Electronegativity is the tendency or ability of an atom to attract electron density to itself.
Numbers for Electronegativity are put on a Periodic Table. For two atoms sharing a bond, the atom with the higher number is better at attracting electron density. That atom will develop a partial negative charge. The other atom, the “loser”, will develop a partial positive charge.
For the extreme cases of Fluorine, Oxygen and Nitrogen, the change is enough to allow hydrogens on any of these three to undergo something called Hydrogen Bonding.
Hydrogen Bonding.
Rob Sterling 