what is the trend for electronegativity as you move down a group

Electronegativity decreases as you move down a group (column) on the periodic table.

Why this happens:

1. Increasing atomic radius – As you descend a group, each successive element has an additional electron shell, making the atom larger.

2. Greater distance from the nucleus – The valence electrons involved in bonding are farther from the positively charged nucleus, so the nucleus's pull on them (and on shared electrons in a bond) is weaker.

3. Increased electron shielding – The inner electron shells "screen" or shield the valence electrons from the full attractive force of the nucleus. Even though the nuclear charge (number of protons) increases going down a group, this increase is largely offset by the added shielding from extra inner shells, so the effective nuclear charge felt by valence electrons stays relatively similar while the distance increases.

The net result is that the nucleus has a diminished ability to attract shared electrons in a chemical bond, so electronegativity falls off as you go from top to bottom within a group.

Example: In Group 17 (the halogens), fluorine is the most electronegative element on the entire periodic table (electronegativity ≈ 3.98 on the Pauling scale), while electronegativity decreases down the group: chlorine (≈3.16), bromine (≈2.96), iodine (≈2.66), and astatine (lower still).

Combined with the periodic trend: Electronegativity increases across a period (left to right) due to increasing nuclear charge with roughly constant shielding, and decreases down a group due to increasing atomic size and shielding. This means the most electronegative elements are found in the upper right of the periodic table (excluding noble gases, which typically aren't assigned electronegativity values since they rarely form bonds), with fluorine at the extreme.

Note: This trend has some exceptions and irregularities, particularly among the transition metals and heavier main-group elements, where relativistic effects and d/f-orbital influences can complicate the simple picture.