Final answer:
Electronegativity decreases down a group in the periodic table due to the increased atomic size and the resulting weaker pull of the nucleus on bonding electrons.
Step-by-step explanation:
The phenomenon of electronegativity refers to the tendency of an atom to attract bonding electrons towards itself. When moving down a group in the periodic table, electronegativity decreases. This is because atomic size increases with each additional electron shell, which places the outer electrons at a greater distance from the positive charge of the nucleus. Consequently, the electrostatic attraction between the nucleus and the bonding electrons weakens, leading to a decrease in electronegativity.
For example, halogens display this trend with fluorine having the highest electronegativity (EN 3.98) at the top of the group, and iodine with a lower electronegativity (EN 2.66) towards the bottom. The general trend across the periodic table is an increase in electronegativity from left to right due to an increase in nuclear charge and a decrease down a group due to the increased atomic size.