Electron geometry (electron-domain geometry) describes the three-dimensional arrangement of regions of electron density around a central atom. In methane, CH4, the central carbon has four equivalent bonding regions (four C–H single bonds) and no lone pairs, so the electron geometry is tetrahedral.
The key count is the steric number: \( \text{steric number} = N_{\text{bonding domains}} + N_{\text{lone pairs}} \). For CH4, \(4 + 0 = 4\), which corresponds to tetrahedral electron geometry.
Lewis structure and electron domains
Methane contains one carbon atom and four hydrogen atoms. Valence-electron accounting supports four single bonds around carbon:
\[ N_{\text{valence}} = 4\;(\text{from C}) + 4 \times 1\;(\text{from four H}) = 8 \]
Four C–H single bonds require \(4 \times 2 = 8\) electrons, so all valence electrons are used in bonding and carbon has no lone pairs in CH4. Each single bond counts as one electron domain in VSEPR.
- Electron domain: a region of high electron density around the central atom (one single bond, one multiple bond, or one lone pair each counts as one domain).
- Bonding domain in CH4: four C–H single bonds, giving four bonding domains.
- Lone pairs on carbon in CH4: none, so there are no nonbonding domains.
VSEPR geometry for methane
VSEPR theory treats electron domains as repelling one another and arranging to maximize separation. With four domains, the maximum-separation arrangement is a tetrahedron (all H–C–H angles equal in the ideal case).
| Steric number | Electron domains around central atom | Electron geometry | Ideal bond angles | Representative examples |
|---|---|---|---|---|
| 2 | 2 bonding, 0 lone pairs | Linear | \(\approx 180^\circ\) | CO2 (about C), BeCl2 |
| 3 | 3 domains total | Trigonal planar | \(\approx 120^\circ\) | BF3 (about B), NO3− (about N) |
| 4 | 4 domains total | Tetrahedral | \(\approx 109.5^\circ\) | CH4, NH4+, CCl4 |
Molecular geometry compared with electron geometry
Electron geometry counts all electron domains (bonding and lone pairs). Molecular geometry describes the arrangement of atoms only (bond directions). In methane, the two geometries coincide because there are no lone pairs on the central carbon:
- Electron geometry (CH4): tetrahedral (four electron domains).
- Molecular geometry (CH4): tetrahedral (four atoms bonded to carbon).
- Bond angle (ideal): H–C–H \(\approx 109.5^\circ\).
Visualization: tetrahedral arrangement for CH4
Related language in general chemistry
The terms electron geometry, electron-domain geometry, and VSEPR geometry refer to the same counting-based classification. For methane, the tetrahedral arrangement is also consistent with sp3 hybridization on carbon, reflecting four equivalent σ bonds directed toward the corners of a tetrahedron.
Common pitfalls
- Electron geometry vs molecular geometry: lone pairs change molecular geometry but still count toward electron geometry; CH4 has none, so both are tetrahedral.
- Domain counting with multiple bonds: a double or triple bond counts as one electron domain; methane contains only single bonds.
- Angle expectations: \(109.5^\circ\) is the ideal tetrahedral angle; real molecules can deviate slightly due to substituent effects, but CH4 is very close to ideal.