Electronegativity is the tendency (or power) of an atom in a molecule to attract electrons to itself.
Electronegativity increases across a period (from left to right) in the Periodic Table.
Electronegativity decreases down a group in the Periodic Table.
Fluorine is the most electronegative of all the elements.
Strictly speaking electronegativity is a measure of how strongly an atom attracts the electrons in bonds involving the atom in question and a second atom. Consequently, it is an undefined quantity for atoms, such as the noble gases helium, neon and argon, that do not form bonds.