Electron Group Geometry Number of Lone Pairs Molecular Geometry Approximate Bond Angles Example Compound 2 Linear 0 Linear 2 180 o carbon dioxide, CO 3 Trigonal Planar 0 Trigonal Planar 120 o formaldehyde, CH 2O 4 Tetrahedral 0 Tetrahedral 109.5 o methane, CH 4 4 Tetrahedral 1 Trigonal Pyramid 107 o ammonia, NH 3 4 Tetrahedral 2 Angular (Bent) Titanium L 3,2 and O K electron energy loss near-edge structures (ELNES) of seven Ti oxides have been measured in a transmission electron microscope to obtain information on the valence state and site geometry of Ti. The coordination of Ti in all phases studied is octahedral, whereas the valence states occurring are Ti 2+, Ti 3+, and Ti 4+.
Excitons trapped at impurity centers in highly ionic crystals were first described by McClure and Pédrini [Phys. Rev. B 32, 8465 (1985)] as excited states consisting of a bound electron-hole pair with the hole localized on the impurity and the electron on nearby lattice sites, and a very short impurity-ligand bond length.