What Is the Dragos Rule and Why Is It Important?

In chemistry, the Dragos Rule is crucial for understanding why certain molecules—especially hydrides of heavier Group 15 and Group 16 elements—display unexpectedly small bond angles. This concept plays a key role when contrasting molecules like ammonia and phosphine, where the usual rules of hybridisation do not explain observed molecular geometries. Learning about Drago's rule clarifies several anomalies in chemical bonding and is particularly important for students preparing for exams covering chemical structure and bonding topics in classes 11 and 12.

What is Drago's Rule in Chemistry?

Drago's Rule describes special cases in chemical bonding where hybridisation is energetically unfavourable. Instead, bonds are formed using pure p-orbitals, resulting in bond angles close to $90^\circ$. This rule is mostly relevant for larger p-block elements found in the 3rd period and below. The central concepts include:

• The rule applies when the central atom comes from Group 15 or Group 16 and is in the 3rd period or lower (e.g., phosphorus, sulfur).
• The surrounding atom must be small and have low electronegativity (usually hydrogen, with EN ≤ 2.5).
• A lone pair exists on the central atom and remains in a non-hybridised s-orbital, making it stereo-chemically inactive.
• Chemical bonds form through the overlap of pure p-orbitals, not through hybrid orbitals.
• Bond angles approach $90^\circ$ because pure p-orbitals are perpendicular to one another.

Drago's Rule Chemistry Definition

• Drago's rule states: “If a molecule’s central atom is from Group 15 or 16, belongs to the 3rd period or below, and is bonded to substituents with low electronegativity (EN ≤ 2.5), its s-orbital lone pair does not hybridise, and bond formation involves only p-orbitals.”

Drago’s Rule: Applicable Molecules and Examples

Not every p-block compound follows Drago's rule. Only specific compounds fulfill the necessary conditions.

• Phosphine (PH₃), Arsine (AsH₃), Stibine (SbH₃): Central atom is from Group 15, 3rd period or lower, bonded to hydrogen.
• Hydrogen Sulfide (H₂S), Hydrogen Selenide (H₂Se), Hydrogen Telluride (H₂Te): Central atom is from Group 16, 3rd period or below, also bonded to hydrogen.

Common characteristics of these Drago’s molecules:

• Non-hybridised structure (i.e., no $sp^3$ hybridisation for the central atom).
• Very small bond angles (approx. $90^\circ$–$92^\circ$).
• Weak bonds due to poor orbital overlap.

Key Applications and Trends Explained by Drago's Rule

• Explaining Bond Angles: Ammonia ($NH_3$) has a bond angle of $107^\circ$ due to $sp^3$ hybridisation, but phosphine ($PH_3$) has a much smaller angle (around $94^\circ$), explained by Drago's rule.
• Predicting Basicity: Molecules with lone pairs in $sp^3$ orbitals (like $NH_3$) are more basic than those with lone pairs in non-hybridized s-orbitals (like $PH_3$).
• Bond Strength Trend: Down the group, bond overlaps become weaker due to the larger atomic size; thus, bond strength decreases and reducing properties increase ($SbH_3 > AsH_3 > PH_3 > NH_3$).
• Electronic Structure: Unhybridised s-orbital lone pairs are held closer to the nucleus and are less available for reactions such as protonation.

Drago's rule is especially important when studying hydrides and chemical bonding in heavier elements—key topics in atomic theory and chemical reactivity.

Illustrative Example: Phosphine (PH₃) Structure

In $PH_3$, Drago's rule applies perfectly:

• Phosphorus (third period, Group 15) has a non-hybridised s-orbital lone pair.
• Three sigma bonds are formed only by the overlap of p-orbitals with hydrogen's s-orbital, producing a trigonal pyramidal shape with bond angles near $90^\circ$.

$$ \text{PH}_3: \ \text{P}~(3s^2,3p^3) + 3\text{H}~(1s^1) \rightarrow \text{pure}~p\text{-orbital~bonding} $$

Noteworthy Points About Drago's Rule

• Typically applies to heavier p-block hydrides (Drago's molecules).
• Bond angle and chemistry deviate strongly from expectations based on lighter elements.
• Understanding this topic is important for mastering atomic spectra and structural trends in advanced chemistry.

To explore related chemical concepts and comparison, check the guides on bonding vs. structure and periodic properties.

In summary, Drago's Rule clarifies why hydrides of heavier Group 15 and 16 elements (like $PH_3$ and $H_2S$) have bond angles close to $90^\circ$ and show little to no hybridisation. This rule only applies when the central atom is large (3rd period/below), has a lone pair, and is bonded to low-electronegativity elements. Recognising Drago’s rule is essential for accurate predictions about molecular structure, basicity, and reactivity for such compounds. By mastering Drago’s rule, students gain deeper insight into chemical bonding exceptions that challenge straightforward valence bond theory.