Diagonal Relationship & Anomalous Behaviour
Periodicity of Elements | Classification of Elements Class 11
1. Anomalous Behaviour of Second Period Elements
The first element of each group (Li, Be, B, C, N, O, F) differs significantly from the rest of the members of the same group. This is called anomalous behaviour.
Reasons for Anomaly:
- Smallest Size: They have the smallest atomic and ionic radii in their respective groups.
- High Electronegativity: They are the most electronegative elements in their groups.
- High Ionization Enthalpy: They require the most energy to remove electrons.
- Absence of d-orbitals: The valence shell ($n=2$) has no d-orbitals, restricting their maximum covalency to 4. Heavier elements can expand their octet.
Examples of Anomaly:
- Covalency: Boron forms $[BF_4]^-$, while Aluminum can form $[AlF_6]^{3-}$ (due to available d-orbitals). Nitrogen cannot form $NCl_5$, but Phosphorus can form $PCl_5$.
- Multiple Bonding: C, N, and O form $p\pi-p\pi$ multiple bonds ($C=C, C=O, N\equiv N$). Heavier elements prefer $d\pi-p\pi$ or single bonds due to poor orbital overlap.
2. Diagonal Relationship
Certain elements of the second period show similarities in properties with the elements of the third period placed diagonally to them. This phenomenon is called Diagonal Relationship.
The Diagonal Pairs:
- Lithium (Group 1) $\longleftrightarrow$ Magnesium (Group 2)
- Beryllium (Group 2) $\longleftrightarrow$ Aluminum (Group 13)
- Boron (Group 13) $\longleftrightarrow$ Silicon (Group 14)
Cause: Similar Ionic Potential ($\phi$)
Ionic Potential = $\frac{\text{Charge on Cation}}{\text{Ionic Radius}}$.
On moving right across a period, charge increases and size decreases (Polarizing power increases). On moving down a group, size increases (Polarizing power decreases). Diagonally, these effects cancel out, leading to similar polarizing power.
Ionic Potential = $\frac{\text{Charge on Cation}}{\text{Ionic Radius}}$.
On moving right across a period, charge increases and size decreases (Polarizing power increases). On moving down a group, size increases (Polarizing power decreases). Diagonally, these effects cancel out, leading to similar polarizing power.
3. Key Examples of Similarity
Lithium (Li) and Magnesium (Mg)
- Both form Nitrides directly with Nitrogen: $6Li + N_2 \rightarrow 2Li_3N$ and $3Mg + N_2 \rightarrow Mg_3N_2$. (Other alkali metals do not).
- Carbonates decompose on heating to give oxide and $CO_2$.
- Both are harder and have higher melting points than their group members.
Beryllium (Be) and Aluminum (Al)
- Both form Amphoteric Oxides ($BeO$ and $Al_2O_3$).
- Both chlorides ($BeCl_2$ and $AlCl_3$) act as Lewis acids and have a polymeric structure with chlorine bridges.
- Both are rendered passive by concentrated Nitric Acid ($HNO_3$).
Practice Quiz
Test your knowledge on Periodic Anomalies.
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