AQA A Level Chemistry 3.1.3 Bonding Exam Questions

About AQA A Level Chemistry 3.1.3 Bonding

Bonding explains why substances have the properties they do. On the AQA A Level course you need ionic, covalent, dative covalent and metallic bonding, the four types of crystal structure, shapes of molecules and ions using electron pair repulsion theory, electronegativity and bond polarity, and the three types of intermolecular force. The topic links to atomic structure, periodicity, the properties of Period 3 elements and oxides, and organic boiling point trends. Examiners commonly ask for a shape and bond angle with a justification, a labelled hydrogen bond diagram, and explanations of melting or boiling points that name the force or bond being overcome.

What you need to know

• Ionic and metallic bonding Ionic bonding is electrostatic attraction between oppositely charged ions in a lattice. Metallic bonding is attraction between positive metal ions and delocalised electrons. Know formulas of compound ions such as sulfate, hydroxide, nitrate, carbonate and ammonium.
• Covalent and dative bonds A covalent bond is a shared pair of electrons. In a dative (coordinate) bond both electrons come from one atom, for example in NH4+ or H3O+, shown with an arrow from the lone pair.
• Crystal structures Ionic (NaCl), metallic (Mg), macromolecular (diamond, graphite) and molecular (I2, ice). Relate melting point, conductivity and solubility to the structure and the forces broken.
• Shapes of molecules Electron pairs repel to be as far apart as possible, and lone pairs repel more than bonding pairs. Learn: linear 180°, trigonal planar 120°, tetrahedral 109.5°, trigonal pyramidal 107°, bent 104.5°, trigonal bipyramidal 90° and 120°, octahedral 90°.
• Electronegativity and polarity Electronegativity is the power of an atom to attract the pair of electrons in a covalent bond. Unequal sharing creates a polar bond with δ+ and δ- ends. Symmetrical molecules such as CO2 and CCl4 have polar bonds but no overall dipole.
• Intermolecular forces Induced dipole-dipole (van der Waals) forces act between all molecules and increase with size. Permanent dipole-dipole forces act between polar molecules. Hydrogen bonds form between an H bonded to N, O or F and a lone pair on N, O or F in another molecule, and explain the high boiling point of water and the low density of ice.

Common mistakes in 3.1.3 Bonding

• Saying covalent bonds break when a simple molecular substance melts or boils. Only the intermolecular forces are overcome.
• Drawing hydrogen bonds without the lone pair and partial charges. Show δ+ on H, δ- on O, N or F, the lone pair, and the hydrogen bond as a dashed line to the lone pair.
• Giving shapes without justification. State the number of bonding pairs and lone pairs and that lone pairs repel more strongly than bonding pairs.
• Calling CO2 or CCl4 polar molecules. They contain polar bonds, but the dipoles cancel because the molecules are symmetrical.
• Describing metallic bonding as attraction between nuclei and electrons. It is attraction between positive ions and a sea of delocalised electrons.

Try three 3.1.3 Bonding exam-style questions