization Enthalpy Trend: — B > Al < Ga > In < Tl (irregular due to d/f shielding effects).

2-Minute Revision

The Group 13 elements, or Boron family (B, Al, Ga, In, Tl), share the $ns^2np^1$ valence configuration. Their most common oxidation state is +3, but the stability of the +1 state increases significantly down the group due to the inert pair effect, making Tl(I) more stable than Tl(III).

Boron is unique: a non-metal, electron-deficient, with a maximum covalency of four, forming covalent compounds. Its hydrides, like **diborane ($B_2H_6$)**, feature unusual 3-center 2-electron 'banana bonds' and $sp^3$ hybridized boron.

**Boric acid ($H_3BO_3$)** is a weak monobasic Lewis acid, accepting $OH^-$ from water. Aluminium is a metal but its oxide and hydroxide are amphoteric, reacting with both acids and bases. Periodic trends for atomic radii and ionization enthalpy are irregular, showing a contraction from Al to Ga and In to Tl, primarily due to the poor shielding of d and f electrons.

The Lewis acid strength of boron trihalides is $BI_3 > BBr_3 > BCl_3 > BF_3$, explained by the effectiveness of $ppi-ppi$ back-bonding.

5-Minute Revision

Group 13 elements, the Boron family, are characterized by their $ns^2np^1$ valence electron configuration. The primary oxidation state is +3, but the inert pair effect causes the +1 oxidation state to become more stable for heavier elements like Indium and Thallium. This effect stems from the poor shielding of the nuclear charge by d and f electrons, leading to the $ns^2$ electrons being tightly held and less available for bonding. For instance, $Tl^+$ is more stable than $Tl^{3+}$.

Boron exhibits significant anomalous behavior due to its small size, high ionization enthalpy, and lack of d-orbitals. It's a non-metal, forms covalent compounds, and is electron-deficient. Its maximum covalency is four. Boron also shows a diagonal relationship with Silicon.

Key compounds include:

Diborane ($B_2H_6$): — An electron-deficient molecule. Its structure involves two $BH_2$ units linked by two bridging hydrogen atoms. It has four terminal 2c-2e B-H bonds and two bridging 3c-2e B-H-B 'banana bonds'. Each boron is $sp^3$ hybridized. Example reaction: $B_2H_6 + 6H_2O \rightarrow 2H_3BO_3 + 6H_2$.
Boric Acid ($H_3BO_3$ or $B(OH)_3$): — A weak, monobasic Lewis acid. It accepts an electron pair from $OH^-$ ions from water, $B(OH)_3 + H_2O \rightleftharpoons [B(OH)_4]^- + H^+$. It has a layered structure held by hydrogen bonds.
Borax ($Na_2B_4O_7 cdot 10H_2O$): — Important mineral, used in borax bead test: $Na_2B_4O_7 xrightarrow{heat} 2NaBO_2 + B_2O_3$ (glassy bead).
Aluminium Oxide ($Al_2O_3$) and Hydroxide ($Al(OH)_3$): — Both are amphoteric, reacting with both acids and bases. E.g., $Al_2O_3 + 6HCl \rightarrow 2AlCl_3 + 3H_2O$ and $Al_2O_3 + 2NaOH + 3H_2O \rightarrow 2Na[Al(OH)_4]$.

Trends: Atomic radii show an anomaly (Al > Ga) due to poor d-electron shielding in Ga. Ionization enthalpy also shows an irregular trend (B > Al < Ga > In < Tl) due to d and f orbital effects. The Lewis acid strength of boron trihalides is $BI_3 > BBr_3 > BCl_3 > BF_3$. This is because effective $ppi-ppi$ back-bonding from halogen to boron decreases from F to I, making boron more electron-deficient in $BI_3$.

Prelims Revision Notes

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General Electronic Configuration: — $ns^2np^1$.
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Oxidation States: — +3 is common. Stability of +1 increases down the group (Ga, In, Tl) due to Inert Pair Effect (poor shielding by d/f electrons). Tl(I) is more stable than Tl(III).
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Atomic Radii: — B (88 pm) < Al (143 pm) > Ga (135 pm) < In (167 pm) < Tl (170 pm). Anomaly (Ga < Al) due to poor shielding of 3d electrons in Ga.
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Ionization Enthalpy: — B > Al < Ga > In < Tl. Irregular trend due to poor shielding of d/f electrons and increased effective nuclear charge.
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Electronegativity: — Decreases from B to Al, then slightly increases (Al < Ga $approx$ In < Tl).
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Anomalous Behavior of Boron:

* Non-metal (others are metals). * Forms covalent compounds (others form ionic/covalent). * Max covalency 4 (no d-orbitals). * Electron-deficient compounds (Lewis acids). * Forms complex hydrides (boranes). * Diagonal relationship with Silicon.

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**Diborane ($B_2H_6$):**

* Electron-deficient, dimeric structure. * 4 terminal B-H bonds (2c-2e). * 2 bridging B-H-B bonds (3c-2e, 'banana bonds'). * Boron is $sp^3$ hybridized. * Reacts with water: $B_2H_6 + 6H_2O \rightarrow 2H_3BO_3 + 6H_2$.

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

* Weak monobasic Lewis acid (accepts $OH^-$ from water, not a protic acid). * Layered structure with planar $BO_3$ units linked by H-bonds.

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**Borax ($Na_2B_4O_7 cdot 10H_2O$):**

* Contains $[B_4O_5(OH)_4]^{2-}$ units. * Borax bead test: $Na_2B_4O_7 xrightarrow{heat} 2NaBO_2 + B_2O_3$ (glassy bead).

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Aluminium:

* Amphoteric nature: $Al_2O_3$ and $Al(OH)_3$ react with both acids and bases. * Acid: $Al_2O_3 + 6HCl \rightarrow 2AlCl_3 + 3H_2O$ * Base: $Al_2O_3 + 2NaOH + 3H_2O \rightarrow 2Na[Al(OH)_4]$ * **Aluminium Chloride ($AlCl_3$):** Anhydrous $AlCl_3$ is covalent, exists as dimer ($Al_2Cl_6$) in vapor, polymer in solid. Strong Lewis acid. Hydrolyzes in moist air.

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Lewis Acid Strength of Boron Trihalides ($BX_3$): — $BI_3 > BBr_3 > BCl_3 > BF_3$. Explained by decreasing effectiveness of $ppi-ppi$ back-bonding from F to I, which reduces electron deficiency on Boron.

Vyyuha Quick Recall

Bright Aliens Gave Indian Tigers 3 electrons. (B, Al, Ga, In, Tl, and their +3 common oxidation state).

For Diborane's bonds: Banana Bonds Have 3 Centers 2 Electrons. (3c-2e bonds).