electronic geometry
Summary
Electronic geometry is the arrangement of all electron domains (bonding groups and lone pairs) around a central atom, predicted by VSEPR. It counts lone pairs, unlike molecular geometry, which describes only the positions of the atoms.
Detail
Meaning: Electron domains repel one another, so they spread as far apart as possible (VSEPR). A domain is a bond (single, double, or triple counts as one) or a lone pair on the central atom. The number of domains sets the electronic geometry: 2 linear (180°), 3 trigonal planar (120°), 4 tetrahedral (109.5°), 5 trigonal bipyramidal, 6 octahedral (90°).
Equation: Steric number = (atoms bonded to central atom) + (lone pairs on central atom). Both terms are counts, so the steric number is dimensionless.
On the MCAT: Questions give a formula and ask for electronic geometry, molecular geometry, bond angle, hybridization, or polarity. Draw the Lewis structure, count domains, then ignore the lone pairs when naming the molecular geometry. CH₄, NH₃ and H₂O all have tetrahedral electronic geometry, but their molecular geometries are tetrahedral, trigonal pyramidal and bent. Lone pairs repel more strongly than bonding pairs, so bond angles shrink below 109.5°.
Common confusion: Students give the molecular shape when asked for electronic geometry. Electronic geometry counts lone pairs; molecular geometry names only the atoms' positions. The two are the same when the central atom has no lone pairs. A double or triple bond is one domain, so CO₂ has two domains and is linear.
Sources
- Chemistry: The Central Science
- Chemistry (Zumdahl)
- General Chemistry (Petrucci)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.