James Richard Fromm
Most chemical bonds are covalent in that electrons are shared, but this sharing need not be equal. The sharing is exactly equal only if the two atoms are identical, as they are in the diatomic elements such as oxygen, O2. When the two atoms are not identical and the sharing is noticeably unequal, the covalent bond is said to have ionic character or to be polar. In the water molecule, the electrons are closer to the oxygen; in the HF molecule, the electrons are closer to the fluorine. An atom which has a greater tendency to pull the electrons in a covalent bond toward itself is called electronegative, while an atom which has a lesser tendency to pull the electrons in a covalent bond toward itself is called electropositive.
The concept of electronegativity was made quantitative by Linus Pauling, an American chemist who has won two Nobel prizes (one for chemistry, one for peace). Pauling compared the bond enthalpies of the known homopolar (two identical atoms) and heteropolar (two different atoms) bonds of various elements, and from them calculated the values we now call the Pauling scale of electronegativities. Similar scales have been calculated by others from ionization potentials and electron affinities, but the Pauling scale has remained as the scale most chemists use.
Any homopolar bond has an electronegativity difference of zero and is purely covalent with equal sharing of electrons. As the difference in electronegativity of the two atoms in a heteropolar bond increases, the bonding electrons are found to lie closer to the more electronegative atom. In HF, with an electronegativity difference of 1.9, one would expect the bond to be polar and the electrons (and therefore also the net negative charge) to lie more toward fluorine than toward hydrogen.