An ion is an atom or molecule that carries a net electrical charge. This charge arises when the particle gains or loses one or more electrons, creating an imbalance between positively charged protons and negatively charged electrons. Ions are the fundamental building blocks of ionic compounds, such as table salt and the minerals in our bodies. Determining whether an ion is positive or negative is a primary skill for interpreting chemical formulas and reactions. This analysis provides methods for distinguishing between the two major types of ions based on their formation, location on the periodic table, and naming conventions.
The Mechanics of Charge Formation
The distinction between the two types of ions rests entirely on the transfer of subatomic electrons. Atoms seek to achieve a stable electron configuration, often called a complete octet, mimicking the noble gases. To reach this stability, some atoms shed electrons, while others acquire them.
When a neutral atom loses one or more outer-shell electrons, the number of protons exceeds the number of electrons. This surplus of positive charge results in the formation of a positively charged ion. The magnitude of the charge corresponds directly to the number of electrons lost; for example, losing two electrons results in a \(2+\) charge.
Conversely, an atom that gains electrons will have more negative particles than positive protons, resulting in a net negative charge. This ion forms when an atom accepts electrons into its valence shell to complete its stable configuration. A gain of a single electron yields a \(1-\) charge, while a gain of two electrons produces a \(2-\) charge.
Identifying Ions Using the Periodic Table
The location of an element on the periodic table is a reliable tool for predicting its ion charge. Elements are categorized into metals (left and center) and nonmetals (right side), separated by the metalloid staircase.
Metals tend to release their outermost electrons, meaning they nearly always form positively charged ions. Group 1 elements (alkali metals) readily lose one electron to achieve a \(1+\) charge, such as \(\text{Na}^+\). Group 2 elements (alkaline earth metals) shed two electrons to form ions with a \(2+\) charge, exemplified by \(\text{Ca}^{2+}\). Transition metals also form positive ions, though their charge can often vary.
Nonmetals, located to the right of the staircase, exhibit a high affinity for electrons, leading them to form negatively charged ions. Halogens in Group 17 need one electron to complete their octet, consistently forming ions with a \(1-\) charge, like \(\text{Cl}^-\). Elements in Group 16, such as oxygen and sulfur, require two electrons for stability and routinely form ions with a \(2-\) charge.
Clues Found in Naming Conventions
The systematic nomenclature of ions provides linguistic clues to determine the charge type from the name alone. Positively charged ions are often the simplest to identify because their name is consistently the same as the parent element, such as the Lithium ion (\(\text{Li}^+\)) or the Zinc ion (\(\text{Zn}^{2+}\)).
When a metal forms multiple positive charges, the name includes a Roman numeral in parentheses to specify the exact charge. Examples include Iron(\(\text{II}\)) for \(\text{Fe}^{2+}\) and Iron(\(\text{III}\)) for \(\text{Fe}^{3+}\). The polyatomic Ammonium ion (\(\text{NH}_4^+\)) is a notable exception, being one of the few common positive ions composed of multiple atoms.
Negatively charged ions made of a single element are signaled by a specific suffix change. The stem of the element’s name is combined with the ending “-ide.” Examples include Fluorine becoming Fluoride (\(\text{F}^-\)) and Oxygen becoming Oxide (\(\text{O}^{2-}\)).
For complex polyatomic negative ions, two different suffixes are commonly used for ions containing oxygen. The “-ate” suffix denotes the form with more oxygen atoms, such as Sulfate (\(\text{SO}_4^{2-}\)). The “-ite” suffix is used for the form with fewer oxygen atoms, exemplified by Sulfite (\(\text{SO}_3^{2-}\)).