4: Periodic Trends in Electronegativity
An atom's electronegativity is affected by its atomic number and the distance that its valence electrons reside from the charged nucleus.
When moving in the periodic table from left to right within a period, the number of protons in the nucleus increases. With increased number of protons, the attraction for electrons in the valence shell increases.
Result: In the periodic table electronegativity increases from left to right (see fig. 1).
When moving in the periodic table from top to bottom within a group, the valence electrons are placed in higher energy shells, which are further away from the nucleus. As the distance from the nucleus increases, the attraction for the valence electrons decreases.
Result: In the periodic table electronegativity decreases from top to bottom (see fig. 1).
Figure 1: Electronegativities of elements according to the Pauling scale
[Todd Helmenstine, http://chemistry.about.com/od/periodictableelements/ss/Printable-Periodic-Table-Of-The-Elements-Electronegativity.htm]
S. Hoenig, 2001. Basic training in chemistry, p. 13. Kluwer Academic/Plenum Publishers, New York.