Ionic compounds are formed between oppositely charged ions that consist of a metal and one or more non-metals. The charges of the ions are separated by the distance between their nuclei (atoms). When two different elements combine to form an ionic compound, the formula is written with the metal element first followed by the charge in Roman numerals, for example: Fe 2+ and Sulfur (CeS)2. The names of the ions are then written as per the systematic name system of the element. The chemical properties of the ions determine how the compound behaves, and are often reflected in the physical properties such as melting and boiling points, electrical conductivity, and density.

Cesium is a soft, gold-colored metal that is very reactive. It reacts quickly with halogens to form the inorganic compounds cesium fluoride, cesium chloride, cesium bromide, and cesium iodide, which are all soluble in water. Cesium also reacts with sulfates to form cesium sulfide and cesium telluride. Cesium sulfide is a whitish-yellow, crystalline compound that is soluble in acid and water. It also binds strongly to glass and is readily released into the air in the form of hydrogen sulfide, which smells like rotten eggs.

Sulfur is found naturally in sulfide minerals such as pyrite, which is known as fool’s gold for its brassy yellow color. It is also a major raw material for the production of sulfuric acid, a key industrial chemical. It is extracted from crude oil in huge quantities and piled up at oil refineries around the world. Sulfur is also deposited around volcanic vents and hot springs and is a common byproduct of some industrial processes.