IB Chemistry: VSEPR & Molecular Shapes — FAQs

VSEPR (Valence Shell Electron Pair Repulsion) theory predicts a molecule's 3D shape by assuming electron domains — bonding pairs and lone pairs — around a central atom arrange themselves to minimise repulsion. In IB Chemistry, it's examined as a model you apply to explain shape, bond angle and polarity together, not shape in isolation.

VSEPR shows up in Paper 1 as multiple-choice questions identifying a shape or bond angle from a formula, and in Paper 2 as structured questions asking you to draw a Lewis structure, name the shape, state the bond angle, and explain the reasoning — usually worth 3-4 marks across the sub-parts.

Electron domain geometry counts every electron domain — bonding pairs and lone pairs — around the central atom. Molecular geometry describes only where the atoms sit, ignoring lone pairs visually. Water's electron domain geometry is tetrahedral, but its molecular shape is bent — and examiners mark the atom-shape name, not the domain shape.

Lone pairs occupy more space than bonding pairs because they're held by only one nucleus rather than shared between two atoms, so they repel neighbouring electron domains more strongly. Each lone pair squeezes bonding pairs closer together, which is why NH3 measures 107° and H2O measures 104.5°, both below the ideal tetrahedral 109.5°.

No — SL Chemistry covers molecules with two to four electron domains (linear, trigonal planar, bent, tetrahedral, trigonal pyramidal), while HL adds five- and six-domain shapes involving expanded octets. Both levels use identical VSEPR repulsion logic; HL students just apply it to a longer, more complex shape list.

HL adds shapes built from five electron domains (trigonal bipyramidal, see-saw, T-shaped, linear) and six electron domains (octahedral, square pyramidal, square planar), used for expanded-octet central atoms such as phosphorus, sulfur and chlorine — think PCl5, SF6 and ClF3 rather than the simple molecules seen at SL.

Draw the Lewis structure first, then count the electron domains — bonding regions plus lone pairs — around the central atom. Apply repulsion rules to arrange those domains as far apart as possible, name that arrangement as the electron domain geometry, then remove the lone pairs from view to state the actual molecular shape.

The single most common error I see in mock scripts is naming the electron domain geometry when the question asks for the molecular shape — writing "tetrahedral" for water instead of "bent". The next most frequent slip is forgetting that lone pairs count as domains at all, which throws off the whole geometry.

Start from the parent electron domain geometry's ideal angle, then reduce it for each lone pair squeezing the bonding pairs closer together. CH4 with no lone pairs stays at 109.5°, NH3 with one lone pair drops to about 107°, and H2O with two lone pairs compresses further to about 104.5°.

For SL you need shapes from two to four electron domains: linear (180°), trigonal planar (120°), bent (around 117° or 104.5°), tetrahedral (109.5°) and trigonal pyramidal (107°). HL adds five- and six-domain shapes with angles around 90° and 120° for expanded-octet molecules such as PCl5 and SF6.

Yes — examiners very often follow a shape question with "hence deduce whether the molecule is polar", so you need the 3D shape and the vector sum of bond dipoles, not just the shape name. A molecule can have polar bonds but be non-polar overall if its shape is symmetric, like CO2 or CCl4.

VSEPR itself isn't conceptually difficult, but it's a frequent source of dropped marks because it demands precise vocabulary — the shape name, the bond angle and the reasoning all correctly matched in one answer. "State and explain" shape questions consistently show weaker performance than pure recall questions, mostly from vague or missing reasoning.

The most effective approach is repeated low-stakes practice: have your child draw the Lewis structure, name the shape, state the angle and justify it out loud for fifteen to twenty different molecules until the pattern becomes automatic. Isolated memorisation of shape names without this repeated reasoning practice tends to fall apart under exam pressure.