Final answer:
Electronegativity decreases within a group as the atomic number increases due to the larger atomic size and increased shielding from added energy levels, which lessen the attraction of bonding electrons.
Step-by-step explanation:
The electronegativity of an element is defined as the ability of an atom to attract electrons towards itself in a chemical bond. When considering how electronegativity changes as we move down a group in the periodic table (which is a vertical column of elements), the trend is that electronegativity tends to decrease.
This decrease in electronegativity with increasing atomic number within a group can be attributed to the atom's increasing size, as well as the increase in the number of occupied principal energy levels, which leads to electrons being further away from the nucleus and less tightly held.
As the atomic size grows and the effective positive nuclear charge felt by the valence electrons becomes less due to this increased distance and electron shielding, the ability of the atom to attract bonding electrons weakens, resulting in lower electronegativities for elements further down a group.