Chemistry · General chemistry I · Worked example
Predict the shape of ammonia with VSEPR
Predict the electron geometry, molecular geometry and polarity of ammonia, NH₃.
Start from the Lewis structure
5 + 3(1) = 8 electrons: three N–H bonds use 6, and the last 2 form a lone pair on nitrogen.
Count the domains
Three bonds and one lone pair make four electron domains around nitrogen.
Find the electron geometry
Four domains point to the corners of a tetrahedron.
Name the molecular geometry
Only three corners hold atoms; the fourth holds the lone pair. The atoms form a trigonal pyramid, with nitrogen at the apex, and the H–N–H angles are a little less than 109.5° because the lone pair pushes the bonds together.
Decide the polarity
The three N–H bond dipoles all tilt toward nitrogen, the more electronegative atom, and the pyramid keeps them from cancelling: ammonia is polar.
Result
Electron geometry tetrahedral; molecular geometry trigonal pyramidal; polar.
Your turn
Predict the electron geometry and molecular geometry of water, H₂O.
Show the answer and explanation
Tetrahedral electron geometry; bent molecular geometry.
Oxygen has two bonds and two lone pairs: four domains in a tetrahedral arrangement. Only two positions hold atoms, so the molecule is bent, with an angle below 109.5°.
Keep exploring
In VSEPR & molecular polarity, add a fourth hydrogen, remove the lone pair and set the charge to +1. The ammonium ion, NH₄⁺, is tetrahedral, with the ideal 109.5° angles.
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