Yes I'll explain. Periodic table must be byheart for this and also the basic properties of periodic table. Atomic size of atoms goes on becoming bigger and bigger down the group. Because of which, the electrons are not so strongly attracted and held on by the power of nucleus. And electronegativity is inversely proportional to size. If the size of an element is small, then it will have more electronegativity. This is the reason of why the electronegativy decreases in a group.