Cesium astatide is not soluble in water. In fact, it is insoluble in all solvents. This is because cesium astatide is an ionic compound, and ionic compounds are generally insoluble in nonpolar solvents like water. The cesium ion (Cs+) is a large, positively charged ion, and the astatide ion (At-) is a small, negatively charged ion. The strong electrostatic attraction between these ions holds the compound together, making it difficult for water molecules to break up the compound and dissolve it.