Polarity and Nonpolarity

[HimaniMadnawat3L]

How can a molecule can have polar bonds but be nonpolar overall. Can the opposite ever be true?

[Sarah H Brown 1L]

In lecture we discussed BeCl₂. This molecule is linear because the chlorines each have 3 lone pairs and are each bonded to Be with a single bond (Be has 0 lone pairs). The furthest apart the two regions of Cl electron density can be is directly opposite with Be in the center. However, there are two dipole moments on BeCl₂. They each point to Cl because Cl is more electronegative than Be. Because the chlorines are 180 degrees apart, they have dipole moment vectors of the same magnitude in the opposite directions and therefore, the dipole moments cancel each other out. Basically, BeCl₂ has polar bonds that cancel each other out so the overall molecule is nonpolar. Addressing your second question, the opposite would be a polar molecule that has all nonpolar bonds. As far as I know, this does not exist.

[HimaniMadnawat3L]

Thank you!