Group 13 Elements: The Boron Family
Group 13 of the periodic table consists of Boron (B), Aluminium (Al), Gallium (Ga), Indium (In), and Thallium (Tl). The general electronic configuration is $ns^2 np^1$. Boron is a typical non-metal, while the others are metals.
1. Atomic Properties & Trends
Atomic Radius (The Gallium Anomaly)
Normally, atomic radius increases down a group. However, Gallium is smaller than Aluminium.
Reason: d-Block Contraction
Gallium follows the d-block elements. The 10 d-electrons in the inner shell provide poor shielding of the nuclear charge. This increases the effective nuclear charge ($Z_{eff}$), pulling the outer electrons closer.
Trend: $B < Ga < Al < In < Tl$
Ionization Enthalpy ($IE_1$)
The trend is irregular due to shielding effects (d- and f-contraction).
Note: $Tl$ has a very high IE due to Lanthanoid contraction (poor shielding by 4f electrons).
Oxidation States & Inert Pair Effect
Group 13 elements show +3 and +1 oxidation states.
+3 Stability: Decreases down the group ($B > Al > Ga > In > Tl$).
+1 Stability: Increases down the group ($B < Al < Ga < In < Tl$).
Inert Pair Effect
In heavier elements (like Thallium), the $ns^2$ electrons are reluctant to participate in bonding due to poor shielding by d/f electrons. Thus, $Tl^{+1}$ is more stable than $Tl^{+3}$.
Example: $TlCl_3$ is unstable and acts as a strong oxidizing agent, reverting to $TlCl$.
2. Important Compounds
A. Borax ($Na_2B_4O_7 \cdot 10H_2O$)
A white crystalline solid. In water, it forms an alkaline solution.
Borax Bead Test: On heating, borax forms a transparent glassy bead ($NaBO_2 + B_2O_3$). This bead reacts with transition metal salts to give characteristic colored metaborates (e.g., Cobalt gives a blue bead).
B. Orthoboric Acid ($H_3BO_3$)
A white crystalline solid with a soapy touch. It has a layered structure held by hydrogen bonds.
- Acidity: It is NOT a protonic acid (it doesn't release $H^+$ directly). It is a weak Lewis Acid. It accepts $OH^-$ from water and releases a proton.
C. Diborane ($B_2H_6$)
The simplest boron hydride. It is electron-deficient.
- Structure: It contains two types of Hydrogen atoms: 4 Terminal Hydrogens (normal covalent bonds) and 2 Bridging Hydrogens.
- Banana Bond (3c-2e): The bridge bond involving $B-H-B$ is a Three-center Two-electron bond. It is bent, resembling a banana.
- Hybridization: Boron in diborane is $sp^3$ hybridized.
3. Lewis Acid Character of Halides
Boron halides ($BX_3$) are electron-deficient and act as strong Lewis Acids.
Acidity Order: $BI_3 > BBr_3 > BCl_3 > BF_3$.
Why is $BF_3$ the weakest?
Due to effective $p\pi - p\pi$ Back Bonding. The small size of Fluorine ($2p$ orbital) allows effective donation of lone pair electron density back to Boron's empty $2p$ orbital. This satisfies Boron's electron deficiency to some extent. In $BI_3$, the large $5p$ orbital of Iodine cannot overlap effectively with Boron's $2p$ orbital.
4. Aluminium: Amphoteric Nature
Unlike Boron, Aluminium metal and its oxide/hydroxide are Amphoteric (react with both acids and bases).
5. Summary Table
| Element | Common Oxidation States | Oxide Nature |
|---|---|---|
| Boron (B) | +3 | Acidic ($B_2O_3$) |
| Aluminium (Al) | +3 | Amphoteric ($Al_2O_3$) |
| Gallium (Ga) | +3, +1 | Amphoteric |
| Indium (In) | +3, +1 | Basic |
| Thallium (Tl) | +1, +3 | Basic ($Tl_2O$) |
Group 13 Quiz
Test your concepts on the Boron Family. 10 MCQs with explanations.
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