The molecular geometry of arsenic trifluoride (AsF3) is described as trigonal pyramidal.
In arsenic trifluoride, the central arsenic atom is bonded to three fluorine atoms. It has a total of four electron pairs surrounding it: three bonding pairs (one for each As-F bond) and one lone pair. The presence of this lone pair influences the molecular geometry.
According to VSEPR (Valence Shell Electron Pair Repulsion) theory, the geometry is determined by the arrangement of electron pairs around the central atom. The three bonding pairs tend to spread out as far as possible to minimize repulsion, leading to a trigonal planar arrangement for the electron pairs. However, due to the lone pair’s presence, the molecular shape changes to trigonal pyramidal as the lone pair occupies one vertex of the tetrahedral arrangement created by the four electron pairs.
This trigonal pyramidal shape is similar to that of ammonia (NH3), where the lone pair creates a noticeable distortion from the ideal tetrahedral geometry. The bond angles in AsF3 will be slightly less than 109.5° due to the repulsive effect of the lone pair compared to the bonding pairs.