Structure 2: Models of bonding and structure practice questions.

According to VSEPR theory, what is the molecular geometry and bond angle of ammonia (NH3)?

• Trigonal planar, with bond angles of exactly 120 degrees
• Trigonal pyramidal, about 107 degrees
• Tetrahedral, 109.5 degrees
• Bent, about 104.5 degrees

Answer: Trigonal pyramidal, about 107 degrees

NH3 has 4 electron domains around N (3 bonding pairs + 1 lone pair). The electron geometry is tetrahedral, but the shape describing atom positions is trigonal pyramidal. The lone pair repels more strongly, compressing the H-N-H angle from 109.5 to about 107 degrees.

What is the shape of a molecule with a central atom surrounded by 5 bonding pairs and no lone pairs, such as PCl5?

• Square-based pyramidal
• Trigonal bipyramidal
• Octahedral
• Seesaw

Answer: Trigonal bipyramidal

Five electron domains with no lone pairs give a trigonal bipyramidal geometry: three equatorial bonds at 120 degrees and two axial bonds at 90 degrees to the equatorial plane. PCl5 is a classic example.

Which of these molecules is polar?

• BF3
• CCl4
• SF6
• SO2

Answer: SO2

BF3 (trigonal planar), CCl4 (tetrahedral) and SF6 (octahedral) are all symmetric, so their bond dipoles cancel and they are non-polar. SO2 is bent (one lone pair on S), so the bond dipoles do not cancel and it has a net dipole, making it polar.

Why does water (H2O) have a much higher boiling point than hydrogen sulfide (H2S)?

• H2O has much stronger London dispersion forces than H2S does
• H2O molecules form hydrogen bonds; H2S does not
• H2S is ionic
• H2O has a larger molar mass

Answer: H2O molecules form hydrogen bonds; H2S does not

Oxygen is far more electronegative than sulfur and small enough for effective hydrogen bonding. Water molecules form strong hydrogen bonds, requiring much more energy to separate. H2S only has weaker dipole-dipole and dispersion forces, so it boils at a much lower temperature despite its larger mass.

Which molecule can act as a hydrogen bond donor?

• CH4
• CH3OCH3 (dimethyl ether)
• CH3OH (methanol)
• CH2F2

Answer: CH3OH (methanol)

A hydrogen bond donor needs an H atom bonded directly to N, O or F. Only methanol (CH3OH) has an O-H bond. Dimethyl ether has O but no O-H, so it can accept but not donate; CH4 and CH2F2 have no N-H, O-H or F-H bonds.

Which type of bonding is present in solid sodium chloride?

• metallic
• ionic
• covalent (non-polar)
• hydrogen bonding

Answer: ionic

NaCl forms a giant ionic lattice of Na⁺ and Cl⁻ held by electrostatic attraction, giving a high melting point.