Diagonal Relationships
The striking chemical similarities between Li & Mg, and Be & Al.
In the periodic table, the first element of a group often exhibits properties that differ significantly from its own group members, but surprisingly resemble the properties of the second element in the next group. This cross-group similarity is known as a Diagonal Relationship.
1. The Cause: Ionic Potential / Polarizing Power
Why does Lithium act like Magnesium instead of Sodium? The answer lies in Fajans' Rules and the concept of Ionic Potential ($\phi$), also known as polarizing power.
The Cancellation Effect:
- Moving across a period (left to right): Charge increases and radius decreases. $\phi$ increases significantly.
- Moving down a group (top to bottom): Radius increases significantly. $\phi$ decreases.
- Moving diagonally (down and right): The increase in charge is almost perfectly canceled out by the increase in radius. As a result, diagonally adjacent elements have nearly identical ionic potentials (polarizing power), electronegativity, and similar atomic/ionic radii.
2. Diagonal Relationship: Lithium (Li) & Magnesium (Mg)
Lithium (Group 1, Period 2) and Magnesium (Group 2, Period 3) share several striking chemical similarities that are highly tested in competitive exams.
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Nitride Formation: Unlike other alkali metals, both Li and Mg react directly with nitrogen gas in the air to form stable, ionic nitrides.
$6Li + N_2 \xrightarrow{\Delta} 2Li_3N$
$3Mg + N_2 \xrightarrow{\Delta} Mg_3N_2$ -
Thermal Decomposition of Carbonates: Other alkali metal carbonates are thermally stable. Both $Li_2CO_3$ and $MgCO_3$ are relatively unstable and decompose easily on heating to form oxides and $CO_2$.
$Li_2CO_3 \xrightarrow{\Delta} Li_2O + CO_2 \uparrow$
$MgCO_3 \xrightarrow{\Delta} MgO + CO_2 \uparrow$ - Covalent and Deliquescent Chlorides: Due to their high polarizing power, both $LiCl$ and $MgCl_2$ have significant covalent character. They are soluble in organic solvents (like ethanol) and absorb moisture from the air (deliquescent), crystallizing as hydrates: $\mathbf{LiCl \cdot 2H_2O}$ and $\mathbf{MgCl_2 \cdot 6H_2O}$.
- Reaction with Water: Both metals react very slowly with cold water, unlike the violent reactions of Sodium, Potassium, Calcium, etc.
- Solid Bicarbonates: Neither Li nor Mg form solid bicarbonates; their bicarbonates exist only in aqueous solutions.
3. Diagonal Relationship: Beryllium (Be) & Aluminum (Al)
Beryllium (Group 2, Period 2) and Aluminum (Group 13, Period 3) share an even more pronounced diagonal relationship.
- Amphoteric Nature: While other Group 2 metals are strictly basic, both $Be$ and $Al$ (and their oxides/hydroxides) are amphoteric. They dissolve in both acids and strong alkalis (like $NaOH$) to form complex ions ($[Be(OH)_4]^{2-}$ beryllate, and $[Al(OH)_4]^-$ aluminate) while releasing $H_2$ gas.
- Passivity with Nitric Acid: Both metals are rendered passive (unreactive) by concentrated Nitric Acid ($HNO_3$) due to the instant formation of a tough, impermeable, protective oxide film on their surface.
- Covalent, Polymeric Chlorides: Both $BeCl_2$ and $AlCl_3$ are predominantly covalent. In the vapor state, they both exist as chloro-bridged dimers ($(BeCl_2)_2$ and $Al_2Cl_6$). They both act as strong Lewis Acids and are frequently used as catalysts in Friedel-Crafts reactions.
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Hydrolysis of Carbides: The carbides of both metals ($Be_2C$ and $Al_4C_3$) are classified as methanides. Upon reaction with water (hydrolysis), they both yield Methane ($CH_4$) gas (unlike Calcium carbide, which yields acetylene).
$Be_2C + 4H_2O \rightarrow 2Be(OH)_2 + CH_4 \uparrow$
$Al_4C_3 + 12H_2O \rightarrow 4Al(OH)_3 + 3CH_4 \uparrow$
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