The Mistake Bank
Chapter 11: P-Block Elements (Group 13 & 14)
Where metal meets non-metal, and logic meets exceptions.
Acidity of Boron Halides
Back BondingScenario: Arrange $BF_3$, $BCl_3$, and $BBr_3$ in increasing order of Lewis Acidic strength.
Student thinks: "Fluorine is most electronegative, so it pulls electrons from Boron, making Boron most electron-deficient."
$$ BF_3 > BCl_3 > BBr_3 $$
(Ignores back-bonding!)
Size Matching Matters!
In $BF_3$, both B and F are $2p-2p$. Effective back-bonding ($p\pi-p\pi$) satisfies Boron's hunger.
In $BBr_3$, the overlap is $2p-4p$ (very poor). Boron stays hungry.
$$ BBr_3 > BCl_3 > BF_3 $$
Hydrolysis of Chlorides
Reaction MechanismScenario: Does $CCl_4$ hydrolyze in water like $SiCl_4$?
Student assumes same group members behave similarly.
Writes: $CCl_4 + H_2O \rightarrow C(OH)_4 + HCl$
(Carbon has no vacancy!)
No d-orbitals in Carbon!
For hydrolysis, water's oxygen needs to attack the central atom. Carbon ($n=2$) has no empty d-orbitals to accept the lone pair.
$SiCl_4$ has empty d-orbitals, so it hydrolyzes.
Answer: $CCl_4$ does NOT hydrolyze.
The Inert Pair Effect
Oxidation StatesScenario: Compare stability of $Pb^{4+}$ and $Pb^{2+}$. Which is a better Oxidizing Agent?
Student assumes group oxidation state (+4) is always most stable.
Thinks $Pb^{4+}$ is stable and inert.
Heavy metals prefer lower states!
Due to poor shielding by f-electrons, the s-electrons of the outer shell ($6s^2$) refuse to participate (Inert Pair).
$Pb^{2+}$ is stable. $Pb^{4+}$ is unstable.
Thus, $Pb^{4+}$ desperately wants to gain electrons to become $Pb^{2+}$, making it a Strong Oxidizing Agent.
Structure of Silica (SiO2)
BondingScenario: Draw the structure of $SiO_2$.
Student thinks it's like $CO_2$.
Draws linear molecule: $O=Si=O$
(Si cannot form stable $\pi$-bonds with Oxygen!)
It's a 3D Network Solid!
Silicon forms single bonds with 4 Oxygens, and each Oxygen bonds with 2 Silicons.
This forms a giant covalent lattice (Quartz), which is why sand is a hard solid, while $CO_2$ is a gas.
Banana Bonds in Diborane
Chemical BondingScenario: In $B_2H_6$, are all B-H bonds equal?
Student assumes symmetry like Ethane ($C_2H_6$).
Answer: "Yes, all bonds are identical single bonds."
Terminal vs Bridge Bonds!
1. 4 Terminal B-H bonds: Normal 2c-2e bonds.
2. 2 Bridge B-H-B bonds: 3c-2e bonds (Banana bonds).
The bridge bonds are longer and weaker than terminal bonds.
AlCl3 in Water
CompoundsScenario: Does $AlCl_3$ exist as a dimer ($Al_2Cl_6$) in water?
Student memorizes "$AlCl_3$ is a dimer" and applies it everywhere.
Answer: "Yes."
High Hydration Energy!
In non-polar solvents (like benzene), it is a dimer.
In water, the high hydration energy breaks the dimer bridges.
It exists as octahedral ions: $[Al(H_2O)_6]^{3+} + 3Cl^-$.
Confess Your Sins!
"P-Block is the wild west of the periodic table. Did you get shot down by an exception?"
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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