If a molecule ${\mathrm{MX}}_3$  has zero dipole moment, the sigma bonding orbitals used by M (atomic number < 21) are

The molecule ${\mathrm{MX}}_3$  has zero dipole moment (𝜇 = 0) the structure of ${\mathrm{MX}}_3$ is as follows below:

The M−X sigma bond orbitals should have " ${\mathrm{sp}}^2$  hybridization.