These are closer to the nucleus, which results in Cl having a higher electronegativity value than I.
What are electrons?
This trend decreases from left to right across a period due to increasing number of valence electrons being pulled closer to the nucleus of an atom.
What is Atomic Radius?
As you move down a group, the ionization energy electrons are further away from the nucleus, which means less energy is required to take the electrons.
What is decreasing?
A greater amount of this causes electrons to be held "tighter" in an atom as you move from left to right across a period for electronegativity.
What is an effective nuclear charge?
These two trends are similar for when higher energy is needed to take an electron from an atom, it signifies how well the atom's nucleus attracts its electrons.
What are electronegativity and ionization energy?