VSEPR Theory
Definition and meaning of VSEPR Theory in chemistry.
VSEPR theory, short for valence shell electron pair repulsion, is a model that predicts the three-dimensional shape of a molecule. It rests on one simple idea. The groups of electrons around a central atom all repel each other, so they spread out as far apart as they can.
In more detail
Electrons carry a negative charge, and negative charges push each other away. Around a central atom those electrons sit in groups. Some groups are bonding pairs, which join the central atom to another atom.
Others are lone pairs, which stay on the central atom and bond to nothing. VSEPR counts both kinds, then arranges them as far apart as possible. The number of groups sets the basic arrangement.
Two groups point in opposite directions, giving a linear shape with 180 degree angles. Three groups spread into a flat triangle at 120 degrees. Four groups form a tetrahedron at 109.5 degrees.
Five groups give a trigonal bipyramid, and six give an octahedron. There is a catch that trips up almost everyone. The arrangement of the electron groups is not always the shape of the molecule.
Shape describes where the atoms sit, and lone pairs are invisible. Methane, ammonia, and water all have four electron groups, so all three share a tetrahedral arrangement. But methane has no lone pairs and is a tetrahedron, ammonia has one lone pair and looks like a pyramid, and water has two lone pairs and looks bent.
Lone pairs also squeeze the angles. A lone pair is held by just one nucleus, so it spreads out and takes up more room than a bonding pair. That extra push shrinks the bond angle from 109.5 degrees in methane to about 107 degrees in ammonia and about 104.5 degrees in water.
One more rule matters: a double or triple bond counts as a single electron group, not as two or three.
Key facts
| Full Name | Valence shell electron pair repulsion theory |
|---|---|
| Core Idea | Electron groups repel and spread as far apart as they can |
| What It Predicts | The three-dimensional shape of a molecule |
| Two Groups | Linear, 180 degrees |
| Three Groups | Trigonal planar, 120 degrees |
| Four Groups | Tetrahedral, 109.5 degrees |
| Lone Pair Effect | Takes up more room, so it shrinks bond angles |
| Multiple Bonds | A double or triple bond counts as one group |
| Field | General Chemistry |
Carbon dioxide, CO2, has a carbon atom joined to two oxygen atoms by double bonds. Each double bond counts as one electron group, and the carbon has no lone pairs, so there are only two groups in total. Two groups push to opposite sides of the atom, so the molecule is linear with a bond angle of 180 degrees. This is also why carbon dioxide is nonpolar overall, even though each carbon to oxygen bond is polar.
Frequently asked questions
What does VSEPR stand for?
It stands for valence shell electron pair repulsion. The valence shell is the outer layer of electrons, and those electron pairs repel one another. Chemists usually say it out loud as vesper.
Why are water and methane different shapes if both have four electron groups?
Because shape describes where the atoms are, and lone pairs cannot be seen. Methane has four bonding pairs and no lone pairs, so it is a tetrahedron. Water has two bonding pairs and two lone pairs, so only the bent arrangement of atoms shows.
Does a double bond count as two electron groups?
No, it counts as one. VSEPR cares about how many separate directions electrons stick out from the central atom, not how many electrons are in each direction. That is why carbon dioxide, with two double bonds, is linear.
Related terms
Sources & references
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