Determine whether the compound is ionic or covalent. In general, compounds where the difference in electronegativity--the element's ability to pull or take electrons from other elements--is greater than 2 are usually considered ionic, while compounds where the difference in electronegativity is less than 2 are usually considered covalent. The first link under the Resources section gives the electronegativity values for all the elements. If the compound is ionic, proceed to the next step. If the compound is covalent, proceed to section 2.
Determine which element in the chemical formula becomes the cation and which becomes the anion. The cation is the positively charged atom that has lost an electron to its partner, while the anion is the atom or group of atoms that gains an electron. The element with the lower electronegativity will be the cation. If there are only two atoms, as in the case of sodium chloride, one will be the anion and the other will be the cation; if a polyatomic anion that contains multiple atoms is involved, however, you may need to determine which atoms are part of the anion in order to choose the correct name. Several common polyatomic anions are listed at the second link under the Resources section.
Write the name for the cation first. The name of the cation is simply the name of the element--in sodium chloride, for example, sodium is the cation, so its name remains unchanged. Some transition metals in the middle block of the periodic table, however, can assume multiple charge states--in other words, they don't always lose the same number of electrons in all the ionic compounds they form. For compounds involving transition metals, write the charge state of the cation in Roman numerals in parenthesis following the cation name. If the chemical formula is CoCl2, for example, we know that the transition metal cobalt is the cation since its electronegativity is less than that of chlorine. We also know that cobalt can assume multiple charge states, so we write the cation name as cobalt (II), since the cobalt atom has lost one electron to each chlorine atom.
Write the name for the anion second. If the anion is a polyatomic ion, write its name without changing the ending. NaClO, for example, is named sodium hypochlorite since the anion is the hypochlorite anion. If the anion is a monoatomic ion, however, change the ending to -ide: chlorine becomes chloride, bromine becomes bromide and iodine becomes iodide.
Determine whether the molecular compound has a common name in use and if so, use the common name. H2O, for example, is generally called water and not dihydrogen oxide. The third link under the Resources section lists the names for many common molecular compounds.
Determine whether the compound is an acid. If the molecular formula does not begin with H, the compound is probably not an acid. In that case, write the name of the first element, followed by the name of the second element, and change the ending of the name on the second element to -ide: oxygen becomes oxide, chlorine becomes chloride,. Next, if more than one atom of a given element is present, add the Greek numerical prefix to each element for the number of atoms of that element present in the compound. A list of numerical prefixes used in chemistry appears at the fourth link under the Resources section.
Name an acid that does not include a polyatomic ion using the procedure outlined in Step 2 with one major difference: Change the ending of the last element to -ic rather than -ide and add hydro to the root of the first element's name. Hydrogen should not be written separately. Hydrochloric acid is a common example. If the acid includes a polyatomic ion/oxyanion, however, name it by changing the ending of the name of the anion instead. If the name of the anion ends in -ate, change it to -ic; if the name of the anion ends in -ite, change it to -ous. Sulfuric acid and nitric acid are two common examples.