Top 15+ Errors Students Make

Error 1: Confusing electron geometry with molecular shape

• Wrong: $XeF_{4}$ is octahedral
• Correct: $XeF_{4}$ has octahedral ELECTRON geometry but SQUARE PLANAR molecular shape
• Fix: Always subtract lone pairs when stating molecular shape

Error 2: Using pi bonds in steric number

• Wrong: $CO_{2}$ (C=O, C=O) has SN = 4
• Correct: $CO_{2}$ has SN = 2 (2 sigma bonds, 0 lone pairs)
• Fix: SN counts only sigma bonds + lone pairs; pi bonds are in the same domain

Error 3: Predicting $O_{2}$ as diamagnetic (VBT trap)

• Wrong: $O_{2}$ has a double bond, so all electrons are paired → diamagnetic
• Correct: MOT shows 2 unpaired electrons in degenerate π*2p → paramagnetic
• Fix: Always use MOT for magnetic predictions of diatomic species

Error 4: Placing lone pairs incorrectly in TBP (SN=5)

• Wrong: Lone pairs at axial positions in $SF_{4}$
• Correct: Lone pairs prefer EQUATORIAL positions in TBP (fewer 90° interactions)
• Fix: Equatorial = 2 interactions at 90°; Axial = 3 interactions at 90°. LP goes equatorial.

Error 5: Wrong MO filling order for $N_{2}$

• Wrong: Using Z > 7 order for $N_{2}$ (σ2p before π2p)
• Correct: $N_{2}$ (Z = 7 ≤ 7): π2p fills BEFORE σ2p (s-p mixing)
• Fix: Remember "Z ≤ 7 → π before σ at 2p level"

Error 6: $BF_{3}$ vs $NF_{3}$ dipole moment

• Wrong: Both $BF_{3}$ and $NF_{3}$ have μ = 0 because they both have 3 identical bonds
• Correct: $BF_{3}$ (trigonal planar, no lp) → μ = 0; $NF_{3}$ (trigonal pyramidal, 1 lp) → μ ≠ 0
• Fix: Check for lone pairs — they break symmetry

Error 7: Bond angle in $XeF_{2}$ (linear trap)

• Wrong: $XeF_{2}$ is bent (V-shaped) like $H_{2}O$
• Correct: $XeF_{2}$ is LINEAR; lone pairs (3) are equatorial, F atoms (2) are axial
• Fix: Count lone pairs and apply equatorial placement rule in TBP

Error 8: All C–Cl bonds in $PCl_{5}$ are equal

• Wrong: $PCl_{5}$ has 5 identical P–Cl bonds
• Correct: Axial P–Cl bonds are LONGER and WEAKER than equatorial ones
• Fix: Axial bonds have 3 repulsors at 90°; equatorial have fewer → axial longer

Error 9: $N_{2}^{+}$ is more stable than $N_{2}$

• Wrong: Adding $N_{2}^{+}$ removes an electron from antibonding → more stable
• Correct: $N_{2}^{+}$ removes an electron from the BONDING σ2p → BO decreases to 2.5 < 3
• Fix: Identify WHICH orbital the electron is removed from in $N_{2}$ (Z ≤ 7)

Error 10: Lattice energy is always the largest term in Born-Haber

• Context: For MgO, the second electron affinity ($EA_{2}$ = +780 kJ/mol) is endothermic, not exothermic
• Fix: $EA_{2}$ for oxygen is endothermic; only $EA_{1}$ is exothermic

Error 11: Hybridization and geometry of S$O_{2}$

• Wrong: S$O_{2}$ is linear like $CO_{2}$ (both are X$O_{2}$)
• Correct: S$O_{2}$ is BENT ($sp^{2}$ on S, 1 lone pair) while $CO_{2}$ is LINEAR (sp on C, 0 lone pairs)
• Fix: Count lone pairs on the central atom; S has 1 lone pair in S$O_{2}$

Error 12: Bond order of CO

• Wrong: CO has bond order 2 (double bond in Lewis structure)
• Correct: CO is isoelectronic with $N_{2}$ (14 electrons) → BO = 3 (triple bond)
• Fix: Use MO theory; CO has a triple bond (dative/coordinate contribution makes it a triple bond)

Error 13: Resonance structures are isomers

• Wrong: The two Kekulé structures of benzene are isomers
• Correct: Resonance structures are NOT real structures; they are hypothetical contributors to the resonance hybrid
• Fix: Isomers can be isolated; resonance structures cannot

Error 14: $SF_{6}$ has lone pairs on S

• Wrong: S uses 6 of 6 valence electrons in 6 bonds; let's say there are leftover electrons
• Correct: S has 6 valence electrons; all 6 are used for 6 S–F bonds → 0 lone pairs
• Fix: Lone pairs = (valence $e^{-}$ − bonds)/2; (6−6)/2 = 0

Error 15: $H_{2}S$ has the same hybridization as $H_{2}O$

• Wrong: $H_{2}S$ is $sp^{3}$ hybridized like $H_{2}O$, so bond angle ≈ 104°
• Correct: $H_{2}S$ uses nearly pure 3p orbitals (poor hybridization in 3rd period), bond angle ~92°
• Fix: Effective hybridization decreases down a group; S doesn't hybridize as effectively as O