Different atoms have different electronegativities. If atoms within one molecule have very different electronegativities, the relative positive and negative regions in the molecule may be shifted. An electrostatic potential map shows regions of relative charge (blue is positive and orange is negative) surrounding a molecule.

Look at the electrostatic potential map of a carbon dioxide (CO₂) molecule shown below. Notice that the carbon atom in the centre is much more positively-charged than the oxygen atoms attached to it.

Molecular oxygen (O₂) is made of two of the same types of atoms (two oxygens). As a result, there is no difference in electronegativity between the two atoms. Would you expect there to be an even or uneven distribution of charge between the two oxygen atoms in molecular oxgyen (O₂)?