Fluorine is the most electronegative element on the periodic table. After Flourine, Oxygen, chlorine and nitrogen are the most electronegative elements, and are in descending order of electronegativity.
Electronegativity is the measure of an atom's ability to attract an electron from another atom. Highly electronegative elements are very reactive due to this tendency to attract electrons. Non-metals are more likely to be electronegative than metals.
Elements with very low electronegativity values are more likely to donate electrons to other atoms, and are equally reactive due to this tendency. Francium, cesium, barium and sodium are all elements with very low electronegativity. The least electronegative elements are also known as electropositive elements.