IB Chemistry: Covalent Bonding & Lewis Structures FAQ

You need to draw Lewis structures showing bonding pairs and lone pairs, use VSEPR to predict shape and bond angle, and explain polarity from electronegativity differences. At HL, add formal charge, expanded octets, sigma/pi bonds and hybridisation. This all sits under Structure 2.2, The covalent model, in the current IB Chemistry guide.

Count every valence electron in the molecule first, then pick a central atom — usually the least electronegative, never hydrogen. Join atoms with single bonds, distribute the rest as lone pairs to complete octets, and convert a lone pair into a second or third bond if any atom is short of eight electrons.

Formal charge is a bookkeeping tool (HL only) used to pick the most realistic Lewis structure when more than one arrangement is possible. It's calculated as valence electrons minus non-bonding electrons minus half the bonding electrons. The best structure usually has formal charges closest to zero, with any negative charge on the more electronegative atom.

An expanded octet is when a central atom has more than eight electrons around it — only possible for elements in period 3 or below, because their larger size and available orbitals can accommodate extra electron pairs. Classic HL examples are SF6, PCl5 and SF4, where sulfur or phosphorus exceeds the octet while surrounding atoms keep normal octets.

VSEPR takes the electron domains from your completed Lewis structure — bonding pairs and lone pairs both count — and arranges them to minimise repulsion. Lone pairs repel more strongly than bonding pairs, which compresses bond angles below the ideal geometric value whenever lone pairs are present on the central atom.

In my experience marking mock papers, the same three errors account for most lost marks: forgetting lone pairs on outer atoms, choosing the wrong central atom, and miscounting electrons for an ion (forgetting to add or remove electrons for the charge). Each one is a quick fix once you know to check for it.

A single Lewis structure shows one fixed arrangement of electrons, but some molecules — ozone, the carbonate ion, benzene — can't be described accurately by just one. Resonance structures are two or more valid Lewis structures with the same atom positions but different electron placement; the real molecule is a hybrid, an average, not a flicker between them.

First check individual bond polarity using electronegativity differences, then look at the molecule's overall shape from VSEPR. If the bond dipoles are arranged symmetrically, they cancel and the molecule is non-polar overall, even if individual bonds are polar. If they don't cancel, the molecule is polar.

Covalent bonding and Lewis structures fall under Structure 2.2, The covalent model, in the current DP Chemistry guide, part of the Structure strand alongside ionic and metallic bonding (2.1) and the periodic table (1.3). According to the IB, this guide was first examined in 2025, replacing the previous topic-based syllabus structure.

Yes. Paper 1 tends to test recognition — picking the correct shape, polarity or bond angle from multiple-choice options — without asking you to draw anything. Paper 2 is where you actually draw full Lewis structures, justify formal charge, name VSEPR shapes and, at HL, explain hybridisation in extended response.

Yes, noticeably — HL keeps everything from SL and adds formal charge, expanded octets, sigma/pi bonding and hybridisation, all of which require more abstract reasoning than simply drawing a structure and naming a shape. Most students I teach find hybridisation the hardest single idea in this whole subtopic.

Broadly similar depth: both IB HL and UK A-Level chemistry cover dot-and-cross (Lewis) diagrams, VSEPR shapes and hybridisation to a comparable standard. The main difference is that IB explicitly assesses formal charge as a decision-making tool for choosing between structures, which most A-Level specifications don't name as a separate skill.

Timed, repeated practice on a mix of neutral molecules, ions and expanded-octet species beats reading notes every time — you need to build the electron-counting habit until it's automatic. Work through past-paper Structure 2.2 questions and mark yourself against the actual mark scheme, not just a model answer.

The most useful thing you can do isn't reteaching the chemistry — it's making sure your child is practising the right way: timed past-paper questions marked against real mark schemes, not just re-reading notes. If formal charge or hybridisation keeps coming up as a weak spot in feedback, that's worth flagging to their teacher directly.