GenChemDrill

General Chemistry · Molecular shape

VSEPR molecular geometry practice, with answers

VSEPR says the electron groups around a central atom spread out as far apart as they can. Count those groups and you know the electron geometry. Then look only at where the atoms sit, ignoring lone pairs, and you have the molecular geometry: the shape the question is usually asking for.

Already know the basics? Jump to the 8 practice questions or the worked examples.

Four steps from formula to shape

  1. Draw the Lewis structure, or at least find the central atom's bonds and lone pairs. This is where most mistakes start, so count valence electrons carefully.
  2. Count electron domains on the central atom. Each bonded atom is one domain, whether the bond is single, double, or triple. Each lone pair is one domain.
  3. Name the electron geometry from the number of domains: 2 linear, 3 trigonal planar, 4 tetrahedral, 5 trigonal bipyramidal, 6 octahedral.
  4. Name the molecular geometry from the atoms only. Use the table below. If there are no lone pairs, the two geometries are the same.

Shortcut for central-atom lone pairs: start with the central atom's valence electrons. Add one for each negative charge on an ion, or subtract one for each positive charge. Subtract one for each single bond to a terminal atom and two for each double bond. Divide what is left by 2. For XeF4: (8 − 4) ÷ 2 = 2 lone pairs. For I3−: (7 + 1 − 2) ÷ 2 = 3. This works when the terminal atoms have zero formal charge. When in doubt, draw the full Lewis structure.

The VSEPR table: 2 to 6 electron domains

Electron domains, lone pairs, and the shapes they give
DomainsLone pairs on central atomElectron geometryMolecular geometryIdeal or typical angleExample
20LinearLinear180°CO2, BeCl2
30Trigonal planarTrigonal planar120°BF3, SO3
31Trigonal planarBenta bit under 120°SO2
40TetrahedralTetrahedral109.5°CH4, NH4+
41TetrahedralTrigonal pyramidalabout 107°NH3
42TetrahedralBentabout 104.5°H2O
50Trigonal bipyramidalTrigonal bipyramidal90°, 120°, 180°PCl5
51Trigonal bipyramidalSeesawunder 90° and under 120°SF4
52Trigonal bipyramidalT-shapedunder 90°ClF3
53Trigonal bipyramidalLinear180°I3−, XeF2
60OctahedralOctahedral90°, 180°SF6
61OctahedralSquare pyramidalunder 90°BrF5
62OctahedralSquare planar90°XeF4

Where lone pairs go with 5 domains: always in the equatorial positions (the triangle around the middle). Equatorial spots have only two neighbors at 90°, while axial spots have three, so a lone pair there feels less crowding. That is why one lone pair gives a seesaw, two give a T, and three give a straight line.

Where lone pairs go with 6 domains: all positions are equivalent for the first one (square pyramidal). A second lone pair goes directly opposite the first, 180° away, which leaves a square planar arrangement of atoms.

Lone pairs squeeze bond angles, and shape decides polarity

A lone pair is held by only one nucleus, so it spreads out closer to the central atom and pushes harder than a bonding pair. Repulsion strength runs lone pair–lone pair > lone pair–bond pair > bond pair–bond pair. Each lone pair squeezes the remaining bond angles below the ideal value:

  • CH4, no lone pairs: 109.5°.
  • NH3, one lone pair: about 107°.
  • H2O, two lone pairs: about 104.5°.

A molecule is polar if it has polar bonds and the bond dipoles don't cancel. Symmetric shapes with identical outer atoms cancel: linear AX2, trigonal planar AX3, tetrahedral AX4, trigonal bipyramidal AX5, octahedral AX6, and square planar AX4E2. Bent, trigonal pyramidal, seesaw, T-shaped, and square pyramidal shapes leave a net dipole when their bonds are polar. Different outer atoms (CH3Cl, for example) can make even a tetrahedral molecule polar.

Worked examples

1. NH3 versus BF3: same formula type, different shape

Both have a central atom bonded to three atoms. Why is NH3 pyramidal while BF3 is flat?

  1. NH3: N has 5 valence electrons. Three N–H bonds use 3, leaving 2, which is one lone pair. Domains: 3 bonds + 1 lone pair = 4.
  2. Four domains give a tetrahedral electron geometry. Ignore the lone pair when naming the shape: the three H atoms and N form a trigonal pyramid. The lone pair squeezes H–N–H to about 107°.
  3. BF3: B has 3 valence electrons, all used in three B–F bonds. No lone pairs, so 3 domains.
  4. Three domains give trigonal planar for both geometries, with 120° angles. The three B–F dipoles cancel, so BF3 is nonpolar. NH3's dipoles don't cancel, so it is polar.

Answer: NH3 has a lone pair (4 domains, trigonal pyramidal, polar); BF3 has none (3 domains, trigonal planar, nonpolar).

2. SF4: where the lone pair goes

Predict the electron geometry, molecular geometry, and polarity of SF4.

  1. Valence electrons: 6 + 4(7) = 34. Four S–F bonds use 8. The F atoms take 24 as lone pairs. 34 − 8 − 24 = 2 electrons left, one lone pair on S.
  2. Domains: 4 bonds + 1 lone pair = 5. Electron geometry: trigonal bipyramidal.
  3. The lone pair takes an equatorial position. Remaining atoms: two axial F and two equatorial F. That shape is called a seesaw.
  4. The lone pair pushes the bonds together, so the axial F–S–F angle is less than 180° and the equatorial F–S–F angle is less than 120°.
  5. The S–F dipoles don't cancel in a seesaw, so SF4 is polar.

Answer: trigonal bipyramidal electron geometry, seesaw molecular geometry, polar

3. XeF4: two lone pairs on opposite sides

Predict the shape and polarity of XeF4.

  1. Valence electrons: 8 + 4(7) = 36. Four Xe–F bonds use 8, the F lone pairs use 24, leaving 4 electrons: two lone pairs on Xe.
  2. Domains: 4 bonds + 2 lone pairs = 6. Electron geometry: octahedral.
  3. The two lone pairs sit opposite each other, 180° apart, to stay as far apart as possible. The four F atoms are left in a square around Xe.
  4. Molecular geometry: square planar, with 90° F–Xe–F angles.
  5. Opposite Xe–F dipoles cancel in pairs, so XeF4 is nonpolar even though it has lone pairs.

Answer: octahedral electron geometry, square planar molecular geometry, nonpolar

Want more molecules to practice on? Work through them with instant feedback in the full General Chemistry course.

Try 3 days free

3-day free trial, no card needed.

Practice questions

Pick an answer, then check it. A wrong answer tells you what went wrong, so you can try again. Every question also has a full solution.

Question 1What is the molecular geometry of SF4?

Show solution

34 valence electrons. Four S–F bonds (8) and F lone pairs (24) leave 2 electrons: one lone pair on S. Five domains give a trigonal bipyramidal electron geometry. The lone pair goes equatorial, so the four F atoms form a seesaw.

Question 2What is the molecular geometry of XeF4?

Show solution

36 valence electrons. Bonds (8) and F lone pairs (24) leave 4, two lone pairs on Xe. Six domains: octahedral electron geometry. The lone pairs go 180° apart, and the four F atoms lie in one plane: square planar.

Question 3What is the molecular geometry of ClF3?

Show solution

28 valence electrons. Three Cl–F bonds (6) and F lone pairs (18) leave 4: two lone pairs on Cl. Five domains: trigonal bipyramidal electron geometry. Both lone pairs take equatorial positions, so the atoms form a T. The F–Cl–F angles are a little under 90°. The molecule is polar.

Question 4What is the approximate H–N–H bond angle in NH3?

Show solution

N has three bonds and one lone pair: 4 domains, tetrahedral electron geometry, trigonal pyramidal shape. The lone pair repels more strongly than the bonding pairs, so the angle drops from 109.5° to about 107°.

Question 5Which of these molecules is nonpolar?

Show solution

All four have polar bonds, so shape decides. CO2 has 2 domains on C (two double bonds, no lone pairs) and is linear: the dipoles cancel. SO2, NH3, and H2O all have lone pairs on the central atom and asymmetric shapes, so each is polar.

Question 6What are the molecular geometry and approximate O–S–O bond angle of SO2?

Show solution

SO2 has 18 valence electrons. Whichever valid Lewis structure you draw, S is bonded to two O atoms and has one lone pair. A double bond still counts as one domain, so S has 3 domains: trigonal planar electron geometry. Ignoring the lone pair, the shape is bent. The lone pair squeezes the angle slightly below 120° (measured value about 119°).

Question 7Which description fits PCl5?

Show solution

P has 5 valence electrons, all used in five P–Cl bonds. Five domains and no lone pairs: trigonal bipyramidal for both the electron and molecular geometry. Angles: 120° in the equatorial triangle, 90° between axial and equatorial, 180° between the two axial bonds. It is nonpolar because the dipoles cancel.

Question 8What is the shape of the triiodide ion, I3−?

Show solution

Valence electrons: 3(7) + 1 = 22. Two I–I bonds use 4, the outer I atoms take 12 as lone pairs, leaving 6 on the central I: three lone pairs. Five domains: trigonal bipyramidal electron geometry. The three lone pairs take the equatorial positions, so the two outer I atoms are axial: a linear ion with a 180° angle.

Answer key

  1. B, seesaw: 4 bonds + 1 lone pair.
  2. D, square planar: 4 bonds + 2 lone pairs.
  3. A, T-shaped: 3 bonds + 2 lone pairs.
  4. C, about 107°: one lone pair compresses 109.5°.
  5. B, CO2: linear, bond dipoles cancel.
  6. D, bent, a little under 120°: 2 bonds + 1 lone pair.
  7. B, trigonal bipyramidal: 5 bonds, no lone pairs.
  8. C, linear: 2 bonds + 3 lone pairs.

Frequently asked questions

What is the difference between electron geometry and molecular geometry?

Electron geometry describes where all the electron domains point, lone pairs included. Molecular geometry describes only where the atoms are. They match when the central atom has no lone pairs.

Does a double or triple bond count as more than one electron domain?

No. Any bond to one atom counts as one domain for predicting shape, whether it is single, double, or triple. Multiple bonds can change bond angles slightly, but not the shape name.

Why do lone pairs go in equatorial positions in a trigonal bipyramid?

An equatorial position has two neighbors at 90 degrees, while an axial position has three. Putting the bulky lone pairs equatorial keeps the strongest repulsions to a minimum.

Is VSEPR on the MCAT?

Yes. The AAMC content outline includes molecular structure and VSEPR among the general chemistry bonding topics, along with polarity. Expect to name common shapes and predict polarity rather than work with rare geometries.

Sources

Ready for the rest of college General Chemistry? Keep practicing with instant feedback on every chapter.

Try 3 days free

3-day free trial, no card needed.