p-Block Elements-I — Important Questions

Na₂B₄O₇·10H₂O

Step 1: Borax is the common name for sodium tetraborate decahydrate. Step 2: Its chemical formula is Na₂B₄O₇·10H₂O, meaning it contains 2 sodium ions, 4 boron atoms, 7 oxygen atoms, and 10 molecules of water of crystallisation. Step 3: Borax is also known to contain tetranuclear units [B₄O₅(OH)₄]²⁻ and this prismatic form has 10 water molecules. Step 4: Na₂B₄O₇·5H₂O is the jeweller's borax (octahedral form), and Na₂B₄O₇ is borax glass (anhydrous form).

Alkaline

Step 1: When borax (Na₂B₄O₇) dissolves in hot water, it undergoes hydrolysis. Step 2: The reaction is: Na₂B₄O₇ + 7H₂O → 4H₂BO₃ + 2NaOH. Step 3: This reaction produces boric acid (H₂BO₃), which is a weak acid, and sodium hydroxide (NaOH), which is a strong base. Step 4: Since NaOH is a strong base and boric acid is a very weak acid, the solution is alkaline (basic) in nature. Step 5: This is a classic example of hydrolysis of salt of weak acid and strong base giving an alkaline solution.

Accepts a hydroxyl ion from water instead of donating a proton

Step 1: Boric acid [B(OH)₃] behaves differently from typical acids. Step 2: A typical Brønsted acid donates a proton (H⁺) to water. However, boric acid instead accepts a hydroxyl ion (OH⁻) from water. Step 3: The reaction is: B(OH)₃ + 2H₂O ⇌ H₃O⁺ + [B(OH)₄]⁻. Step 4: Boron is electron-deficient (has an empty p orbital), so it acts as a Lewis acid by accepting OH⁻ from water, indirectly releasing H₃O⁺. Step 5: Because this reaction is incomplete (equilibrium lies to the left), boric acid is a very weak monobasic acid.

Three-centre two-electron bonds (3c-2e)

Step 1: Diborane (B₂H₆) has 12 valence electrons in total (3 from each B + 1 from each of 6 H). Step 2: Four of the B-H bonds are normal terminal bonds — these are regular 2-centre 2-electron (2c-2e) bonds. Step 3: The remaining 4 electrons are used for two bridging B-H-B bonds. Step 4: Each bridging bond involves three atoms (B-H-B) sharing only 2 electrons. This is called a three-centre two-electron (3c-2e) bond, also called a banana bond or half bond. Step 5: These unusual bonds arise because boron is electron-deficient. The boron in diborane is sp³ hybridised.