Molecular Shape

Theory — Valence Shell Electron Pair Repulsion

The VSEPR model, short for valence shell electron pair repulsion, says that the groups of valence electrons around a central atom arrange themselves to be as far apart as possible, because like charges repel. Each group, whether it is a bond or a lone pair, is called an electron domain. A single, double, or triple bond all count as one domain.

From Lewis structure to domains

Start by drawing the Lewis structure. Add up the valence electrons from every atom, connect the atoms with bonds, and place the remaining electrons as lone pairs so that each atom reaches a full shell. Then count, on the central atom, the number of bonding groups and the number of lone pairs. Their sum is the steric number, the total number of electron domains.

Steric numbersteric number = (bonding groups) + (lone pairs on the central atom)

A double or triple bond still counts as a single bonding group

Electron-domain geometry and molecular shape

The steric number fixes the electron-domain geometry, the arrangement of all the domains. The molecular shape is the arrangement of just the atoms, found by ignoring the lone pairs but keeping the positions they force. Lone pairs take up more room than bonds, so they squeeze the bond angles slightly smaller than the ideal value.

Steric number	Electron-domain geometry	Ideal angle	Molecular shapes (by lone pairs)
2	Linear	180°	linear
3	Trigonal planar	120°	trigonal planar (0), bent (1)
4	Tetrahedral	109.5°	tetrahedral (0), trigonal pyramidal (1), bent (2)
5	Trigonal bipyramidal	90° and 120°	trigonal bipyramidal (0), seesaw (1)
6	Octahedral	90°	octahedral (0), square planar (2)

Polarity

A bond between two different atoms is polar. A whole molecule is polar only if those bond dipoles do not cancel. When the shape is symmetric and all the outer atoms are the same, the dipoles cancel and the molecule is nonpolar, as in carbon dioxide or methane. When a lone pair makes the shape lopsided, as in water or ammonia, the dipoles do not cancel and the molecule is polar.

Count the domains

Bonding groups plus lone pairs on the central atom give the steric number, which sets the geometry.

Atoms, not pairs

The molecular shape describes where the atoms sit; the lone pairs are invisible but still push the atoms into place.

Symmetry sets polarity

If the bond dipoles cancel by symmetry the molecule is nonpolar; a lone pair that breaks the symmetry makes it polar.

Apparatus

The equipment a real molecular-shape experiment uses. In the simulation these are modelled for you, but the readings correspond to what each instrument would measure.

Molecular model kit

Builds the three-dimensional shape of each molecule.

VSEPR balloon model

Shows how electron pairs spread out to set the geometry.

Protractor

Measures the bond angles of the built shapes.

Periodic table chart

Gives valence-electron counts for the Lewis structures.

Lone-pair model

Distinguishes bonding pairs from lone pairs that shape the molecule.

Sample vials

Hold real substances whose shapes are being modelled.

Instructions — Running the Virtual Experiment

This is a predict, reveal, and compare lab. For each molecule you predict the answer yourself, enter it, and only then does the simulation reveal the result so you can compare. Work through at least six of the molecules and record every prediction in your worksheet.

Part A — Lewis Structure and Electron Count (Lewis & Domains tab)

Open Simulation → Lewis & Domains and choose a molecule. Draw its Lewis structure on paper.

Predict the total number of valence electrons, the number of bonding groups on the central atom, and the number of lone pairs on the central atom, enter all three, and click Check.

Part B — Electron-Domain Geometry (VSEPR Geometry tab)

Open VSEPR Geometry. From your counts, predict the steric number, the electron-domain geometry, and the actual bond angle, then click Check. The lab draws the domain arrangement and reveals both the ideal angle for that geometry and the actual bond angle, showing how lone pairs squeeze it below the ideal (for example 104.5° in water and 107° in ammonia, below the ideal 109.5°).

Part C — Molecular Shape and Polarity (Shape & Polarity tab)

Open Shape & Polarity. Predict the molecular shape (ignoring the lone pairs) and whether the molecule is polar or nonpolar, then click Check. The shape, with its lone pairs, is then drawn for you.

For your reportInclude your Lewis structures, a table of your predicted and revealed geometry and polarity for each molecule, screenshots, and a short discussion of how lone pairs change the molecular shape and the bond angle.

Simulation — The VSEPR Bench

Molecule	Your VE	VE	Your groups	Bonding groups	Your LP	LP
No rows yet — choose a molecule, predict the counts, and check.

Molecule

Choose

Total valence electrons

Bonding groups on central atom (atoms bonded; a double or triple bond counts as one)

Lone pairs on central atom

Bond type present

Actual bonding pairs (double = 2, triple = 3)

Central atom: C

Total valence electrons— hidden

Bonding groups— hidden

Bond type— hidden

Actual bonding pairs— hidden

Lone pairs (central)— hidden

Molecule

Choose

Your steric number (bonds + lone pairs)

Electron-domain geometry

Actual bond angle

Result

Steric number— hidden

Electron geometry— hidden

Ideal angle— hidden

Actual bond angle— hidden

Molecule

Choose

Molecular shape

Polarity

Result

Molecular shape— hidden

Polarity— hidden

violet lobe = lone pair · teal = bonded atom · drag to rotate, scroll to zoom

3D model

Example molecule

Build your own

Central atom (label only)

Bonding groups (X)

Lone pairs (E)

Result

Shape—

Ideal angle—

Steric number—

Hybridisation—

Team Questions

Question 1. What does VSEPR stand for? (valence shell electron pair ___)

Question 2. What is the steric number of the carbon atom in CH₄? (one number)

Question 3. What is the molecular shape of water, H₂O? (one word)

Question 4. CO₂ and H₂O both have two bonded atoms, yet only one is polar. Which one is polar? (formula)

Question 5. The electron-domain geometry of NH₃ is tetrahedral, but its molecular shape is what? (two words)

Question 6. What is the ideal bond angle for a tetrahedral electron-domain geometry? (in degrees)

Question 7 — Challenge. XeF₄ has six electron domains, two of them lone pairs. What is its molecular shape? (two words)

Example Lab Report

A worked example showing the expected format and the predict, reveal, and compare workflow.

Molecular Shape

Chemistry | Section: [Your Section] | Date: [Date]

Lab Members: [Names of all members present]

Objective

To draw Lewis structures, predict the electron-domain geometry and molecular shape of a set of molecules using VSEPR, and decide the polarity of each, comparing every prediction with the simulation.

Results Table (worked example)

Molecule	Bonds / LP	Steric no.	Electron geometry	Molecular shape	Polar?
CO₂	2 / 0	2	Linear	Linear	No
BF₃	3 / 0	3	Trigonal planar	Trigonal planar	No
H₂O	2 / 2	4	Tetrahedral	Bent	Yes
NH₃	3 / 1	4	Tetrahedral	Trigonal pyramidal	Yes
CH₄	4 / 0	4	Tetrahedral	Tetrahedral	No
SF₆	6 / 0	6	Octahedral	Octahedral	No

Worked example for water: O contributes 6 valence electrons and each H contributes 1, giving 8 in total. The central oxygen has 2 bonding groups and 2 lone pairs, so the steric number is 4. Four domains give a tetrahedral electron geometry with an ideal angle of 109.5°, but with two lone pairs the molecular shape is bent and the angle is squeezed to about 104.5°. Because the two O–H dipoles do not cancel, water is polar.

Discussion and Conclusion

Every prediction agreed with the simulation. The steric number set the electron-domain geometry, and removing the lone pairs gave the molecular shape: the same tetrahedral arrangement produced tetrahedral methane, trigonal pyramidal ammonia, and bent water as lone pairs were added. Symmetric molecules with identical outer atoms were nonpolar, while lone pairs that broke the symmetry made the molecule polar.

Practice Questions

Question 1

Predict the electron-domain geometry, molecular shape, and polarity of SO₂.

Hint: S has 2 bonding groups and 1 lone pair, so steric number 3, trigonal planar electron geometry, bent shape, and the molecule is polar.

Question 2

How many valence electrons are in BF₃, and what is its molecular shape?

Hint: B(3) + 3 × F(7) = 24 electrons; with 3 bonding groups and no lone pairs the shape is trigonal planar.

Question 3

What is the molecular shape of PCl₅, and what are its bond angles?

Hint: 5 bonding groups, no lone pairs, steric number 5, trigonal bipyramidal, with angles of 90° and 120°.

Question 4

SF₄ has 4 bonding groups and 1 lone pair. Give its electron geometry and molecular shape.

Hint: steric number 5, trigonal bipyramidal electron geometry, seesaw molecular shape; the molecule is polar.

Question 5 — Challenge

Explain why CH₄ is nonpolar but NH₃ is polar, even though both have a steric number of 4.

Hint: methane is symmetric, so its four bond dipoles cancel; ammonia has a lone pair that makes it trigonal pyramidal, so its dipoles do not cancel.

Molecular Shape

Theory — Valence Shell Electron Pair Repulsion

From Lewis structure to domains

Electron-domain geometry and molecular shape

Polarity

Count the domains

Atoms, not pairs

Symmetry sets polarity

Apparatus

Instructions — Running the Virtual Experiment

Simulation — The VSEPR Bench

Molecule

Molecule

Molecule

3D model

Build your own

Team Questions

Example Lab Report

Molecular Shape

Objective

Results Table (worked example)

Discussion and Conclusion

Practice Questions

Welcome — one quick step before you start