Phosphorous trihydride (PH₃), or phosphine, has a central phosphorous (P) atom. It is bonded to three hydrogen (H) atoms through single covalent bonds. Phosphorous has a lone electron pair that repels the bonding pairs. As a result, the PH₃ molecule becomes asymmetric, resulting in a bent structure. The molecular geometry of PH₃ has a deviation from the trigonal pyramidal structure, with a bond angle of 93.5°.

The electronegativity of phosphorous is 2.19, and that of hydrogen is 2.2. Since the electronegativity difference between the two atoms is very low, the P-H bond is nonpolar. However, the molecule is polar. It is because of both the lone pair and the bent structure. There is a region of negative charge on phosphorous and a region of positive charge on the hydrogen.