Why do periodic trends exist for electronegativity?
Electronegativity is the relative force of attraction by an atom on electrons involved in a chemical bond. This is determined by two key factors:
1. How large is the (effective) nuclear charge?
2. How close are the bonding electrons to the nucleus?
As we move down a Group on the Periodic Table of Elements, we observe that EN decreases. This is because, although there is a dramatic increase in nuclear charge, the bonding electrons are in much higher energy levels so are much farther from the nucleus. There is also more shielding of the attractive force (protons in the nucleus attracting bonding electrons) by electrons in the lower energy levels; this decreases the effect of the nuclear charge.
As we move across a Period, we observe that EN generally increases. We also notice that atomic radius tends to decrease. This decrease in radius, coupled with an increase in nuclear charge makes this trend quite intuitive.