How electronegativity varies

Electronegativity varies across the table in a pattern of its own — and it is the mirror image of the ionization energy pattern.

The two directions

Along a period, going left to right, electronegativity increases.

Down a group, going top to bottom, electronegativity decreases.

Check both in the table on the previous page. Across period 3: sodium 0.9, then 1.2, 1.5, 1.8, 2.1, 2.5, and chlorine 3.0 — climbing all the way. Down group I: lithium 1.0, sodium 0.9, potassium 0.8 — falling.

Figure to be added

Figure 3.10: a graph of electronegativity on the vertical axis against atomic number 1 to 20 on the horizontal axis, with each point labelled by element symbol — rising to a peak at F within each period and dropping back at Li, Na and K.

Two properties, one explanation

Both patterns come from the same thing: how strongly the nucleus holds the outer electrons. Down a group, the outer electrons are further away and the hold weakens — so the atom both loses its own electron more easily (lower ionization energy) and pulls another atom's electrons less strongly (lower electronegativity).

What this shows

That is why the group-I metals, at the far left and the bottom, are the elements that give electrons away most readily — a fact the next section puts to work.