VSEPR shape explorer
Electron pairs around a central atom push each other as far apart as possible — that's the whole idea. But a lone pair pushes harder than a bonding pair, so pick a molecule and watch the real 3D shape. Same number of electron domains, wildly different bond angles.
The central atom sits at the origin. A cabinet-projection camera angle simulates 3D on this 2D canvas, and the molecule slowly turns so you can see it from every side — pause anytime to read angles precisely. Bonded atoms are solid circles joined by a bond line; lone pairs are the pale dashed lobes with no bond line, since there's no second atom there.
The rules you're seeing
The core VSEPR idea
Electron domains — bonding pairs and lone pairs alike — repel each other around a central atom and arrange themselves to be as far apart as possible. That resulting 3D arrangement is the electron-pair geometry.
Electron geometry vs molecular geometry
These aren't always the same thing. Electron geometry counts all electron domains, including lone pairs. Molecular geometry — the shape chemists actually name, like "bent" or "pyramidal" — only describes where the atoms are. Lone pairs stay invisible in the name even though they still push the atoms around.
Why lone pairs shrink bond angles
A lone pair is attracted to only one nucleus — the central atom's — so its electron cloud spreads out more than a bonding pair's, which is pulled toward two nuclei. That extra "elbow room" pushes neighbouring bonding pairs closer together — why NH₃ (107°) and H₂O (104.5°) both come in under the ideal tetrahedral 109.5°.
Same electron count, different shape
CH₄, NH₃ and H₂O all have exactly 4 electron domains — tetrahedral electron geometry — but look completely different because they carry 0, 1 and 2 lone pairs respectively. The molecular shape name changes even though the underlying electron arrangement doesn't.
Part of the Chemical Bonding and Molecular Structure chapter — read the notes, grab the formula sheet and take the quiz. One of Priodemy for School, free with every EduSuite school.
Predicting shape from electron pairs
One principle, many shapes
VSEPR rests on a single idea: electron pairs in the valence shell repel one another, so they arrange themselves as far apart as possible. Count the groups around the central atom, spread them out, and the shape follows. Two groups go to opposite sides at 180° giving a linear arrangement; three spread into a trigonal plane at 120°; four move into three dimensions as a tetrahedron at 109.5°; five give a trigonal bipyramid and six an octahedron.
A double or triple bond counts as a single group for this purpose, because its electrons occupy one region of space. This is why CO₂, with two double bonds and no lone pairs, is linear.
Electron geometry is not molecular geometry
This distinction accounts for most of the marks in the topic. The electron geometry describes where all the groups sit, lone pairs included. The molecular geometry describes only where the atoms are, because that is what we can observe — lone pairs are invisible to a structural measurement.
Compare three molecules that all have four electron groups and therefore identical tetrahedral electron geometry. Methane has four bonding pairs and no lone pairs, so it is tetrahedral. Ammonia has three bonds and one lone pair, so the atoms form a trigonal pyramid. Water has two bonds and two lone pairs, so the atoms lie in a bent shape. Same underlying arrangement, three different observed shapes. Toggle the lone pairs in the explorer and watch the molecular shape change while the electron geometry does not.
Why lone pairs squeeze the angles
A lone pair is held by only one nucleus, so it spreads out closer to the central atom and takes up more room than a bonding pair, which is pulled between two nuclei. The repulsion order is therefore lone-pair–lone-pair > lone-pair–bond-pair > bond-pair–bond-pair.
The consequence is visible in the bond angles. Methane holds the ideal 109.5°. Ammonia, with one lone pair pressing down, closes to about 107°. Water, with two, closes further to about 104.5°. The progression is not arbitrary — each additional lone pair squeezes the remaining bonds a little more.
Mistakes that cost marks
Reporting the electron geometry when asked for shape. "What is the shape of NH₃" expects trigonal pyramidal, not tetrahedral. Read which one the question wants.
Counting a multiple bond as two or three groups. One bond, however many pairs it contains, occupies one region and counts once.
Assuming polar bonds make a polar molecule. CO₂ has two strongly polar bonds but is non-polar, because its linear symmetry makes the dipoles cancel. Water's bent shape prevents cancellation, which is why it is polar. Shape decides polarity, not bond type alone.