VSEPR Steric Number Calculator
Add bonded atoms and lone pairs to get the steric number and the VSEPR electron-domain geometry of a central atom.
How it works
Steric number = number of bonded atoms + number of lone pairs on the central atom
The steric number counts the electron domains around a central atom — one for each bonded atom and one for each lone pair — and a multiple bond counts as a single domain. VSEPR theory says these domains spread out to minimise repulsion, so the steric number fixes the electron-domain geometry: 2 is linear, 3 trigonal planar, 4 tetrahedral, 5 trigonal bipyramidal and 6 octahedral. Lone pairs then push the visible molecular shape away from the ideal, but the electron geometry follows the steric number directly.
Worked example
In methane (CH₄) the carbon has 4 bonded atoms and 0 lone pairs, so the steric number is 4 — tetrahedral. In water the oxygen has 2 bonded atoms and 2 lone pairs, also steric number 4, giving a tetrahedral electron geometry but a bent molecular shape.
Sources: VSEPR theory — Wikipedia
Frequently asked questions
Do double bonds add to the steric number?
No. A double or triple bond is still one electron domain, so it counts as a single bonded atom. In CO₂ the carbon has two bonded oxygens and a steric number of 2 despite the double bonds.
What is the difference between electron and molecular geometry?
Electron geometry counts all domains including lone pairs; molecular geometry describes only the positions of the atoms. Water’s electron geometry is tetrahedral, but its molecular shape is bent because two domains are lone pairs.
How do I count lone pairs?
Draw the Lewis structure and count the non-bonding electron pairs on the central atom. Nitrogen in ammonia has one lone pair; oxygen in water has two.
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This calculator is for educational purposes. Double-check important results before acting on them.