Different elements have different electro-negativities. Electro-negativity is the ability of an atom to withdraw 'electron density' towards itself, i.e. it makes electrons come closer to it. Fluorine...
Water is called a polar molecule because of the three atoms in a water molecule are not arranged in a straight line as you might expect.
Nonpolar Molecules
It consists of a slightly negative oxygen atom and 2 slightly positive hydrogen atoms
Cohesion.