Diagonal Relationship
Why Boron behaves more like Silicon than Aluminum.
In the periodic table, the first element of a group often shows a strong resemblance to the second element of the next higher group. Boron (Group 13, Period 2) exhibits a striking diagonal relationship with Silicon (Group 14, Period 3), meaning they share physical and chemical properties that Boron does NOT share with Aluminum.
1. The Root Cause
Why do elements placed diagonally apart behave similarly? It comes down to a balance of opposing periodic trends.
- Similar Electronegativity: As you move right across a period, electronegativity increases. As you move down a group, it decreases. Moving diagonally cancels these out. Boron's electronegativity ($2.0$) is very close to Silicon's ($1.8$).
- Similar Charge-to-Size Ratio (Polarizing Power): Boron is small with a $+3$ charge state; Silicon is larger but has a $+4$ charge state. Their resulting polarizing power (ability to distort electron clouds) is nearly identical, causing them to form primarily covalent bonds.
2. Physical Similarities
Because of their high ionization energies and similar electronegativities, both Boron and Silicon are distinct non-metals/metalloids.
Unlike Aluminum, which is a soft metal with a metallic lattice, both Boron and Silicon form extremely hard, high-melting, giant 3-dimensional covalent network lattices.
Furthermore, both elements behave as semiconductors, whereas Aluminum is a highly efficient electrical conductor.
3. Chemical Similarities: Oxides and Acids
The chemical reactions of Boron and Silicon are remarkably parallel, specifically regarding oxygen and water.
A. Acidic Oxides:
Both form exclusively acidic oxides: $B_2O_3$ (Boric oxide) and $SiO_2$ (Silica). (Recall: Aluminum's oxide, $Al_2O_3$, is amphoteric).
Both oxides dissolve in strong alkalis to form salts:
$SiO_2 + 2NaOH \rightarrow Na_2SiO_3 \text{ (Sodium Silicate)} + H_2O$
B. Formation of Weak Acids:
Both elements form weak acids when their oxides are hydrated: Orthoboric acid ($H_3BO_3$) and Orthosilicic acid ($H_4SiO_4$).
4. Chemical Similarities: Hydrides and Chlorides
Their interactions with Hydrogen and Halogens further cement their diagonal relationship.
A. Volatile Covalent Hydrides:
Both Boron and Silicon form a series of volatile, covalent, molecular hydrides known as Boranes (e.g., $B_2H_6$, $B_4H_{10}$) and Silanes (e.g., $SiH_4$, $Si_2H_6$). Both types of hydrides are highly reactive and catch fire spontaneously upon exposure to air.
B. Hydrolysis of Chlorides:
Both form liquid, covalent chlorides ($BCl_3$ and $SiCl_4$) that hydrolyze completely and rapidly in water to yield their respective weak acids and $HCl$ fumes.
$SiCl_4 + 4H_2O \rightarrow \mathbf{H_4SiO_4} + 4HCl$
(Note: Carbon tetrachloride, $CCl_4$, does not hydrolyze at all, which is the Carbon/Silicon anomaly we studied previously).
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