In order to understand the concept of electronegativity, it is necessary to take into account the following idea... The molecules are highly dynamic, and electrons are in constant motion around the atoms, representing the so-called "electron cloud". The movement of electrons around an atoms is directly conditioned by its characteristics. That is, if an atom has a greater ability to take over the electronic cloud, the electrons will be located predominantly on him. Electronegativity concerns exactly with this capability. Therefore, the more electronegative an atom is, the greater the portion of the electron cloud on it. Consequently, the more electronegative atoms tend to have negative partial charges, because at every moment there will be more electrons on them than on other atoms .
It is this asymmetry that is created in the distribution of the electron cloud that causes the molecules to become polar. Thus, in general , the presence of different atoms with different electronegativities in a molecule causes the molecule becomes polar, or at least the region where this happens to become polar .
In the case of biochemistry , since carbon and hydrogen have similar electronegativities, the regions of the molecules that contain only these two atoms are nonpolar , while the presence of oxygen, nitrogen , fluorine , phosphorus , etc., tend to render that region polar .