The term "Sn number" in chemistry most likely refers to the Steric Number (SN) used in Valence Shell Electron Pair Repulsion (VSEPR) theory.
Steric Number Explained
The steric number helps predict the geometry of a molecule by considering the number of atoms bonded to the central atom and the number of lone pairs of electrons around the central atom.
Definition
According to the provided reference, the steric number (SN) is calculated as:
- SN = Number of attached atoms + Number of lone pairs
How to Determine Steric Number
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Identify the Central Atom: Determine the central atom in the molecule. This is typically the least electronegative atom (excluding hydrogen).
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Count the Attached Atoms: Count the number of atoms directly bonded (attached) to the central atom. A double or triple bond counts as one attached atom.
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Count the Lone Pairs: Count the number of lone pairs of electrons present on the central atom.
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Calculate the Steric Number: Add the number of attached atoms and the number of lone pairs.
Examples
Here are a few examples demonstrating how to calculate the steric number:
| Molecule | Central Atom | Attached Atoms | Lone Pairs | Steric Number (SN) |
|---|---|---|---|---|
| Methane (CH₄) | Carbon | 4 | 0 | 4 |
| Ammonia (NH₃) | Nitrogen | 3 | 1 | 4 |
| Water (H₂O) | Oxygen | 2 | 2 | 4 |
| Carbon Dioxide (CO₂) | Carbon | 2 | 0 | 2 |
Significance of Steric Number
The steric number is crucial because it correlates directly with the electron-pair geometry around the central atom. This geometry then influences the molecular shape. The following table gives the basic electron-pair geometries based on steric number.
| Steric Number | Electron-Pair Geometry |
|---|---|
| 2 | Linear |
| 3 | Trigonal Planar |
| 4 | Tetrahedral |
| 5 | Trigonal Bipyramidal |
| 6 | Octahedral |
