An ion is an atom or molecule that carries a net electrical charge because its total number of electrons is unequal to its total number of protons. A neutral atom contains an equal number of protons and electrons, resulting in a zero charge. A negative ion, specifically called an anion, forms when a neutral atom gains one or more electrons, giving it a surplus of negative charge.
The Drive for Electron Gain
Atoms are chemically driven to achieve maximum stability, generally reached when their outermost electron shell (the valence shell) is completely full. This pursuit is summarized by the Octet Rule, which states that atoms tend to react to gain eight electrons in this outer layer. This configuration mimics the highly stable electron arrangement of the noble gases.
For atoms possessing five, six, or seven valence electrons, gaining a few electrons is more energetically favorable than losing many. For example, an atom with seven valence electrons only needs to gain one to reach the stable count of eight. Losing those seven electrons would require significantly more energy. The exception to the Octet Rule is hydrogen, which only needs two electrons to fill its first shell (the Duet Rule).
The Elements That Form Negative Ions
The elements most likely to form negative ions are the non-metals located on the right side of the periodic table, closest to the noble gases. These elements have an almost complete outer shell, meaning they only need a small number of electrons to achieve stability. The charge an anion carries is directly related to the number of electrons it needs to gain.
The Halogens (Group 17) are the most prominent anion-formers, possessing seven valence electrons. They readily gain a single electron to form an ion with a negative one charge (X\(^-\)), such as chloride (Cl\(^-\)) and fluoride (F\(^-\)). The Chalcogens (Group 16) have six valence electrons and typically gain two electrons to form a negative two charge (X\(^{2-}\)). Oxygen (O) and sulfur (S) frequently form the oxide (O\(^{2-}\)) and sulfide (S\(^{2-}\)) ions.
The Pnictogens (Group 15), such as nitrogen (N) and phosphorus (P), have five valence electrons and gain three electrons to achieve a negative three charge (X\(^{3-}\)). The non-metal hydrogen is unique because it can also gain a single electron to form a hydride ion (H\(^-\)). This strong tendency to gain electrons defines the chemistry of these non-metal groups.
Electronegativity and Electron Affinity
The properties that dictate the electron-gaining behavior of non-metals are electronegativity and electron affinity. Electronegativity measures an atom’s ability to attract a shared pair of electrons toward itself within a chemical bond. This property is measured on a relative scale, increases from left to right across the periodic table, and means non-metals have a stronger attraction than metals.
Electron affinity is a specific energy measurement quantifying the energy change when an electron is added to a neutral atom in the gaseous state. A highly negative value indicates that energy is released when the atom gains an electron, signifying a favorable and spontaneous process. Elements on the far right of the periodic table exhibit the highest electron affinity values.
Both properties peak in the upper-right corner of the periodic table, excluding the noble gases which are already stable. The high effective nuclear charge of atoms in this region, combined with a relatively small atomic size, allows the nucleus to exert a strong attraction on incoming electrons. This powerful attraction is the physical reason why non-metals readily accept electrons and form stable negative ions.
Naming Negative Ions
When a single atom gains electrons to form a negative ion, a specific naming convention is used to distinguish the ion from its neutral atom counterpart. For all monatomic (single-atom) negative ions, the name of the parent element is modified by dropping its original ending and adding the suffix “-ide.”
When these anions combine with positive ions (cations), the resulting compound is named by listing the positive ion first, followed by the newly named negative ion, such as in sodium chloride (NaCl).
Examples of Anion Naming
- Oxygen becomes the oxide ion (O\(^{2-}\)).
- Bromine becomes the bromide ion (Br\(^-\)).
- Nitrogen forms the nitride ion (N\(^{3-}\)).
- Phosphorus forms the phosphide ion (P\(^{3-}\)).