Structure: Models of Matter, Bonding and Classification
Structure is the first core area of IB Chemistry HL and is examined across all papers. It covers the particulate model of matter, bonding and structure, and the classification of matter. HL questions ask you to move between representations-electron configurations, Lewis structures, molecular shapes, and macroscopic properties-and to explain trends rather than just state them.
Core rules and concepts
- Electron configuration follows the Aufbau principle, Pauli exclusion principle and Hund's rule. Chromium is [Ar]3d⁵4s¹ and copper is [Ar]3d¹⁰4s¹. When forming cations, remove 4s electrons before 3d electrons.
- Periodic trends: atomic radius decreases across a period and increases down a group. First ionisation energy generally increases across a period but dips at groups 13 and 16; it decreases down a group. Electronegativity increases across a period and decreases down a group.
- VSEPR: count electron domains (bonding regions plus lone pairs). Lone pairs repel more strongly than bonding pairs, so bond angles compress: CH₄ 109.5°, NH₃ 107°, H₂O 104.5°.
- Hybridisation: steric number 4 → sp³, 3 → sp², 2 → sp. Sigma bonds are end-on; pi bonds are side-on and restrict rotation.
- Polarity is the vector sum of bond dipoles. Lone pairs can make a molecule polar even when the bonds themselves are non-polar, as in H₂O.
- Intermolecular forces: London dispersion (all molecules, stronger with more electrons and greater surface area), dipole–dipole, and hydrogen bonding (H bonded to N, O or F).
Worked example
Explain why the first ionisation energy of Mg is higher than that of Al. Mg is 1s²2s²2p⁶3s²; Al is 1s²2s²2p⁶3s²3p¹. The electron removed from Al is in the higher-energy 3p subshell, which is shielded by the 3s electrons. Mg's 3s electron is closer to the nucleus and less shielded, so more energy is needed to remove it. This earns two marks: configuration and shielding/energy level.
Traps that cost marks
Do not write “Mg has a full outer shell” without specifying 3s². Do not confuse electron-pair geometry with molecular geometry. Do not ignore lone pairs when assigning polarity. Do not claim hydrogen bonding for HCl-it only has dipole–dipole forces.