The Mistake Bank
d and f Block Elements
Transition metals are colorful, but don't let their exceptions fade your marks.
Is Zinc a Transition Element?
DefinitionsScenario: Classify Zinc ($Zn$), Cadmium ($Cd$), and Mercury ($Hg$).
Student sees them in the d-block.
Answer: "Yes, they are transition elements."
(Incorrect Definition!)
Check for Incomplete d-orbitals!
Transition elements must have incompletely filled d-orbitals in their ground state or any common oxidation state.
$Zn$ ($3d^{10}$) and $Zn^{2+}$ ($3d^{10}$) are fully filled.
Answer: They are d-block elements, but NOT transition elements.
Stability of Copper Ions
Oxidation StatesScenario: Which is more stable in Aqueous solution? $Cu^+$ or $Cu^{2+}$?
Student looks at electronic config:
$Cu^+ = 3d^{10}$ (Full, Stable)
$Cu^{2+} = 3d^9$ (Unstable?)
Answer: "$Cu^+$ is more stable."
(Ignores solvent effect!)
Hydration Energy Wins!
In water, $Cu^{2+}$ has a much higher (negative) Hydration Enthalpy due to higher charge density.
This energy release compensates for the second ionization enthalpy.
Answer: $Cu^{2+}$ is more stable in aqueous solution.
Lanthanoid Contraction
Atomic RadiiScenario: Compare the atomic size of Zirconium ($Zr$, 4d) and Hafnium ($Hf$, 5d).
Student assumes size increases down the group.
Answer: "Hf is significantly larger than Zr."
They are almost identical!
Before Hf, 14 electrons fill the 4f orbitals (Lanthanoids).
f-electrons shield the nucleus poorly, causing the outer shell to be pulled in tightly.
Result: Size of $Zr \approx$ Size of $Hf$ ($160$ pm vs $159$ pm).
Acidifying KMnO4
TitrationScenario: Which acid is used to acidify $KMnO_4$ during titration? $HCl$ or $H_2SO_4$?
Student thinks: "Both are strong acids, so it doesn't matter."
Chooses $HCl$ because it's common.
Never use HCl with Permanganate!
$KMnO_4$ is a strong oxidizing agent and will oxidize $Cl^-$ (from $HCl$) to Chlorine gas ($Cl_2$).
This interferes with the titration reading.
Answer: Use Dilute $H_2SO_4$ (stable).
Chromate-Dichromate pH
CompoundsScenario: What happens when you add alkali ($OH^-$) to Orange Dichromate solution?
Student forgets the equilibrium color change.
Answer: "Nothing happens" or "It turns green ($Cr^{3+}$)."
Orange to Yellow Shift!
$$ Cr_2O_7^{2-} (\text{Orange}) + OH^- \rightarrow CrO_4^{2-} (\text{Yellow}) $$
Basic pH: Favors Chromate (Yellow).
Acidic pH: Favors Dichromate (Orange).
Gadolinium Configuration
Electronic ConfigScenario: Write the electronic configuration of Gadolinium ($Gd$, Z=64).
Student fills f-orbitals sequentially:
$$ [Xe] 4f^8 6s^2 $$
(Misses the stability of half-filled f-orbital!)
Half-Filled Stability!
The 8th electron enters the 5d orbital to keep the 4f orbital half-filled ($f^7$).
$$ [Xe] 4f^7 5d^1 6s^2 $$
Confess Your Sins!
"Transition metals are complex. Did you forget an exception or fall for a trap?"
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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