Be-Cl is polar but Becl₂ is not why

BeCl₂ is non polar due to its symmetric linear structure. Resultant dipole moment in symmetric molecules is always 0 and hence symmetric molecules are non polar in nature. In BeCl_​2, the dipole moment of one chlorine cancels out the dipole moment of other chlorine and hence the net dipole moment is 0. This is shown below:

iOn the other hand, BeCl is also a linear molecule (if it exists) but the net dipole moment is non zero as only one bond is present and its dipole does not get cancelled.