Chromium (Cr), element number 24, is a silvery, hard transition metal. Unlike elements that form only one type of ion, chromium exists in multiple chemical states, meaning its charge (oxidation state) is not fixed. Its chemical behavior and biological effects depend entirely on which charge it possesses. This variability makes understanding the specific charge necessary for considering its use in industry or its impact on health.
Chromium’s Common Oxidation States
Chromium’s charge is most commonly found in three main positive oxidation states: +2, +3, and +6. The oxidation state represents the number of electrons an atom has gained or lost to form a chemical bond. In naming compounds, the charge is typically indicated by a Roman numeral in parentheses, such as Chromium(III) or Chromium(VI).
Chromium in the +3 state (\(\text{Cr}^{3+}\)) is the most stable form in the environment, often seen in compounds like the green pigment chromium(III) oxide (\(\text{Cr}_2\text{O}_3\)). The +6 state (\(\text{Cr}^{6+}\)) is a powerful oxidizing agent found in compounds such as potassium dichromate (\(\text{K}_2\text{Cr}_2\text{O}_7\)). The +2 state (\(\text{Cr}^{2+}\)) is the least common and highly reactive because it readily oxidizes to the more stable +3 form. These differences in charge are responsible for the vast differences in color, solubility, and reactivity of the resulting compounds.
The Chemical Basis for Variable Charges
Chromium’s position in the d-block of the periodic table accounts for its multiple charges. As a transition metal, chromium possesses valence electrons in the outer 4s orbital and the inner 3d orbital. The small energy difference between these two orbitals allows different numbers of electrons to be removed or shared during chemical reactions.
The neutral chromium atom has one electron in the 4s orbital and five electrons in the 3d orbital. When forming the \(\text{Cr}^{2+}\) ion, it loses two electrons. To reach the stable \(\text{Cr}^{3+}\) state, it loses three electrons (the 4s electron and two 3d electrons), resulting in a half-filled 3d orbital that imparts greater stability.
The highest oxidation state, \(\text{Cr}^{6+}\), occurs when the atom loses all six outer electrons (one from 4s and five from 3d). Involving various combinations of 4s and 3d electrons in bonding allows chromium to form compounds with different charges. This flexibility is characteristic of d-block transition metals.
Biological Role Versus Toxicity
The specific charge of the chromium atom determines its biological activity, contrasting the benign \(\text{Cr}(\text{III})\) form with the toxic \(\text{Cr}(\text{VI})\) form. \(\text{Cr}(\text{III})\) is often considered a trace mineral involved in the metabolism of glucose, protein, and fat. Although its necessity for human health is debated, it is commonly included in dietary supplements and occurs naturally in many foods.
\(\text{Cr}(\text{III})\) compounds exhibit low toxicity because they are relatively inert and poorly cross cell membranes. Its large positive charge makes it difficult for the body to absorb. This form is mainly found in nature, such as in rocks and soil.
\(\text{Cr}(\text{VI})\) is classified as a known human carcinogen, particularly when inhaled. It is highly mobile and easily enters cells by mimicking essential ions like sulfate and phosphate, which are transported through common cellular channels. The primary sources of this toxic form are industrial processes, including chrome plating, leather tanning, and the manufacture of dyes.
Once inside the cell, \(\text{Cr}(\text{VI})\) undergoes reduction back to \(\text{Cr}(\text{III})\). This chemical change generates reactive intermediate species and oxygen radicals. These intermediates cause toxicity by binding to and damaging DNA, which can lead to mutations and lung cancer from chronic inhalation exposure. The three-electron charge difference dictates why one form is promoted for health and the other is an environmental contaminant.