Electronegativity
How strongly an atom pulls shared electrons toward itself when it's bonded to another atom.
See electronegativity colored on the interactive table →Across a period (left → right)
Rises as you move left to right across a period.
Down a group (top → bottom)
Falls as you move down a group.
Why it happens
Same underlying cause as ionization energy — a smaller atom with a strong, close-range nuclear pull attracts bonding electrons more forcefully than a larger atom does. Fluorine, at the top right of the main table, is the most electronegative element for exactly this reason: small radius, high nuclear charge, minimal shielding. Francium, bottom-left, is the least.
Noble gases are usually left off electronegativity scales entirely, since they essentially don't form conventional bonds.