Watch electron geometry and molecular shape come apart as you add lone pairs.
Molecular Geometry simulator
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This simulation needs a modern browser with canvas support. The measurements and formula below still describe the physics.
Live measurements
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Formula
SN = X + E
- SN Steric number
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Observations
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Real-world examples
Carbon dioxide
Two bonding pairs and no lone pairs. Steric number 2, electron geometry linear, molecular shape linear, angle 180°. The two polar bonds cancel, which is why CO2 has no dipole.
Water
Two bonding pairs and two lone pairs. Steric number 4, so the electrons sit tetrahedral, but the atoms make a bent shape at 104.5°. That is the case that shows the two names coming apart.
Ammonia
Three bonding pairs and one lone pair. Steric number 4 again, electron geometry tetrahedral, molecular shape trigonal pyramidal, angle 107°. One lone pair, one step of compression.
Xenon tetrafluoride
Four bonding pairs and two lone pairs. Steric number 6, electron geometry octahedral, molecular shape square planar. The two lone pairs sit opposite each other, which is why the fluorines land in a plane.
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Uses of molecular geometry
- Deciding whether a molecule with polar bonds has a net dipole
- Checking a VSEPR exam answer against both names, not one
- Explaining why CO2 is nonpolar and water is polar from the same kind of bond
molecular geometry pitfalls
- Reporting electron geometry where molecular shape was asked
- Forgetting lone pairs when counting the steric number
- Quoting 109.5 for water and ammonia
molecular geometry questions (8)
What is molecular geometry?
Molecular geometry is the arrangement of the bonded atoms around a central atom. It is not the arrangement of all the electron groups. Water has four electron groups, so the electrons sit tetrahedral, but only two of those groups are bonds, so the molecular shape is bent. The name you want on an exam is almost always this one, the shape of the atoms.
What is VSEPR theory?
VSEPR means valence shell electron pair repulsion. Electron groups around a central atom get as far from each other as they can, and the arrangement they settle into is the electron geometry. Linear, trigonal planar, tetrahedral, trigonal bipyramidal, octahedral: those five are the whole table for steric numbers 2 through 6. Molecular shape is then what is left after you ignore the lone pairs.
What is steric number?
Steric number is the count of electron groups around the central atom: bonding pairs plus lone pairs. Carbon dioxide is 2, water is 4, sulfur hexafluoride is 6. That integer picks the electron geometry. Used as a steric number calculator, this page is just X plus E, and the shape names follow from that sum.
What is the difference between electron geometry and molecular geometry?
Electron geometry counts every group, bonds and lone pairs. Molecular geometry, also called molecular shape, counts only the bonded atoms. They match when there are no lone pairs (methane is tetrahedral both ways) and they split as soon as a lone pair occupies a site (water, ammonia, xenon tetrafluoride). A chart that prints one name is conflating the two.
How do you find the molecular shape of a molecule?
Count bonding pairs and lone pairs on the central atom, add them to get the steric number, read the electron geometry from that number, then drop the lone pairs to name the shape. Water is AX2E2: steric number 4, tetrahedral electrons, bent atoms. The calculator does the table lookup. The step people skip is counting the lone pairs in the first place.
Why is water bent, not linear?
Oxygen has two bonds and two lone pairs, so four electron groups. Four groups sit tetrahedral, not in a line. The two hydrogen atoms occupy two of those four sites, so they sit 104.5° apart rather than 180°. Carbon dioxide is linear because its central carbon has no lone pairs. Same kind of polar bond, opposite shapes, which is why one molecule has a dipole and the other does not.
How do lone pairs affect bond angles?
Lone pairs take more room than bonding pairs, so they compress the angle between the bonds. On a tetrahedral electron geometry the ideal is 109.5°. One lone pair (ammonia) brings it to 107°. Two (water) bring it to 104.5°. Quoting 109.5 for either molecule is the ideal, not the molecule.
What is the bond angle of a tetrahedral molecule?
The ideal tetrahedral angle is 109.5°, and that is the angle in methane, where every group is a bonding pair. Ammonia and water share the same electron geometry and do not share that angle, because their lone pairs compress it. A bond angle calculator that always prints 109.5 for steric number 4 is printing the electron geometry, not the molecule.