Transition metals are a unique group of elements found in the center of the periodic table (d-block, Groups 3 through 12). Unlike main group elements, which form ions with a single, predictable charge, transition metals exhibit variable charges. This means a single element can form multiple different positive ions, or oxidation states, depending on the chemical environment.
The Reason Transition Metals Have Varying Charges
The ability of transition metals to form multiple stable positive ions is rooted in their electron configuration. These elements have valence electrons in both the outermost \(s\) orbital and the inner \(d\) orbital. For example, in the first transition series, electrons fill the \(4s\) orbital and then the \(3d\) orbital.
When a transition metal atom forms a positive ion, it always loses electrons from the outermost \(s\) orbital first. The initial loss of these two \(s\) electrons results in a common +2 oxidation state across many elements. The term oxidation state refers to the charge an atom would have if all its bonds were completely ionic.
After the \(s\) electrons are removed, the energy difference between the remaining \(d\) electrons is small enough that additional electrons can be lost from the \(d\) orbital. This allows a metal like Iron to exist as a +2 ion (losing only the \(s\) electrons) or as a +3 ion (losing the \(s\) electrons and one \(d\) electron).
How to Determine the Charge in Chemical Compounds
Since the charge of a transition metal is not fixed, its specific charge must be determined when it is part of a chemical compound. Chemists use the Stock system of nomenclature, which employs Roman numerals in parentheses immediately following the metal’s name to indicate its specific positive charge, such as Iron(II) or Copper(I).
To determine the metal’s charge, one must use the principle that all stable compounds are electrically neutral. This means the total positive charge must equal the total negative charge, relying on knowing the fixed negative charge of the nonmetal ion (anion). For example, the Chloride ion (Cl-) always carries a -1 charge, and the Oxide ion (O2-) always carries a -2 charge.
In the compound \(\text{CuCl}_2\), the two Chloride ions contribute a total negative charge of -2. Since the compound must be neutral, the single Copper ion must carry a +2 charge to balance the negative charge. The compound is therefore named Copper(II) Chloride, with the Roman numeral indicating the calculated charge of the metal ion.
Common Oxidation States and Important Exceptions
Many transition metals exhibit a relatively small number of common oxidation states in their most stable compounds. Iron is most frequently observed with charges of +2 and +3, while Copper commonly forms ions with charges of +1 and +2. Chromium forms a variety of ions, with +2, +3, and +6 being the most prevalent charges in different chemical settings.
There are a few notable exceptions among the transition metals that do not exhibit variable charges. Scandium always forms a +3 ion, and Zinc consistently maintains a single charge, forming only the +2 ion. Silver is typically found with a +1 charge in most of its stable compounds.
Because these exceptions have only one possible charge, the Stock system using Roman numerals is not necessary when naming their compounds.