An ion is an atom or molecule that carries a net electrical charge due to the gain or loss of electrons. Identifying these charged components is foundational to understanding how chemical compounds are formed and named. An ion with a positive charge is a cation, resulting from the loss of electrons. Conversely, an ion with a negative charge is an anion, formed when an atom gains electrons. These oppositely charged species are drawn together by electrostatic forces, forming the stable structure of an ionic compound.
Identifying Cations and Anions in Fixed-Charge Compounds
Fixed-charge compounds are the simplest ionic compounds, formed between metals and non-metals whose ionic charges are predictable based on their position in the Periodic Table. In the chemical formula, the positively charged cation is always listed first, followed by the negatively charged anion. For instance, in sodium chloride (\(\text{NaCl}\)), sodium (\(\text{Na}\)) is the cation and chlorine (\(\text{Cl}\)) is the anion.
The charge relates directly to the number of electrons lost or gained to achieve a stable electron configuration. Group 1 metals, such as sodium and potassium, lose one electron to form a \(+1\) cation. Group 2 metals, like magnesium and calcium, lose two electrons to form a \(+2\) cation.
Non-metals gain electrons to form anions. Group 17 elements (halogens) gain one electron for a \(-1\) charge. Group 16 elements, such as oxygen and sulfur, gain two electrons, resulting in a \(-2\) charge. This systematic pattern allows for the straightforward determination of both the cation and anion, along with their specific charges, in binary compounds like \(\text{KCl}\) or \(\text{MgS}\).
How to Identify Polyatomic Ions
Identifying compound components becomes more complex when one or both ions consist of multiple bonded atoms. These charged groups are known as polyatomic ions; they behave chemically as a single, indivisible unit with an overall electrical charge. For example, the sulfate ion (\(\text{SO}_4^{2-}\)), which contains one sulfur atom and four oxygen atoms, carries a collective \(-2\) charge.
Polyatomic ions are often indicated by parentheses in the chemical formula, especially when multiple units are needed for charge balance, such as in \(\text{Ca}(\text{NO}_3)_2\). Here, the calcium ion (\(\text{Ca}^{2+}\)) is the cation, and the nitrate group (\(\text{NO}_3^-\)) is the polyatomic anion. Recognizing these units requires familiarity, as their charges cannot be determined simply from the Periodic Table location of a single element.
Most polyatomic ions are anions, but the ammonium ion (\(\text{NH}_4^+\)) is the most common exception, acting as a polyatomic cation with a \(+1\) charge. This positively charged group is unique because it is listed first in a formula, like in \(\text{NH}_4\text{Cl}\), yet it does not involve a metal atom. Other frequently encountered polyatomic anions include carbonate (\(\text{CO}_3^{2-}\)), phosphate (\(\text{PO}_4^{3-}\)), and hydroxide (\(\text{OH}^-\)).
Determining Cation Charge in Variable-Charge Metals
A different procedure is necessary when the compound contains a metal that can form ions with multiple possible charges. These variable-charge metals are typically found in the transition metal block of the Periodic Table. Unlike fixed-charge metals, the charge of these cations must be determined indirectly by relying on the known charge of the anion.
This determination is founded on the principle of charge neutrality: a stable ionic compound must have a net overall charge of zero. The total positive charge contributed by the cations must exactly cancel out the total negative charge contributed by the anions. The process begins by identifying the fixed charge of the anion, whether it is a single element like chloride (\(\text{Cl}^-\)) or a polyatomic group like sulfate (\(\text{SO}_4^{2-}\)).
For instance, in the compound \(\text{FeCl}_3\), the anion is chloride, which always carries a \(-1\) charge. Since there are three chloride ions present, the total negative charge is \(-3\). To achieve neutrality, the single iron cation (\(\text{Fe}\)) must contribute a total positive charge of \(+3\), meaning the cation is \(\text{Fe}^{3+}\).
Conversely, in \(\text{FeCl}_2\), the two chloride ions produce a total negative charge of \(-2\), requiring the iron cation to be \(\text{Fe}^{2+}\) to balance the compound. This calculated charge is then represented by a Roman numeral in the compound’s formal name, such as Iron(III) Chloride for \(\text{FeCl}_3\), which serves as a definitive identifier for the specific cation present.