Enter the number of atoms bonded to the central atom (B) and the number of lone pairs on the central atom (N) into the calculator to determine the molecular geometry.
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VSEPR Theory Formula
The following relationship is used in VSEPR (Valence Shell Electron Pair Repulsion) Theory to find the steric number (the number of electron domains) around a central atom.
SN = B + N
Variables:
- SN is the steric number (total number of electron domains around the central atom)
- B is the number of bonding domains on the central atom (often the number of atoms bonded to the central atom; each single/double/triple bond counts as one electron domain)
- N is the number of lone pairs on the central atom
After finding SN, use it to determine the electron geometry (SN = 2 linear, 3 trigonal planar, 4 tetrahedral, 5 trigonal bipyramidal, 6 octahedral). The molecular geometry (molecular shape) is then determined by how many of those electron domains are lone pairs (N) versus bonding domains (B).
What is VSEPR Theory?
VSEPR (Valence Shell Electron Pair Repulsion) Theory is a model used in chemistry to predict the geometry of individual molecules based on the number of electron pairs (electron domains) surrounding their central atoms. It postulates that these electron domains, including both bonding and lone pairs, will arrange themselves in three-dimensional space to minimize their mutual repulsion, thus determining the shape of the molecule.
How to Calculate VSEPR Theory?
The following steps outline how to determine the molecular geometry using VSEPR Theory.
- Identify the central atom in the molecule.
- Determine the number of bonding domains around the central atom (B). (In basic VSEPR counting, a single, double, or triple bond counts as one electron domain.)
- Determine the number of lone pairs on the central atom (N).
- Calculate the steric number: SN = B + N.
- Use SN to determine the electron geometry, then use B and N together (AXmEn notation) to determine the molecular geometry.
- After inserting the variables and calculating the result, check your answer with reference materials or a reliable source (especially for resonance structures, formal charges, and expanded octets).
Example Problem:
Use the following variables as an example problem to test your knowledge.
Number of bonded atoms (B) = 2 (e.g., H2O has two O–H bonds)
Number of lone pairs on the central atom (N) = 2 → SN = 4 → electron geometry: tetrahedral; molecular geometry: bent
