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HS Chemistry - Chemical Bonding

Electron & Molecular Geometry

Overview of The Page

This page will cover:

  • What is electron geometry? How does it relate to molecular geometry?

Molecular Geometry

Electron geometry is the positioning of valence electrons in an atom. Molecular geometry is the positioning of atoms in a molecule. Molecular geometry is usually determined from the central atom. To understand molecular geometry, first we need to look at electron geometry.

Electron Geometry

Atoms use their valence electrons to bond to other atoms. However, they don't just do so randomly. There are specific ways and positions in which they bond to these atoms.

The VSEPR Theory - The Valence Shell Electron Pair Repulsion Theory - is the theory that the valence electron pairs of an atom involved in bonding, regardless of whether those specific electrons are involved in bonding or not, repel each other as much as possible. They will get as far away from each other as possible to maximize separation.

The electron geometry is determined by the number of valence electrons there are - that determines how many atoms our central atom can bond with, and that gives the electron geometry. From there, we can determine the molecular geometry.

A lone pair of electrons is a pair of electrons that isn't involved in bonding.

The electron geometry of the central atom of a molecule, as well as the molecular geometry of the whole molecule, can be determined through these steps:

  1. Determine which atom is the central one.
  2. Draw the Lewis Dot structure for all the atoms
  3. Determine how the valence electrons arrange themselves.
  4. Determine how the other atoms connect to the central one.
  5. Count the number of atoms bonded to the central atom, and the number of lone electron pairs on the central atom.

Once you've done this, and found the number of bonding pairs and the number of loose atoms, you can find the electron and molecular geometries. You kind of have to memorize which ones will be formed given the number of bonding atoms and the number of lone pairs.

A more through explanation for why this only happens in bonding can be found on the next page: Sigma and Pi Bonds.

Practice