Electronegativity is the power of an atom to attract the pair of electrons in a covalent bond. It is measured on the Pauling scale, which runs from about 0.7 for caesium and francium up to 4.0 for fluorine, the most electronegative element.The trends in electronegativity are that it increases across a period and decreases down a group.Across a period the nuclear charge increases while the shielding stays about the same, because electrons are added to the same main energy level. The atomic radius decreases, so the nucleus attracts the bonding pair more strongly.Down a group the atomic radius increases and there are more inner shells of electrons shielding the nucleus. The bonding pair is further from the nucleus and is attracted less strongly, even though the nuclear charge is larger.The most electronegative elements are therefore at the top right of the periodic table (ignoring the noble gases), such as fluorine, oxygen and nitrogen. The least electronegative are at the bottom left.The difference in electronegativity between two bonded atoms decides how polar the bond is. Carbon (2.5) and hydrogen (2.1) have similar values, so the C-H bond is almost non-polar.