To determine whether phosphorus trioxide (PO3) is polar or nonpolar, we need to consider its molecular geometry and the electronegativity of its atoms.
Phosphorus trioxide has a trigonal planar molecular geometry due to the presence of three oxygen atoms bonded to a central phosphorus atom with no lone pairs. In a trigonal planar structure, the bond angles are approximately 120 degrees.
Next, we look at the electronegativity values. Oxygen is more electronegative than phosphorus, meaning that the P-O bonds will have polar character. However, because the structure is symmetrical and the dipole moments of the three P-O bonds are oriented in such a way that they cancel each other out, the overall molecule does not have a net dipole moment.
Thus, despite having polar bonds, the symmetry of the trigonal planar shape leads us to conclude that phosphorus trioxide is nonpolar.