An ion is an atom or molecule that carries a net electrical charge due to the gain or loss of one or more electrons. Atoms are naturally neutral, containing an equal number of positively charged protons and negatively charged electrons, but this balance is easily disrupted in chemical reactions. When an atom gains an electron, it becomes a negatively charged ion, known as an anion; when it loses an electron, it becomes a positively charged ion, called a cation.
Chemical nomenclature is a standardized system for naming charged particles, ensuring clarity and universal understanding in chemistry. This convention quickly communicates the ion’s composition and charge. For anions, a specific naming rule applies, involving the addition of a suffix that distinguishes the ion from its neutral atomic form.
The Rule of the -ide Suffix
The suffix “-ide” serves as a fundamental signal in chemical nomenclature, almost exclusively designating an anion. This negative charge arises when atoms gain electrons to achieve a stable electron configuration. The use of “-ide” is primarily associated with ions formed from a single atom, which are classified as monatomic ions.
To name a monatomic anion, the root of the element’s name replaces its original ending with the “-ide” suffix. For example, the element chlorine (Cl) becomes the chloride ion (\(\text{Cl}^{-}\)), and oxygen (O) is converted into the oxide ion (\(\text{O}^{2-}\)).
Common Monatomic -ide Ions
The most frequently encountered monatomic “-ide” ions come from the nonmetal elements in Groups 15, 16, and 17 of the periodic table. These elements are highly electronegative, meaning they readily attract and gain electrons to complete their octet. The halogens, located in Group 17, are particularly prone to this, forming ions with a 1- charge by gaining a single electron.
This group includes fluoride (\(\text{F}^{-}\)), chloride (\(\text{Cl}^{-}\)), bromide (\(\text{Br}^{-}\)), and iodide (\(\text{I}^{-}\)), which are collectively known as the halide ions. Similarly, elements in Group 16, such as oxygen and sulfur, must gain two electrons to reach a stable state, resulting in ions with a 2- charge. This yields the common ions oxide (\(\text{O}^{2-}\)) and sulfide (\(\text{S}^{2-}\)). Nitrogen and phosphorus, from Group 15, gain three electrons to form nitride (\(\text{N}^{3-}\)) and phosphide (\(\text{P}^{3-}\)) ions.
Important Polyatomic -ide Ions
While the “-ide” suffix predominantly names monatomic ions, a few highly important exceptions exist where the suffix is applied to a polyatomic ion. A polyatomic ion is a tightly bound group of two or more atoms that acts as a single unit and possesses an overall electrical charge. The naming of these specific polyatomic ions often stems from historical conventions rather than strict adherence to modern naming rules.
The most notable exception is the hydroxide ion (\(\text{OH}^{-}\)), which consists of oxygen and hydrogen atoms carrying a 1- charge. Another key polyatomic ion is cyanide (\(\text{CN}^{-}\)), which also carries a 1- charge. Finally, the peroxide ion (\(\text{O}_2^{2-}\)) involves two bonded oxygen atoms sharing a 2- charge, distinguishing it from the monatomic oxide ion.