Shapes of molecules (VSEPR): CO₂, BF₃, CH₄, NH₃, H₂O, PCl₅, SF₆, XeF₄

Turn eight molecules in 3D and see how bond pairs and lone pairs push each other apart: linear, trigonal planar, tetrahedral, pyramidal, bent, bipyramidal, octahedral and square planar shapes, bond angles, hybridisation and the net dipole. Free for every student.

Also searched as: vsepr · vsepr theory simulation · shapes of molecules class 11 · molecular geometry · hybridisation of molecules · bond angle of h2o nh3 ch4

About this experiment

Aim

To see how the shape of a molecule follows from the number of bond pairs and lone pairs around the central atom (VSEPR theory), and how shape decides bond angles, hybridisation and polarity.

Theory

VSEPR stands for valence shell electron pair repulsion. The electron pairs around the central atom (bond pairs and lone pairs) repel each other and so they stay as far apart as possible. The number of pairs (the steric number) decides the electron geometry: 2 pairs are linear, 3 trigonal planar, 4 tetrahedral, 5 trigonal bipyramidal and 6 octahedral.

The shape of the molecule is described by the position of the atoms only, not the lone pairs. CH₄, NH₃ and H₂O all have 4 electron pairs (tetrahedral arrangement), but CH₄ has 4 bond pairs (tetrahedral), NH₃ has 3 bond pairs and 1 lone pair (pyramidal) and H₂O has 2 bond pairs and 2 lone pairs (bent).

A lone pair is held by one nucleus only, so it spreads out and takes more space than a bond pair. The repulsion order is lone pair–lone pair > lone pair–bond pair > bond pair–bond pair. This squeezes the bond angle: 109.5° in methane, 107° in ammonia, 104.5° in water.

Formulas used

• steric number = bond pairs + lone pairs on the central atom
• 2 → linear 180° (sp) 3 → trigonal planar 120° (sp²) 4 → tetrahedral 109.5° (sp³)
• 5 → trigonal bipyramidal 90°, 120° (sp³d) 6 → octahedral 90° (sp³d²)
• lone pair–lone pair > lone pair–bond pair > bond pair–bond pair (repulsion)
• dipole moment μ = charge × distance (unit: debye, D)
• net dipole = vector sum of the bond dipoles

Procedure: what to do in the virtual lab

1. Choose CH₄ and drag the picture around with the mouse. Count the bond pairs and check the angle.
2. Now choose NH₃ and then H₂O. Switch the lone pairs on and off and see how the angle decreases when lone pairs are added.
3. Look at CO₂ and BF₃ with the dipole arrow on. Why is there no arrow?
4. Compare SF₆ (octahedral, no lone pairs) with XeF₄ (two lone pairs). Where do the lone pairs go?
5. Fill the table: molecule, bond pairs, lone pairs, shape, hybridisation, polar or non-polar.

Observations to make

• Molecules with the same number of electron pairs can have different shapes because of lone pairs.
• The yellow net dipole arrow points from the positive end to the negative end. It is zero when the molecule is symmetrical.
• In XeF₄ the two lone pairs sit above and below the plane, as far apart as possible, which gives a flat square molecule.

What you can change in this experiment

• Molecule: CO₂ (carbon dioxide) / BF₃ (boron trifluoride) / CH₄ (methane) / NH₃ (ammonia)
• Show lone pairs (blue clouds) (on / off)
• Show the net dipole (yellow arrow) (on / off)
• Turn the molecule slowly (on / off)

Viva questions on Shapes of molecules (VSEPR)

Questions an examiner may ask in the Class 11 chemistry practical. Run the experiment in the lab to find the answers yourself, then check them in the Learn tab.

1. What does VSEPR stand for? State the main idea.
2. Why is the H–O–H angle in water (104.5°) smaller than the tetrahedral angle?
3. Why is CO₂ non-polar but H₂O polar?
4. What is the hybridisation and shape of PCl₅ and SF₆?
5. Why is XeF₄ square planar?

Frequently asked questions

Syllabus: Class XI Chemistry, Chapter 4 — Chemical Bonding and Molecular Structure, §4.5 (VSEPR theory), §4.6 (valence bond theory, hybridisation)

What you do in the lab

Change the controls and watch the chemistry behave in real time in 3D. Live readouts and graphs show the numbers behind it, and the Learn tab has the theory, procedure, observations and viva questions. Works on a phone or a laptop, in English and Hindi.

Topics: VSEPR · molecular shape · bond angle · lone pair · hybridisation · dipole moment · polarity · tetrahedral · chemical bonding

More from this unit

• Covalent bond: sharing electrons, bond length and bond energy
• Ionic bond: how sodium and chlorine make NaCl

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