Chemical Bonding and Molecular Structure — Important Questions

Ice is less dense than liquid water due to hydrogen bonding

Step 1: In ice, each H₂O molecule forms 4 hydrogen bonds in a tetrahedral arrangement, creating an open cage-like lattice structure with vacant spaces. Step 2: This open structure means for a given mass, ice occupies MORE volume than liquid water → lower density. Hence ice floats on water. Step 3: Option A is wrong — HCl does NOT show H-bonding because Cl is large in size (despite being electronegative). Small size is required for effective H-bonding. Step 4: Option B is wrong — H-bond energy is only 3.5–40 kJ/mol while covalent bonds are ~400 kJ/mol. H-bonds are much weaker. Step 5: Option D

Trigonal planar geometry

Step 1: B–F bonds are polar (F is more electronegative than B), so each bond has a dipole moment. Step 2: The net dipole moment of the molecule is the vector sum of all individual bond dipoles. Step 3: In trigonal planar geometry, the three B–F bonds are at 120° to each other. The resultant of any two bond dipoles exactly cancels the third one. Step 4: This vector cancellation gives a net dipole moment of ZERO, even though individual bonds are polar. Step 5: Pyramidal geometry (like NH₃) would NOT give zero dipole because the lone pair adds an additional dipole component and the vectors don't

O₂ has two unpaired electrons in degenerate antibonding π* orbitals

Step 1: O₂ has 16 electrons. The MO configuration is: KK(σ₂s)²(σ*₂s)²(σ₂pz)²(π₂px)²(π₂py)²(π*₂px)¹(π*₂py)¹ Step 2: The last two electrons go into the degenerate (equal energy) π*₂px and π*₂py antibonding orbitals. Step 3: By Hund's rule, they occupy these two orbitals SINGLY (one in each) rather than pairing up. Step 4: These two unpaired electrons make O₂ paramagnetic (attracted to magnetic fields). Step 5: This is a triumph of MOT — valence bond theory incorrectly predicted O₂ would be diamagnetic. Option A is about Lewis structure (doesn't explain paramagnetism). Option C is wrong — O₂ has

AlCl₃

Step 1: Fajan's rules state that covalent character increases with higher cation charge, smaller cation size, and larger anion size. Step 2: Compare the cations — Na⁺ (charge +1), K⁺ (charge +1), Mg²⁺ (charge +2), Al³⁺ (charge +3). Step 3: The ionic potential φ = charge/size. Al³⁺ has the highest charge (+3) and a small size among these. Step 4: High ionic potential means Al³⁺ strongly attracts the electron cloud of Cl⁻, distorting it (polarising the anion). Step 5: This polarisation leads to electron sharing (covalent character). The order of covalent character is: AlCl₃ > MgCl₂ > NaCl ≈ KCl.