"The centre of positive and the centre of negative coincide" What does that mean? I'm doing AS level chemistry right now and my teacher told us that when a molecule is non polar , the centre of positive and the centre of negative coincide , I don't really get what he means by that .
Each molecule has negatively charged electrons, and positively charged nuclei.  

If the centroid of the electrons' charges (electron probability density) is different than the centroid of the nuclear charges, then the molecule will have a permanent electric dipole moment.