An atom is the smallest unit of an element and is electrically neutral, possessing an equal number of positively charged protons and negatively charged electrons. When this balance is disrupted, the atom becomes an ion, a particle carrying a net electrical charge. An anion is a type of ion that possesses a net negative charge. This negative charge is created when a neutral atom gains one or more electrons, resulting in more electrons than protons.
Why Atoms Gain Electrons
The fundamental driving force behind anion formation is the pursuit of stability. Atoms are generally more stable when their outermost electron shell, the valence shell, is completely full. For most elements, this means having eight electrons in the valence shell, a principle known as the Octet Rule.
Nonmetal atoms, which typically form anions, are located on the right side of the periodic table and possess many valence electrons. It is energetically easier for these atoms to gain a small number of electrons to reach the stable eight-electron configuration than to lose many. For example, a chlorine atom has seven valence electrons, so gaining just one electron allows it to achieve the stable electron arrangement of a noble gas.
The Process of Electron Gain
The physical process of an atom gaining an electron is quantified by Electron Affinity (EA). This property measures the change in energy that occurs when an electron is added to a neutral atom in the gaseous state. For atoms that readily form anions, the addition of the first electron is an energetically favorable process, meaning energy is released. The chlorine atom, for instance, has a strong attraction for an extra electron, and the reaction \(\text{Cl}(\text{g}) + \text{e}^- \rightarrow \text{Cl}^-(\text{g})\) releases energy.
Adding a second or third electron to form multi-charged anions, like the oxide ion (\(\text{O}^{2-}\)), is more complicated. Once the atom gains the first electron, it becomes a negatively charged ion. The incoming second electron must then overcome a strong repulsive force from this existing negative charge. The energy required for this step is so high that it is energetically unfavorable, or endothermic, requiring an input of external energy.
While the first electron gain is a spontaneous, energy-releasing event, subsequent electron gains for species like oxygen or sulfur require energy to proceed. The overall formation of a stable multi-charged anion, such as \(\text{O}^{2-}\), is only possible when the energy released from forming a compound, like an ionic solid, compensates for the energy penalty of adding the second electron.
Where Anion Formers Live on the Periodic Table
Elements most likely to form anions are concentrated in the upper right section of the Periodic Table, among the nonmetals. These elements exhibit high electronegativity, which measures an atom’s ability to attract electrons toward itself. They also tend to have the most favorable Electron Affinity values, indicating a strong drive to gain electrons.
The most common anion-forming groups are Group 17 (halogens) and Group 16 (chalcogens). Halogens, including fluorine and chlorine, need only one electron to complete their octet, forming ions with a \(\text{1}^-\) charge, such as chloride (\(\text{Cl}^-\)) and bromide (\(\text{Br}^-\)). Elements in Group 16, such as oxygen and sulfur, require two electrons and commonly form \(\text{2}^-\) ions, like oxide (\(\text{O}^{2-}\)) and sulfide (\(\text{S}^{2-}\)). Similarly, Group 15 elements, like nitrogen and phosphorus, gain three electrons to form \(\text{3}^-\) ions, such as nitride (\(\text{N}^{3-}\)) and phosphide (\(\text{P}^{3-}\)).
How Anions Are Named
The naming of simple, single-atom anions, known as monatomic anions, follows a consistent rule set in chemical nomenclature. To name a monatomic anion, the ending of the original element’s name is dropped, and the suffix “-ide” is added in its place.
For example, a neutral chlorine atom that gains one electron becomes the chloride ion, and a neutral oxygen atom that gains two electrons becomes the oxide ion. This rule applies consistently across all monatomic anions formed by nonmetals. Thus, sulfur becomes sulfide, phosphorus becomes phosphide, and bromine becomes bromide. The “-ide” ending signals that the particle is a simple ion derived directly from a single element.