In these semiconductors, that polarity can flip under an electric field—and stay that way even after the field is gone.

Electrons are attracted to the protons of other atoms. This is what causes atoms to bond and what holds a molecule together. Electrons can be shared unequally in a molecule, creating a polar molecule.

A substance that contains polar covalent bonds may not be overall polar. This is due to the shape of the molecule. Water molecules are polar molecules. Both of the bonds inside the molecule are ...