Ionic character is a measure of the polarity of a bond. The greater the difference in electronegativity between two atoms, the more polar the bond will be. In general, bonds between metals and non-metals are more polar than bonds between two non-metals.

The electronegativity of cesium is 0.79, while the electronegativity of fluorine is 3.98. This gives a difference in electronegativity of 3.19, which is the greatest difference in electronegativity of any pair of elements. Therefore, cesium fluoride has the greatest ionic character.