Molecular vs. Electron Geometry

Imagine you are arranging five people around a circular table. If all five are visible guests, you would describe the seating arrangement based on all five positions. But what if two of those seats are occupied by invisible people? The physical chairs are still there, still taking up space, still forcing the visible guests to adjust - but when someone asks you to describe the arrangement, you only describe where the visible people are sitting. That is the difference between electron geometry and molecular geometry.

Electron geometry describes the arrangement of all electron groups around a central atom - bonding pairs and lone pairs alike. Molecular geometry describes only where the atoms are. The lone pairs are still present and still influencing shape, but they are invisible when naming the molecular geometry.

Why Lone Pairs Compress Bond Angles

Not all electron groups are equal. A lone pair of electrons sits closer to the central atom than a bonding pair does, because it is not being shared with another nucleus that would pull it outward. Because the lone pair is held closer, its electron cloud spreads out more and demands extra space.

The key consequence: every lone pair on a central atom compresses the bond angles between the remaining bonded atoms. A perfect tetrahedral angle is 109.5 degrees, but add one lone pair (as in NH3) and the angles drop to about 107 degrees. Add two lone pairs (as in H2O) and they fall further to about 104.5 degrees.

The Two-Step Process

To determine molecular geometry on the MCAT, always follow two steps:

1. Count all electron groups around the central atom (bonds + lone pairs). This gives you the electron geometry.
2. Ignore the lone pairs and describe only the positions of the atoms. This gives you the molecular geometry.

A single bond, double bond, and triple bond each count as one electron group. Only the number of positions matters for geometry, not the bond order within each position.

Geometries Derived from Tetrahedral (4 Electron Groups)

When a central atom has four electron groups, the electron geometry is always tetrahedral. The molecular geometry depends on how many of those groups are lone pairs.

CH4 (methane): Four bonding pairs, zero lone pairs. Electron geometry = tetrahedral. Molecular geometry = tetrahedral. Bond angles are a perfect 109.5 degrees.

NH3 (ammonia): Three bonding pairs plus one lone pair. Electron geometry = tetrahedral. Molecular geometry = trigonal pyramidal. The lone pair squeezes the H-N-H angles down to about 107 degrees.

H2O (water): Two bonding pairs plus two lone pairs. Electron geometry = tetrahedral. Molecular geometry = bent. Two lone pairs compress the H-O-H angle even further to about 104.5 degrees.

Geometries Derived from Trigonal Bipyramidal (5 Electron Groups)

Five electron groups create a trigonal bipyramidal electron geometry with two distinct positions: three equatorial positions (in the plane, 120 degrees apart) and two axial positions (above and below, 90 degrees from equatorial). Lone pairs always occupy equatorial positions first because there is more room there.

SF4 (sulfur tetrafluoride): Four bonding pairs plus one equatorial lone pair. The resulting shape looks like a playground seesaw - two atoms up and down (axial) and two out to the sides (equatorial), with a gap where the lone pair sits.

ClF3 (chlorine trifluoride): Three bonding pairs plus two equatorial lone pairs. The remaining three atoms form a T-shape - one axial up, one axial down, and one equatorial out to the side.

XeF2 (xenon difluoride): Two bonding pairs plus three equatorial lone pairs. The three lone pairs fill the entire equatorial plane, leaving only the two axial fluorines. Molecular geometry = linear, even though the electron geometry is trigonal bipyramidal. This is a favorite MCAT question - a molecule that looks linear but has five electron groups.

Geometries Derived from Octahedral (6 Electron Groups)

Six electron groups form an octahedral electron geometry where all positions are equivalent (90 degrees apart). When lone pairs appear, they position themselves opposite each other to minimize repulsion.

BrF5 (bromine pentafluoride): Five bonding pairs plus one lone pair. Remove one vertex from the octahedron and you get a square base with one atom on top - square pyramidal.

XeF4 (xenon tetrafluoride): Four bonding pairs plus two lone pairs. The two lone pairs sit opposite each other (trans positions, both axial), leaving four fluorines in a flat square - square planar. This is another heavily tested geometry.

The Complete Geometry Reference

Here is a summary of all five base electron geometries and every molecular geometry that derives from them:

Notice that “bent” appears twice - once from trigonal planar (3 groups, 1 LP, ~120 degrees) and once from tetrahedral (4 groups, 2 LP, ~104.5 degrees). Similarly, “linear” appears in multiple rows. Always specify the electron geometry if asked to distinguish them.

Common MCAT Traps

• Confusing electron geometry with molecular geometry. Water has a tetrahedral electron geometry but a bent molecular geometry. If an answer choice says “tetrahedral” for water, it is only correct if the question specifically asks about electron geometry.
• Forgetting that multiple bonds count as one group. CO2 has two double bonds, giving it two electron groups - not four. Its electron geometry and molecular geometry are both linear.
• Assuming bent always means the same angle. Bent from a trigonal planar parent has angles near 120 degrees (like SO2), while bent from a tetrahedral parent has angles near 104.5 degrees (like H2O).