<P> Although it may seem that Fluorine should have the greatest electron affinity, the small size of fluorine generates enough repulsion that Chlorine has the greatest electron affinity . </P> <P> Electronegativity is a measure of the ability of an atom or molecule to attract pairs of electrons in the context of a chemical bond . The type of bond formed is largely determined by the difference in electronegativity between the atoms involved, using the Pauling scale . Trend-wise, as one moves from left to right across a period in the periodic table, the electronegativity increases due to the stronger attraction that the atoms obtain as the nuclear charge increases . Moving down in a group, the electronegativity decreases due to the longer distance between the nucleus and the valence electron shell, thereby decreasing the attraction, making the atom have less of an attraction for electrons or protons . </P> <P> However, in the group 13 elements electronegativity increases from aluminium to thallium, and in group 14 electronegativity of lead is lower than that of tin . </P> <P> Valence electrons are the electrons in the outermost electron shell of an isolated atom of an element . Sometimes, it is also regarded as the basis of Modern Periodic Table . In a period, the number of valence electrons increases (mostly for light metal / elements) as we move from left to right side . However, in a group this periodic trend is constant, that is the number of valence electrons remains the same . </P>

Metallic and nonmetallic properties in modern periodic table