Electronegativity is the measure of how much an atom "needs" electrons in
order to be in its most stable state. Generally, the if one removes the
nobel gasses from the periodic table (He, Ne, Ar, Kr, Xe, Rn), the most
electronegative elements are in the upper right hand corner of the periodic
table. If one looks at water (H2O), oxygen is very close to the upper
right hand corner of the periodic table, where as hydrogen is not.
Therefore, oxygen "needs" the electrons more then hydrogen in order to be
stable and hogs the electrons. This makes the oxygen have a slightly more
negative charge, and the hydrogens slightly more positive.
Just for fun, Fluorine is the most electronegative, Francium is the least
electronegative and thus electropostive.
It is thought extreame differences in electronegativity give compounds
which are more likely to be ionic bonds. Atoms with electronegativities
closure to eachother are more likely to have covalent bonds. Polar
compounds have covalent bonds with one atom being slightly more negative
then the rest.
