Draw methane, CH₄, on paper and the easy picture is a flat cross: a carbon in the middle and a hydrogen at each point, 90° apart. The real molecule is not flat. Its four hydrogens sit at the corners of a tetrahedron, a pyramid with four triangular faces, and every H–C–H angle is 109.5°.
This section turns the flat Lewis structures of the previous sections into objects in space.
The idea
Methane's four C–H bonds are four pairs of electrons. Electrons repel each other, so the four pairs move as far apart as they can. Around one atom, the arrangement that puts four things farthest apart is a tetrahedron.
That is VSEPR, short for valence shell electron pair repulsion: the electron groups around a central atom arrange themselves as far apart as possible. An electron group is either a bond to one neighbor or a lone pair. A single, double or triple bond counts as one group, because it points in one direction.
- Methane, CH₄: the carbon has four single bonds, so 4 groups.
- Formaldehyde, H₂C=O: the carbon has two C–H bonds and one C=O, so 3 groups.
- Carbon dioxide, O=C=O: the carbon has two double bonds, so 2 groups.
This is the same count you used for hybridization in the Hybridization section. Hybridization tells you which orbitals the central atom uses. VSEPR tells you the shape those orbitals settle into. They are two descriptions of one physical arrangement.
One point is easy to miss. The name of a shape describes only where the atoms are. A lone pair still takes a corner and still pushes, but it is invisible in the name.
Drawing three dimensions on paper
Look again at methane in the figure above. Two of its C–H bonds are plain lines: they lie in the plane of the paper. One is a solid wedge, which widens toward you: that bond comes out of the page. One is a dashed wedge: that bond goes behind the page.
That is the standard convention for a tetrahedral center. A correctly drawn center has exactly two plain bonds, one solid wedge and one dashed wedge, with the wedge and dash adjacent to each other rather than opposite.
Two mistakes fail in different ways. Swap which bond is the wedge and which is the dash, and you draw the mirror image of the molecule. That is a legal structure; whether it is a different molecule is the subject of Stereochemistry. Put the wedge and the dash on opposite sides, and the two plain bonds end up opposite each other too, 180° apart. No two bonds of a tetrahedron are 180° apart, so that drawing shows no tetrahedron at all.
Practice reading these drawings now. In Stereochemistry you will assign R and S configurations from them, and that whole exercise depends on reading wedges and dashes correctly. The 3D viewer tool lets you rotate real structures, which is the fastest way to see that a flat drawing is a projection of something solid.
The shapes you need
| Groups | Lone pairs | Shape | Angle | Example |
|---|---|---|---|---|
| 2 | 0 | Linear | 180° | CO₂ |
| 3 | 0 | Trigonal planar | 120° | BF₃, an alkene carbon |
| 3 | 1 | Bent | <120° | SO₂ (rare for carbon) |
| 4 | 0 | Tetrahedral | 109.5° | CH₄ |
| 4 | 1 | Trigonal pyramidal | 107° | NH₃ |
| 4 | 2 | Bent | 104.5° | H₂O |
Methane, CH₄. Four bonds, no lone pairs. All four groups are atoms, so the shape is tetrahedral, 109.5°.
Ammonia, NH₃. Three bonds plus one lone pair: still four groups, so still a tetrahedral arrangement. The name counts only the three N–H bonds. With the nitrogen at the top and the three hydrogens below, they form a trigonal pyramidal shape, like a tripod.
Water, H₂O. Two bonds plus two lone pairs: four groups again. Only the two O–H bonds count in the name, so the shape is bent.
All three are sp³, because all three have four groups.
Reading geometry off an organic structure
In practice you rarely apply VSEPR to a whole molecule. You apply it atom by atom, at whichever center you care about, and a molecule of any size has several different local geometries at once.
Take ethanol, CH₃CH₂OH. The methyl carbon has 4 groups: tetrahedral. The CH₂ carbon has 4 groups: tetrahedral. The oxygen has 2 bonds plus 2 lone pairs: bent, with a C–O–H angle near 108°.
Now take acetic acid, CH₃C(=O)OH. The methyl carbon is tetrahedral. The carbonyl carbon has 3 groups and is trigonal planar, 120°. The C=O oxygen has one double bond and two lone pairs: 3 groups. The hydroxyl oxygen has two neighbors, so its shape is bent. By the Hybridization section's rule for a lone pair beside a π bond, that oxygen is sp²: one of its lone pairs overlaps the C=O π bond next door. Its shape is bent either way. The molecule as a whole has no single shape — it has a shape at every atom.
The trigonal planar carbons are the flat parts, and flatness has a consequence. Take the carbon of formaldehyde, H₂C=O. Its three groups lie in one plane, so the carbon has two faces, one above that plane and one below, and both are open. Anything approaching can arrive from above or from below. Unless something else in the molecule makes the two sides different, both routes are equally available.
A tetrahedral carbon offers no such choice. All four of its positions are already occupied, so nothing can add to it unless one of its groups leaves.
That difference, a site with two open faces against a site with none, decides the three-dimensional outcome of a great many reactions later on, and every one of them traces back to a group count you can do in five seconds.
Beyond four groups
Phosphorus pentachloride, PCl₅, has five P–Cl bonds and no lone pair on phosphorus: five groups. They take a trigonal bipyramidal shape, two three-sided pyramids joined at their base. Three chlorines sit around the middle in one plane, 120° apart. These are the equatorial positions, around the "equator". The other two point straight up and straight down, at 90° to the middle three. These are the axial positions, along the axis.
Swap one of the five groups for a lone pair and the name changes, just as it did for four groups. Sulfur tetrafluoride, SF₄, has four S–F bonds and one lone pair on sulfur. The lone pair takes an equatorial position, where it has fewer neighbors at 90°, and the four fluorines form a seesaw.
Six groups, as in sulfur hexafluoride, SF₆, give an octahedral shape: four fluorines around the middle, one above and one below, with every neighboring pair at 90°. All six positions are equivalent, so here "axial" and "equatorial" only name where each fluorine sits in the drawing.
These shapes are rare in day-to-day organic chemistry but worth recognizing in sulfur and phosphorus compounds. Why period 3 atoms can have five or six groups at all, and period 2 atoms never can, is set out once in the Lewis Structures section under the octet-rule exceptions; nothing in this section depends on it beyond the shapes themselves.
What carries forward
Geometry is the input to the next section, Bond Polarity, where bond dipoles either cancel or add depending on shape. It is the whole of Alkanes & Conformations, where the question is which rotational and ring shapes a molecule actually adopts, and why a ring prefers one shape over another.
It is the foundation of Stereochemistry, since chirality, the difference between a molecule and its mirror image, is a purely geometric property. That difference is why an enzyme can bind a drug and do nothing with the drug's mirror image.
And it explains mechanism after mechanism in Substitution & Elimination and beyond: the backside attack of SN2, where the new group arrives opposite the one leaving and turns the carbon inside out like an umbrella in the wind; the anti-periplanar requirement of E2, where the two bonds that break must point in opposite directions; and the face selectivity of carbonyl addition, which is the two open faces above at work.