According to Molecular Orbital Theory (MOT), atomic orbitals must possess the same symmetry about the molecular axis to combine and form molecular orbitals . By convention, the z-axis is taken as the internuclear (molecular) axis . The $p_x$ and $p_y$ orbitals are mutually perpendicular, oriented along the x and y axes respectively . Because they have different symmetries relative to the z-axis, their overlap is zero, and they cannot undergo linear combination to form a bond .