How to Write Chemical Formulas for Transition Metals

You may be used to writing ionic compounds and balancing formulas, but transition metals present new challenges. With these problems, you are suddenly faced with Roman numerals in parentheses and different possible charges for the same element. On top of all that, sometimes the resulting compound is an ion itself. Though these components may be new to you, you can conquer them by understanding a few simple principles.

Transition metals are the elements that make up the middle section of the periodic table, beginning with the group comprised of Sc, Y, La and Ac and ending with Zn, Cd and Hg. Transition metals can form positive ions called cations just like the elements on the left of the periodic table, but they can also form complexes with a net negative or positive charge like FeCl4- or [Fe(H2O)6]2+, respectively.

The ions formed by transition metals are different from other cations because many transition metals can form more than one cation, with different oxidation numbers, or number of electrons lost when forming an ion. For example, chromium can form Cr2+, Cr3+ or Cr6+. Thus when you see the name of a compound involving chromium, it should have the oxidation number written in Roman numerals in parentheses. For example, if Cr2+ formed a compound with one atom of oxygen, the name would be chromium(II) oxide, and the formula would be CrO. If Cr3+ formed a compound with oxygen, the name would be chromium(III) oxide, and the formula would be Cr2O3, since two Cr3+ ions would balance the charge of three O2- atoms. So whenever you see the name of a compound involving a transition metal, write the metal ion with the positive charge indicated in parentheses, and then write the ion of the second element -- in this case, oxygen -- and balance the charges to create a compound with zero net charge.

Transition metals do not have to form neutral compounds; they can also form complexes with a net negative charge. To write the formula for such complexes, you need to know the net charge as well as the oxidation number of the transition metal. Balance the charges just like you would with a neutral compound, except this time you will sum the individual ions' charges so that they equal the total charge of the complex. For example, to create the permanganate ion, you need to know that the overall charge of the ion is -1, the oxidation number of manganese is +7 and the other atom in the compound is oxygen. You know oxygen has an oxidation number of -2, so you would combine one Mn7+ ion and four O2- ions to get a net charge of -1; thus the formula of the permanganate ion is MnO4-. You can usually tell whether a particular substance containing a transition metal is a neutral compound or a charged complex by its name. The names of complex ions usually consist of one word, whereas neutral compounds containing transition metals are usually named by listing the element farther to the left in the periodic table -- or the cation -- first, followed by the element farther to the right -- or the anion -- as separate words. For example, permanganate is one word, and it is a complex; sodium permanganate is two words and is a neutral compound with sodium -- the element farthest to the left -- named first followed by the name of the anion.

An atom's electronic configuration is the way electrons are distributed in the atom. For example, cobalt is the seventh transition metal atom, or d-block atom, in its period -- row in the periodic table -- after the two main group, or s-block, elements K and Ca. Thus, its electronic configuration is [Ar]4s23d7; the [Ar] indicates that Co is in the period below Ar, 4s2 represents the two electrons in the s-block, and 3d7 represents the 7 atoms in the d-block. If, instead, you were given that cobalt had the electronic configuration [Ar]3d7, you would know that this is the Co2+ ion, since two electrons have been removed from the normal configuration. If you were given [Ar]3d6, you would write the chemical formula Co3+, since now three electrons have been removed from the normal configuration.