Final answer:
The electron domain geometry for SF3- is tetrahedral due to the four electron domains around the central sulfur atom, with the molecular geometry being trigonal pyramidal because of the lone pair of electrons causing a distortion.
Step-by-step explanation:
The student asked about the electron domain geometry for SF3-. The electron domain geometry is determined by the number of electron regions (domains) around the central sulfur (S) atom, which could be bonding pairs or lone pairs of electrons. In SF3-, sulfur is surrounded by three bonding pairs connected to fluorine atoms and one lone pair of electrons due to the extra electron that gives the molecule a negative charge.
In accordance with the VSEPR theory (Valence Shell Electron Pair Repulsion theory), the presence of four electron domains around the central sulfur atom implies a tetrahedral electron domain geometry. However, because one of these domains is a lone pair and lone pairs exert more repulsion than bonding pairs, the actual shape of the molecule will be distorted. When we take into account this distortion caused by the lone pair, the molecular geometry is described as trigonal pyramidal, which is the arrangement of atoms when a tetrahedral arrangement has one vertex missing (where the lone pair is located).
Illustration of SF4 for Comparison
As a point of comparison, we can look at SF4. In SF4, sulfur has five regions of electron density: one lone pair and four bonding pairs, resulting in a trigonal bipyramidal electron domain geometry, with the molecular shape being a 'seesaw'. This is due to the fact that lone pairs take up more space than bonding pairs, leading to a distortion in the symmetrical shape.