The molecular shape of phosphine (PH3) is described as trigonal pyramidal. This is primarily due to the presence of a lone pair of electrons on the phosphorus atom, which affects the geometry of the molecule.
In more detail, phosphorus has five valence electrons and forms three covalent bonds with hydrogen atoms. The remaining lone pair of electrons occupies space around the phosphorus atom, creating a bent arrangement of the hydrogen atoms. The ideal bond angles in a trigonal planar arrangement are 120 degrees, but the lone pair pushes the hydrogen atoms closer together, resulting in approximately 93.5 degrees.
To summarize, the trigonal pyramidal shape of PH3 is a result of the three bonding pairs of electrons and one lone pair, leading to a distinct geometric configuration.