Bonding & Structure

Which compound contains a dative (coordinate) covalent bond?

• ACH₄
• BNaCl
• CNH₄⁺✓
• DO₂

Answer: In the ammonium ion, NH₄⁺, the nitrogen lone pair forms a bond with an H⁺ ion that brings no electrons of its own. Both bonding electrons originate from nitrogen, making one of the four N–H bonds a dative covalent bond.

Which of the following correctly describes metallic bonding?

• AElectrostatic attraction between oppositely charged ions in a lattice
• BThe electrostatic attraction between positive metal ions and a sea of delocalised electrons✓
• CShared pairs of electrons localised between two atoms
• DWeak attraction between temporary dipoles

Answer: Metallic bonding is the electrostatic attraction between a lattice of positive metal ions and the delocalised (mobile) outer electrons shared throughout the structure. This explains conductivity and malleability.

Which molecule is polar overall (has a net dipole moment)?

• ACO₂
• BCCl₄
• CBF₃
• DNH₃✓

Answer: NH₃ is trigonal pyramidal with a lone pair, so its polar N–H bond dipoles do not cancel, giving a net dipole. CO₂ (linear), CCl₄ (tetrahedral) and BF₃ (trigonal planar) are all symmetrical, so their bond dipoles cancel and the molecules are non-polar.

What is the shape of, and the bond angle in, a molecule of sulfur hexafluoride, SF₆?

• AOctahedral, 90°✓
• BTrigonal bipyramidal, 120°
• CTetrahedral, 109.5°
• DSquare planar, 90°

Answer: Sulfur in SF₆ has 6 bonding pairs and no lone pairs. Six electron pairs repel to positions as far apart as possible, giving an octahedral shape with bond angles of 90°.

Which sequence correctly ranks these three molecules in order of INCREASING boiling point?

• AHF < HCl < HBr
• BHCl < HBr < HF✓
• CHBr < HCl < HF
• DHF < HBr < HCl

Answer: HCl and HBr have only permanent dipole–dipole and London forces; HBr has more electrons so stronger London forces and a higher boiling point than HCl. HF can form hydrogen bonds (F is highly electronegative with a lone pair), giving it the highest boiling point of the three: HCl < HBr < HF.

Why does ice (solid H₂O) have a LOWER density than liquid water?

• AThe covalent O–H bonds become longer on freezing
• BLondon forces increase, packing the molecules more closely
• CWater molecules become non-polar when frozen
• DHydrogen bonds hold the molecules in an open, regular lattice with larger spaces between molecules✓

Answer: In ice, each water molecule forms four hydrogen bonds, arranging the molecules into an open hexagonal lattice. This holds the molecules slightly further apart than in liquid water, so ice is less dense and floats.

Which statement correctly compares the bond angles in CH₄, NH₃ and H₂O?

• AAll three have a bond angle of 109.5°
• BNH₃ (120°) > CH₄ (109.5°) > H₂O (104.5°)
• CH₂O (109.5°) > NH₃ (107°) > CH₄ (104.5°)
• DCH₄ (109.5°) > NH₃ (107°) > H₂O (104.5°)✓

Answer: All three have four electron pairs around the central atom. Lone pairs repel more strongly than bonding pairs. CH₄ has 0 lone pairs (109.5°), NH₃ has 1 lone pair (107°), and H₂O has 2 lone pairs (104.5°). Each additional lone pair reduces the bond angle by roughly 2.5°.

Diamond and graphite are both giant covalent forms of carbon, yet graphite conducts electricity and diamond does not. What is the reason?

• AGraphite contains ionic bonds that carry charge
• BIn graphite each carbon bonds to only three others, leaving one delocalised electron per atom that is free to move✓
• CDiamond contains weak London forces between layers
• DGraphite has a higher melting point than diamond

Answer: In graphite each carbon forms three covalent bonds within layers, leaving one electron per atom delocalised between the layers. These mobile electrons conduct electricity. In diamond all four outer electrons are used in localised covalent bonds, so there are no free charge carriers.

Which of the following has the greatest difference in electronegativity between its bonded atoms, giving the most polar bond? (Electronegativity values: H 2.2, C 2.6, N 3.0, O 3.4, F 4.0)

• AC–H
• BN–H
• CO–H
• DF–H✓

Answer: Bond polarity increases with the difference in electronegativity (Δ). C–H Δ = 0.4; N–H Δ = 0.8; O–H Δ = 1.2; F–H Δ = 1.8. The largest difference is for H–F, so it is the most polar bond.

Magnesium oxide (MgO) has a much higher melting point than sodium chloride (NaCl). Which factor best explains this?

• AMgO has covalent bonds whereas NaCl has ionic bonds
• BNaCl has a simple molecular structure
• CMg²⁺ and O²⁻ ions are 2+/2− charged and smaller, giving stronger electrostatic attraction than Na⁺/Cl⁻✓
• DMgO contains hydrogen bonds

Answer: Both are giant ionic lattices. MgO contains 2+ and 2− ions which are also smaller than Na⁺ and Cl⁻. Higher ionic charges and smaller ionic radii give a much greater lattice enthalpy and stronger electrostatic forces, so MgO needs far more energy to melt.