the electronegativity difference

When two different atoms bond, one of them is usually greedier for the shared electrons than the other. Electronegativity is a number that ranks that greed: fluorine is the greediest at about 4.0, oxygen next at about 3.5, while metals like sodium (0.9) or magnesium (1.2) barely pull at all. The electronegativity difference between the two bonded atoms, the bigger minus the smaller, is the single most useful clue to whether their bond is ionic, covalent, or a blend.

Linus Pauling built the scale so that the difference predicts bond character. A large difference (roughly above 1.7 to 2.0) means the electrons are dragged almost entirely onto one atom, making the bond mostly ionic; a small difference (near zero) means the electrons are shared evenly, a covalent bond; in between you get a mix. For oxides the anion is oxygen at 3.5, so the difference is essentially set by the metal: MgO has delta-X = 3.5 - 1.2 = 2.3 (strongly ionic), while SiO2 has delta-X = 3.5 - 1.8 = 1.7 (about half-and-half). Feeding delta-X into Pauling's exponential gives the percent ionic character directly.

Electronegativity is not a directly measured quantity like mass; it is an assigned number, and several scales exist (Pauling, Mulliken, Allred-Rochow) that disagree slightly. It also ignores the atoms' oxidation states and their environment in the crystal. So the difference is a superb back-of-envelope guide, but two solids with the same delta-X can still behave differently once real structure and defects enter the story.