posted by michael .
i know that there's a difference between electron group arrangmenets and molecular shape. i know how to find the molecular shape of a compound like h3o+ for example, but how do i find the geometric arrangement of the electron groups???
If I understand you correctly, you simply add another group for each pair of unshared electrons. For example, H3O^+ has one shared pair of electrons so there are 3 bonded pairs and 1 unpaired which makes four regions of high electron density and four is tetrahedral. The VSEPR theory does it by hybridization of orbitals.