The Mistake Bank
P-Block Elements (Groups 15, 16, 17, 18)
From Nitrogen's bonding to Xenon's reactions—handle with care.
Nitrogen Pentahalide ($NCl_5$)
Group 15 - ValencyScenario: Why does $PCl_5$ exist but $NCl_5$ does not?
Student thinks: "Nitrogen is too small to hold 5 chlorines."
Or: "Nitrogen is not electronegative enough."
(Size is a factor, but not the primary reason for valency limit.)
Absence of d-orbitals!
Nitrogen is in the 2nd period ($2s^2 2p^3$). It has NO 2d orbitals.
It cannot expand its covalency beyond 4 (using one 2s and three 2p).
Phosphorus ($3s, 3p, 3d$) can expand its octet.
Bond Strength: $F_2$ vs $Cl_2$
Group 17 - HalogensScenario: Which bond is stronger: $F-F$ or $Cl-Cl$?
Student applies size trend: "Smaller bond length = Stronger bond."
Answer: "$F-F$ is stronger."
(The Fluorine Anomaly strikes!)
Electron-Electron Repulsion!
Fluorine is so small that the lone pairs on the two F atoms repel each other strongly.
This repulsion weakens the bond significantly.
Order: $Cl-Cl > Br-Br > F-F > I-I$.
Boiling Points of Hydrides
Group 16 - TrendsScenario: Arrange Boiling Points: $H_2O, H_2S, H_2Se, H_2Te$.
Student follows molecular mass trend purely.
$$ H_2O < H_2S < H_2Se < H_2Te $$
(Water is liquid, Hydrogen Sulfide is gas!)
Hydrogen Bonding Anomaly!
Water forms strong intermolecular H-bonds, making its BP exceptionally high.
For the rest, BP increases with mass (Van der Waals forces).
Correct Order: $H_2S < H_2Se < H_2Te < H_2O$.
Hydrolysis of Xenon Fluorides
Group 18 - Noble GasesScenario: Complete reaction: $XeF_4 + H_2O \rightarrow$ ?
Student assumes simple substitution like $XeF_6$.
$$ XeF_4 + H_2O \rightarrow XeO_2 + HF $$
(Incomplete!)
Disproportionation!
$XeF_4$ hydrolysis is a redox reaction.
Xenon goes to both $Xe$ (0) and $XeO_3$ (+6).
$$ 6XeF_4 + 12H_2O \rightarrow 4Xe + 2XeO_3 + 24HF + 3O_2 $$
Acidity of Oxoacids
Group 17 - AcidityScenario: Which is more acidic? $HClO$ or $HClO_4$?
Student gets confused between oxidizing power and acidity.
Thinks: "$HClO$ is a strong oxidizer, so maybe it's a strong acid?"
Oxidation Number Rule!
Higher Oxidation State of Central Atom = More Acidic.
$HClO_4$ ($Cl=+7$) is one of the strongest acids known.
$HClO$ ($Cl=+1$) is very weak.
Order: $HClO < HClO_2 < HClO_3 < HClO_4$.
Reaction of $Cl_2$ with Ammonia
Group 17 - ReactionsScenario: Product of $NH_3 (\text{Excess}) + Cl_2$.
Student writes the explosive product: $NCl_3$.
(That happens when $Cl_2$ is in excess!)
Ammonium Chloride!
If Ammonia is in excess, it acts as a base to neutralize the acid formed.
$$ 8NH_3 + 3Cl_2 \rightarrow 6NH_4Cl + N_2 $$
Product: $NH_4Cl + N_2$.
Confess Your Sins!
"P-Block reactions are all about conditions. Excess, dilute, hot, cold... did you miss a keyword?"
Did one of these catch you? Or do you have a different horror story from your last exam?
Scroll down to the comments section below and tell us:
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