Molecular Geometry: The Shapes of Molecules

Naming atom positions, not electron clouds

Molecular geometry answers a simple question: where are the atoms? It names the arrangement of nuclei, and nuclei only. Lone pairs bend the shape but never appear in its name — which is why ammonia, with its four electron domains, is called trigonal pyramidal rather than tetrahedral.

This naming convention exists because experiments see atoms, not electron pairs. X-ray crystallography and spectroscopy locate nuclei; the lone pairs are inferred. So the vocabulary describes the observable skeleton.

The workflow is fixed: draw the Lewis structure, count domains to get the electron geometry, then erase the lone pairs mentally and name what remains. Tetrahedral electrons minus one lone pair gives trigonal pyramidal; minus two gives bent. Learn to perform that mental erasure and every shape name follows.

The complete geometry chart

The complete catalog, organized by steric number:

• Steric 2: AX2 — linear (CO2, 180°).
• Steric 3: AX3 — trigonal planar (BF3); AX2E — bent (SO2, about 119°).
• Steric 4: AX4 — tetrahedral (CH4); AX3E — trigonal pyramidal (NH3); AX2E2 — bent (H2O, 104.5°).
• Steric 5: AX5 — trigonal bipyramidal (PCl5); AX4E — seesaw (SF4); AX3E2 — T-shaped (ClF3); AX2E3 — linear (XeF2).
• Steric 6: AX6 — octahedral (SF6); AX5E — square pyramidal (BrF5); AX4E2 — square planar (XeF4).

Notice the pattern: each lone pair removes one atom-position from the parent shape. The parent geometries (linear, trigonal planar, tetrahedral, trigonal bipyramidal, octahedral) are the electron geometries; everything else is a parent with invisible lone pairs subtracted. Memorizing the parents plus the subtraction rule replaces memorizing the whole list.

Worked predictions: SF4 and XeF4

Two molecules test whether you understand the system or merely memorized it: SF4 and XeF4.

Sulfur tetrafluoride: sulfur brings 6 valence electrons, four fluorines bring 28, totaling 34 — wait, recount: 6 + 4(7) = 34. Four S−F bonds use 8 electrons; the fluorines' octets consume 24 more; 2 electrons remain as one lone pair on sulfur. That's AX4E, steric 5, from a trigonal bipyramidal parent.

Where does the lone pair sit? The bipyramid has two kinds of slots: axial (90° neighbors) and equatorial (120° neighbors). The bulky lone pair takes an equatorial slot for breathing room, leaving the four fluorines in a seesaw arrangement.

Xenon tetrafluoride: Xe has 8 valence electrons, four fluorines add 28, total 36. Four bonds plus full fluorine octets leave 4 electrons — two lone pairs on xenon: AX4E2, steric 6, octahedral parent. The two lone pairs position themselves opposite each other to minimize repulsion, and the four fluorines define a perfect square plane. Logic, not memory, delivers both answers.

Why lone pairs choose their seats

The trigonal bipyramid deserves special attention because its positions are not equivalent. Three equatorial slots ring the middle with 120° between neighbors; two axial slots cap the top and bottom with tighter 90° angles to the equator.

Lone pairs always claim equatorial slots first, where repulsion is weakest. SF4's single lone pair goes equatorial (giving the seesaw); ClF3's two lone pairs take two equatorial slots (giving the T-shape); XeF2's three lone pairs fill all three equatorial slots, leaving the two fluorines exactly opposite — so XeF2 is linear despite having five domains.

This seating rule is another worked-logic tool: place lone pairs equatorially, then read off the remaining atom positions. Students who master it can name any steric-5 shape on sight, while those who memorize shapes one by one invariably mix up seesaw and T-shaped.

Why shape matters

Shape is not academic decoration — it governs how molecules behave. Polarity depends on shape: identical polar bonds cancel in linear CO2 but reinforce in bent H2O, deciding solubility and boiling points (the next articles build on this).

Reactivity follows shape too. A nucleophile attacks a carbonyl carbon from a specific angle; an SN2 reaction demands backside attack along the bond axis. Catalysts are engineered with pocket shapes that admit only correctly shaped substrates.

Biology is shape all the way down. Enzymes bind substrates by shape complementarity — the lock-and-key and induced-fit models. Smell works because receptor proteins distinguish molecular shapes; limonene's two mirror-image forms smell of orange and lemon respectively despite identical formulas.

Macroscopic properties — what dissolves, what boils when, what reacts — are shadows cast by these three-dimensional arrangements. Learn the shapes and the behaviors start looking inevitable.

A reliable prediction routine

A dependable routine beats memorization. For any molecule:

1. Draw the Lewis structure. Get connectivity and lone pairs right — everything downstream depends on this.
2. Count domains. Steric number = sigma bonds + lone pairs on the central atom. Multiple bonds count once.
3. Name the electron geometry. 2 linear, 3 trigonal planar, 4 tetrahedral, 5 trigonal bipyramidal, 6 octahedral.
4. Name the molecular geometry. Subtract the lone pairs from the parent shape using the chart.
5. Estimate the angle. Start from the ideal, then squeeze for each lone pair: roughly 2–2.5° per lone pair in tetrahedral systems.

Practice the chain on four molecules: H2O (bent, 104.5°), NH3 (trigonal pyramidal, 107°), CO2 (linear, 180°), BF3 (trigonal planar, 120°). If you can walk each one from Lewis structure to angle estimate without notes, the system is yours.

Common mistakes

Real-world applications

• Predicting whether a molecule is polar

• Designing drug molecules that fit protein pockets

• Interpreting infrared and microwave spectra

• Engineering catalysts with the right approach angles