When sulfur and cesium react they form an ionic compound. The chemical formula for this compound is Ce2S. Cesium sulfide is a brittle, brownish-black solid. It is toxic if ingested and can cause respiratory problems. The toxicity of cesium sulfide can be minimized by adding a small amount of sulfur dioxide, SO2. The reaction between cesium and sulfur takes place in a polar covalent bond. This type of bond is stronger than a nonpolar covalent bond, because the electrons in a polar covalent bond are shared equally by the bonded atoms. The electronegativity of sulfur is 2.5, which means that it is more negative than hydrogen, which has a positive electronegativity of 2. Hence, the bonded atoms have equal attraction for the shared electrons.
Binary Ionic Compounds containing a Metal and a Nonmetal
In a binary ionic compound the more electropositive metal is named first, followed by the nonmetal. If the metal is a main-group element (A groups), it usually forms only one type of ion. But many transition elements (B groups) can form more than one type of ion. Therefore, their compounds often have Roman numerals added in parentheses immediately after the name of the metal to indicate their ionic charge.
A polyatomic ion is a group of ions that has the same charge, so it stays together in an ionic compound. For example, calcium nitrate is CaNO3, because each Ca2+ ion balances two NO3- ions. However, when the same ion has more than one charge, it is named using a Greek numerical prefix and a subscript, such as NaNO3 or Na2CO3. When a polyatomic ion has no fixed number of atoms, it is named as a monatomic ion without any parentheses or subscript.