Electronegativity. The covalent bond

The outer electrons of metals are held weakly, so metals give electrons away easily; nonmetals tend to accept electrons. Electronegativity is the ability of an atom in a chemical bond to attract the shared electron pair. Its absolute value is awkward to calculate, so relative values are used; on a convenient scale fluorine is 4.0, oxygen 3.44, chlorine 3.16, nitrogen 3.04, carbon 2.55, hydrogen 2.20, sodium 0.93 and caesium 0.79. Along a period from left to right the nuclear charge grows, the radius shrinks and electronegativity rises; down a main subgroup the radius grows and electronegativity falls. So the most electronegative element is fluorine and the lowest are caesium and francium. The shared electron pair shifts from the atom with lower electronegativity to the one with higher; the latter becomes partly negative, the former partly positive.