Electronegativity is a measure of the tendency of an atom to attract a bonding pair of electrons.
Rule 1: Since opposite charges attract, elements that have more protons in their nucleus, like Cl, can attract electrons more strongly than lighter elements in their period, like S.
Rule 2: The positive attractiveness of the nucleus is mitigated by the negative non-valence electrons. However, valence electron number does not affect electronegativity. So if C and Si were the same size, they would have the same electronegativity, because they would seem to have the same amount of charge to outside electrons. Because of this rule, noble gases tend not to have electronegativity values.
Rule 3: The larger the atom, the farther away it is from outside electrons, and therefore the less electronegative it is. Thus, Si is less electronegative than C.
These trends are summarized in the graphic.