How to calculate the number of valence electrons
In chemistry, calculation of valence electron pairs is a key step in understanding molecular structure and chemical bonds. This article will introduce in detail the calculation method of valence electron logarithm and illustrate it with actual cases.
1. Definition of valence electron logarithm
The number of valence electron pairs refers to the number of valence electron pairs around the central atom in a molecule or ion, including bonding electron pairs and lone electron pairs. It is an important basis for predicting molecular geometric configuration.
2. Calculation formula of valence electron logarithm
The calculation formula for the number of valence electron pairs (VSEPR number) is:
Number of valence electron pairs = (number of valence electrons of the central atom + number of electrons provided by the coordinating atom ± number of ion charges) / 2
The following are the number of electrons provided by common coordinating atoms:
| Coordinating atoms | Number of electrons provided |
|---|---|
| H | 1 |
| F | 1 |
| Cl | 1 |
| O | 0 |
| S | 0 |
3. Calculation steps
1. Determine the number of valence electrons in the central atom (main group elements are equal to the group number)
2. Calculate the total number of electrons provided by the coordinating atoms
3. Process the ion charge (positive ions minus the charge number, negative ions plus the charge number)
4. Divide the sum by 2 to get the number of valence electron pairs
4. Example analysis
| Molecule/Ion | Calculation process | Valence electron pairs |
|---|---|---|
| CH₄ | (4+1×4)/2=4 | 4 |
| NH₃ | (5+1×3)/2=4 | 4 |
| H₂O | (6+1×2)/2=4 | 4 |
| SO₄²⁻ | (6+0×4+2)/2=4 | 4 |
5. The relationship between the number of valence electron pairs and molecular configuration
| Valence electron pairs | electron pair arrangement | Molecular configuration examples |
|---|---|---|
| 2 | Straight type | CO₂ |
| 3 | plane triangle | BF₃ |
| 4 | tetrahedron | CH₄ |
| 5 | triangular bipyramidal | PCl₅ |
| 6 | octahedron | SF₆ |
6. Precautions
1. For transition metal complexes, the calculation methods are different.
2. Treat multiple bonds as single electron pairs
3. Lone electron pairs will affect the actual molecular configuration
4. When a resonance structure exists, all possible contribution forms need to be considered
7. Application value
Valence electron logarithm calculation has important applications in the following aspects:
- Predict molecular geometry
- Explain molecular polarity
- Understand chemical reaction mechanisms
- Design the molecular structure of new materials
By mastering the calculation method of valence electron logarithm, we can better understand the three-dimensional structure and chemical properties of molecules, laying a solid foundation for subsequent chemical learning and research.
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