Question

What is the electronegativity trend for elements in the same chemical group that form anions?

Answer 1

AS we look at the Table `"electronegativity"` increases across the Period from left to right, but decreases DOWN a Group.

Explanation:

As we face the Table,

`"electronegativity increases across the Table from left to right"`, `"but decreases down the Group"`.

Two factors are important here: (i) nuclear charge; and (ii) shielding by other electrons.

Across the Period, nuclear charge increases sequentially, and the incomplete electronic shell shields the nuclear charge very imperfectly. And thus as we go across the Period from left to right as face the Table, electronegativity sequentially increases, to reach a maximum at fluorine, which is the most electronegative element. This trend is also observed in the marked decrease in atomic radius across the Period in the same direction.

Down the Group, the nuclear charge is shielded somewhat by complete electronic shells. Of course, as `Z` increases, electronegativity increases, but the observed electronegativity is LESS than that of the congeneric element in the same Group but is one Period above that of the one observed.

On the Pauling scale, which used actual ionization enthalpies, and electron affinities, fluorine at `4.0` is the most electronegative element; oxygen, and chlorine were approx. `3.5`, and bromine was approx. `2.8`.

Electronegativities thus follow a pronounced Periodic trend.