Question

Which element from following exhibits the highest number of allotropes?

• A. O
• B. S
• C. Se
• D. Te

Hint:

Chemistry Tip: Sulfur's remarkable allotropy is a direct consequence of its high catenation tendency, driven by the strong S-S single bond energy ($266 \text{ kJ/mol}$) which is significantly stronger than the O-O single bond ($142 \text{ kJ/mol}$).

Answer 1

The Correct Option is B

Concept: Chemistry (p-Block Elements) - Allotropy in Group 16 Elements.

Step 1: Define the concept of allotropy. Allotropy is the phenomenon where a single chemical element can exist in two or more structurally different forms in the same physical state. These different forms, called allotropes, vary in their bonding arrangements.

Step 2: Evaluate the allotropes of Oxygen (O). Oxygen is a period 2 element. It primarily exists in only two distinct allotropic forms at standard conditions: dioxygen ($O_2$) and ozone ($O_3$).

Step 3: Evaluate the allotropes of Selenium (Se) and Tellurium (Te). As we move down the group, the tendency for catenation generally decreases due to weakening single bond strengths (except the O-O vs S-S anomaly). Selenium exhibits a few allotropes, such as red selenium (contains $Se_8$ rings) and gray/metallic selenium (helical polymer chains). Tellurium exhibits even fewer, essentially having only one stable crystalline form.

Step 4: Evaluate the allotropes of Sulfur (S). Sulfur has an exceptionally strong S-S single bond, which grants it an outstanding ability to undergo catenation (forming chains and rings). Because of this, sulfur forms a vast array of allotropes.

Step 5: Compare the quantities to establish the maximum. Sulfur exhibits numerous well-characterized forms including rhombic sulfur ($\alpha$-sulfur), monoclinic sulfur ($\beta$-sulfur), various cyclo-sulfur rings ranging from $S_6$ to $S_{20}$ (like Engel's sulfur $S_6$), and plastic sulfur (polymeric chains).
Comparing this immense structural diversity to O, Se, and Te, sulfur clearly possesses the highest number of allotropes. $$ \therefore \text{Sulfur (S) exhibits the highest number of allotropes.} $$