Finding the Number of Electron Pairs

This section outlines a step-by-step process for determining the number of electron pairs around a central atom, which is crucial for predicting molecular shape using VSEPR theory:

1. Identify the central atom
2. Determine the number of valence electrons on the central atom
3. Add one electron for each bonded atom
4. Account for charge in ions
5. Calculate total electrons and divide by 2 for number of pairs
6. Compare to number of bonds to find lone pairs

Two detailed examples are provided:

1. Carbon tetrafluoride (CF₄)

   • 4 bonding pairs, 0 lone pairs
   • Tetrahedral shape

2. Phosphorus trihydride (PH₃)

   • 3 bonding pairs, 1 lone pair
   • Pyramidal shape

Example: In PH₃, phosphorus has 5 valence electrons + 3 from hydrogen bonds = 8 total electrons or 4 pairs.

Highlight: Understanding how to count electron pairs is essential for applying VSEPR theory to predict molecular geometries.

Electron Pair Arrangements

This page focuses on the fundamental principle of electron pair repulsion theory - that electron pairs arrange themselves to minimize repulsion. Key points include:

• Two electron pairs arrange 180° apart, resulting in a linear shape
• Three electron pairs form a trigonal planar arrangement at 120° angles
• Four electron pairs adopt a tetrahedral geometry with 109.5° angles

Definition: The tetrahedral arrangement is formed when four electron pairs surround a central atom, with bond angles of 109.5°.

Highlight: The tetrahedral geometry is particularly important as it forms the basis for understanding more complex molecular shapes.