By David Stewart • Updated Mar 24, 2022
Electrons orbit an atomic nucleus in discrete energy levels. The outermost electrons—known as valence electrons—determine how an atom interacts with others. Atoms strive to reach a stable configuration akin to the nearest noble gas, typically by donating, accepting, or sharing these electrons. This behavior is referred to as an atom’s valency.
Variable valency describes an element’s ability to exhibit different charge states depending on the reaction. For instance, iron forms FeO with a +2 valency and Fe2O3 with a +3 valency.
When an atom donates or accepts valence electrons, it often creates an ionic bond. Conversely, when atoms share these electrons, a covalent bond forms. Both types of bonds are central to the structure and properties of chemical compounds.
Some elements do not adhere to a single valence. Their oxidation states vary with the chemical environment, influencing the properties of the resulting compounds. Iron’s shift from +2 in ferrous oxide (FeO) to +3 in ferric oxide (Fe2O3) exemplifies this phenomenon. The magnetic behavior of these oxides differs accordingly.
Examples include hydrogen peroxide (H2O2), where hydrogen has a +2 valency, versus water (H2O), where hydrogen is +1. Nitrogen appears in ammonium (NH4+), where it has a -3 valency, and in nitrous oxide (N2O), where it is +1.
Chemists denote an element’s oxidation state with a Roman numeral in superscript next to its symbol. For instance, PVCl5 indicates phosphorus is in the +5 state in phosphorus pentachloride.
Data sourced from the International Union of Pure and Applied Chemistry (IUPAC) and standard inorganic chemistry textbooks.
