What is diagonal relationship?

Short Answer

Diagonal relationship is the similarity in chemical properties between elements diagonally positioned in the periodic table, usually from top-left to bottom-right in the s- and p-block.

  • Examples include lithium (Li) and magnesium (Mg) or beryllium (Be) and aluminium (Al).
  • These elements show similarities in atomic size, charge density, electronegativity, and chemical reactivity, despite being in different groups.
Detailed Explanation :

Definition of Diagonal Relationship

Diagonal relationship refers to the chemical similarity between elements that are diagonally adjacent in the periodic table.

  • Typically observed in first-period and second-period elements with elements in the next group but one period down.
  • This relationship occurs because certain periodic properties compensate, making their chemical behavior unexpectedly similar.

Factors Leading to Diagonal Relationship

  1. Atomic Size:
    • Atomic size decreases across a period and increases down a group.
    • Diagonal elements have similar atomic radii.
    • Example: Li (152 pm) ≈ Mg (160 pm).
  2. Electronegativity:
    • Similar values → similar tendency to attract electrons in bonding.
  3. Charge Density / Polarizing Power:
    • Small cations with high charge → strong polarizing ability.
    • Li⁺ and Mg²⁺ have similar polarizing effects → form similar covalent compounds.
  4. Ionization Energy and Electron Affinity:
    • Values of diagonal elements are comparable → similar reactivity patterns.
  5. Hydration and Solubility Behavior:
    • Diagonal elements show similar solubility in water, hydroxide formation, and complex formation.

Examples of Diagonal Relationships

  1. Lithium (Li) and Magnesium (Mg):
    • Both form carbonates and nitrates that decompose on heating.
    • Both form acetylides with carbon.
    • Oxides show amphoteric behavior: Li₂O + water → LiOH; MgO + water → Mg(OH)₂.
  2. Beryllium (Be) and Aluminium (Al):
    • Both form amphoteric oxides (BeO, Al₂O₃).
    • Both form tetrahalides (BeCl₂, AlCl₃) that are covalent and soluble in organic solvents.
  3. Boron (B) and Silicon (Si):
    • Form covalent compounds and oxides with acidic behavior.

Reasons for Diagonal Relationship

  1. Compensation of Trends:
    • Atomic size increases down a group → reduces reactivity.
    • Electronegativity increases across a period → increases reactivity.
    • Diagonal elements balance these trends → similar chemical properties.
  2. Similar Bonding Nature:
    • Both elements form covalent compounds rather than ionic compounds.
  3. Polarizing Ability:
    • Cations like Li⁺ and Mg²⁺ strongly polarize anions → similar hydroxide and carbonate behavior.

Significance

  • Explains exceptions to periodic trends.
  • Helps predict chemical behavior of lesser-known elements based on their diagonal counterpart.
  • Useful in understanding compounds formation, solubility, and reactivity across the periodic table.
Conclusion

Diagonal relationship is a phenomenon where elements diagonally placed in the periodic table show similar chemical and physical properties. This occurs due to comparable atomic size, electronegativity, charge density, and bonding behavior, despite being in different groups. Examples such as Li-Mg and Be-Al illustrate how diagonal relationships help explain chemical similarities and deviations from expected periodic trends, providing valuable insight into predicting element behavior.