Naming Ions and Ionic Compounds

Ions

The name of an ionic compound is simply the combination of the names of its constituent ions: the cation (positive ion) and the anion (negative ion). Therefore, before we can assemble the full compound names, we must first master the specific nomenclature rules for the ions themselves. These rules depend on whether the ion is formed from a single atom (monatomic) or from a group of covalently bonded atoms (polyatomic). In the following sections, we will cover the conventions for naming each type.

Monatomic Ions

The nomenclature for a monatomic ion is determined by the element it is formed from and whether it carries a positive (cation) or negative (anion) charge.

• Naming Cations: For invariant metals (primarily those in Groups 1 and 2 of the periodic table, along with Al, Zn, Ag, Cd, and Sc), the name of the cation is simply the name of the element followed by the word “ion”.

  Examples:

  • Na⁺: sodium ion
  • Mg²⁺: magnesium ion
  • Al³⁺: aluminum ion

  For multivalent metals (such as many transition metals), the oxidation state (which is equal to the charge on a monatomic ion) must be specified using the Stock system. This system uses a Roman numeral in parentheses immediately following the element’s name to indicate the charge. Notice how there is no space between the element name and the opening parenthesis. Other non-transition metals that follow the Stock system include In, Tl, Sn, Pb, Sb, and Bi.

  Examples:

  • Fe²⁺: iron(II) ion
  • Fe³⁺: iron(III) ion
  • Pb⁴⁺: lead(IV) ion

• Naming Anions: The nomenclature for monatomic anions is consistent. The name is formed by taking the root of the element’s name and changing the suffix to –ide. For example, F⁻ is the “fluoride ion,” and S²⁻ is the “sulfide ion.”

  Examples:

  • F⁻: fluoride ion
  • O²⁻: oxide ion
  • N³⁻: nitride ion

Polyatomic Ions

Polyatomic ions are charged species composed of two or more atoms linked by covalent bonds. Unlike monatomic ions, the names of most polyatomic ions are not derived systematically from their constituent elements and must be memorized. When naming compounds and writing formulas, it is critical to treat these ions as single, unbreakable units with a specific name and charge. The table below lists common polyatomic ions that are essential for mastering ionic nomenclature.

NoteA Note on ‘Retained Names’

You will notice that while some polyatomic ion names follow a clear, systematic pattern (like hydrogen carbonate or dihydrogen phosphate), other fundamental names like “sulfate” or “ammonium” do not seem to be derived from a rule. Why are these names still correct?

These are examples of retained names. A retained name is a historical or common name that is so widespread and deeply embedded in the chemical literature that IUPAC has officially accepted it as a valid, and often preferred, name. Instead of replacing these universally understood names with new systematic ones, IUPAC has chosen to “retain” them to avoid confusion.

Therefore, when you memorize names like sulfate (SO₄²⁻), nitrate (NO₃⁻), phosphate (PO₄³⁻), carbonate (CO₃²⁻), and ammonium (NH₄⁺), you are learning official, IUPAC-approved nomenclature.

WarningWhat is the deal with Hg₂²⁺?

The ion Hg₂²⁺ has the name “mercury(I) ion.” A student might reasonably look at the 2+ charge and assume the name should be “mercury(II) ion.” The reason it is not is because the Roman numeral in the Stock system does not represent the total charge of the ion; it represents the oxidation state of a single metal atom within that ion.

This can be confusing because for simple, monatomic cations, the oxidation state and the ion’s charge are the same number. For example, in the iron(II) ion (Fe²⁺), the ion’s charge is 2+ and the iron atom’s oxidation state is +2. We get used to this simple pattern.

The mercury(I) ion, however, is a special case. It is a diatomic cation, meaning it consists of two mercury atoms covalently bonded together: [Hg–Hg]²⁺. The total charge of 2+ is shared across both atoms. Therefore, to find the oxidation state of a single mercury atom, we divide the total charge by the number of atoms:

(Total Charge) / (Number of Hg atoms) = Oxidation State
(+2) / (2) = +1

This is why it is named mercury(I). It should not be confused with the monatomic mercury(II) ion, which has the formula Hg²⁺ and consists of a single mercury atom with an oxidation state of +2.

The Rule to Remember: The Roman numeral in a cation’s name always refers to the oxidation state of an individual atom, not the overall charge of the ion. Additionally, when writing an oxidation state, the sign comes before the numeral whereas with charge, the sign comes after the numeral.

Polyatomic Ions with –ide suffix

Typically, the –ide suffix indicates a monatomic anion, such as chloride (Cl⁻) or oxide (O²⁻). However, a few very common polyatomic ions also use this ending. Because they are exceptions to the general rule, it is important to recognize and memorize them as distinct units.

Examples:

• Hydroxide: OH⁻
• Cyanide: CN⁻
• Peroxide: O₂²⁻
• (Less common) Azide: N₃⁻

Oxyanions

Many polyatomic ions belong to a family called oxyanions. These are ions containing a central atom bonded to one or more oxygen atoms. Fortunately, these ions follow a clear naming pattern that simplifies memorization.

The system is based on a pair of suffixes: –ate and –ite. For a given central atom that forms two different oxyanions, the –ate suffix is assigned to the ion with the greater number of oxygen atoms, and the –ite suffix is assigned to the ion with one fewer oxygen atom. For example, NO₃⁻ is the nitrate ion, while NO₂⁻ is the nitrite ion. This pattern helps you name a wide range of ionic compounds.

NoteStrategy for Memorizing Oxyanions

At first glance, the list of oxyanions can seem like a long and random collection of names and formulas to be memorized. Fortunately, that is not the case. Nearly all of them belong to predictable families that follow a simple, systematic set of rules. By learning this system, you can reduce the task of pure memorization to just a handful of the most common ions.

The key is to memorize the “–ate” ions first. These ions serve as the “base” or reference point for their entire family. Once you know the formula and charge for the base “–ate” ion, you can derive the names of the others by simply changing the number of oxygen atoms.

Here are the four rules for naming an oxyanion series, starting from the memorized –ate ion:

Your Memorization List

Instead of memorizing a huge list, focus on these seven essential “–ate” ions. Know their formula and charge.

By memorizing only these seven base ions, you can apply the rules to name dozens of different oxyanions.

Ionic Compounds

Ionic compounds are formed when a cation (a positive ion, typically a metal) and an anion (a negative ion, typically a nonmetal or polyatomic ion) are joined by an ionic bond. You will often hear these compounds referred to by the general term salt. For instance, the common name for sodium chloride (NaCl) is “table salt.”

While nearly all salts are ionic compounds, the formal definition of a salt is the ionic product of an acid-base neutralization reaction. This means that a few specific types of ionic compounds, such as metal hydroxides (like NaOH), which are themselves considered bases, are not technically classified as salts. However, for the purposes of this chapter, which focuses on the structural rules of nomenclature, you can consider the terms “ionic compound” and “salt” to be largely interchangeable.

Binary Ionic Compounds

A binary ionic compound is composed of ions from only two elements: a metal cation and a nonmetal anion, held together by ionic bonds. The fundamental principle for naming any ionic compound is to name the cation first, followed by the anion (with its suffix changed to –ide for monatomic anions).

However, the rule for naming the cation depends on whether the metal can form more than one type of positive ion. To address this, we classify metal cations into two groups:

• Invariant-Charge Metals: These metals form only one stable positive ion. This group includes the metals in Groups 1 and 2 of the periodic table, as well as five additional metals outside these groups: aluminum (Al³⁺), zinc (Zn²⁺), silver (Ag⁺), cadmium (Cd²⁺), and scandium (Sc³⁺). Their cation name is simply the name of the element.

• Multivalent Metals: These metals can form more than one stable positive ion. This category includes most transition metals and some heavier main-group metals (like tin and lead). To name these, we must use the Stock system, which specifies the charge using a Roman numeral in parentheses.

To determine the correct Roman numeral for a multivalent metal, we use the principle of charge neutrality. Since all ionic compounds are electrically neutral, the total positive charge from the cations must exactly balance the total negative charge from the anions. For instance, in the compound FeCl₂, we know that the chloride ion (Cl⁻) always has a 1− charge. Since there are two chloride ions, the total negative charge is 2−. To achieve neutrality, the single iron cation must therefore have a 2+ charge, and its name is the iron(II) ion. The full compound is named iron(II) chloride.

Invariant metal + nonmetal

This is the simplest category of ionic compounds to name. It involves a metal that forms only one stable cation (an invariant-charge metal) bonded to a monatomic nonmetal anion. Because the metal’s charge is always known, we do not need Roman numerals in the name.

Naming Rules

The naming process:

1. The cation (metal) is named first, using the full name of the element.
2. The anion (nonmetal) is named second, by taking the root of the element’s name and changing the suffix to –ide.

For example, for the compound CaBr₂, the cation is Ca²⁺ (calcium) and the anion is Br⁻ (bromide). Combining them gives the name calcium bromide.

Examples:

• NaCl: sodium chloride
• KBr: potassium bromide
• MgCl₂: magnesium chloride
• CaBr₂: calcium bromide
• Al₂O₃: aluminum oxide
• CdBr₂: cadmium bromide
• AgCl: silver chloride
• ZnCl₂: zinc chloride

Multivalent metal + nonmetal

We now address a more complex category of ionic compounds: those containing a multivalent metal. These are metals that can form more than one stable positive ion, a characteristic of most transition metals as well as some heavier main-group metals like tin (Sn) and lead (Pb).

Because these metals can form cations with different positive charges, simply stating the element’s name is ambiguous. For instance, the name “iron chloride” could refer to either FeCl₂ or FeCl₃. To resolve this ambiguity, we must use the Stock system of nomenclature, which explicitly states the metal’s charge.

Naming Rules

The process for naming these compounds adds one step to the previous rules:

1. The cation (metal) is named first, using the full name of the element.
2. The charge of the metal cation is written as a Roman numeral in parentheses immediately following the metal’s name.
3. The anion (nonmetal) is named second, by taking the root of the element’s name and changing the suffix to –ide.

NoteHow to Determine the Roman Numeral

The key to finding the correct Roman numeral is to remember that the total charge of an ionic compound must be zero. Use the known charge of the anion to deduce the charge of the multivalent cation.

To name Co₂O₃:

1. Start with the anion: We know the oxide ion always has a charge of 2−.
2. Calculate total negative charge: There are three oxide ions, so the total negative charge is 3 × (2−) = 6−.
3. Balance the charge: To make the compound neutral, the total positive charge from the cobalt ions must be 6+.
4. Find the charge per cation: This 6+ charge is shared between two cobalt ions. Therefore, the charge on a single cobalt ion is (6+) / 2 = 3+.
5. Write the name: The cation is cobalt(III). The full name is cobalt(III) oxide.

Examples:

• FeCl₂: iron(II) chloride
• FeCl₃: iron(III) chloride
• Co₂O₃: cobalt(III) oxide
• CoCl₂: cobalt(II) chloride
• Hg₂Cl₂: mercury(I) chloride

Ionic Compounds with Polyatomic Ions

When a polyatomic ion is part of an ionic compound, the compound contains more than two elements and is therefore not a binary compound. The naming rules are the same: the cation is named first, followed by the anion.

Invariant metal + polyatomic ion

Here, an invariant-charge metal cation is bonded to a polyatomic anion. The key is to treat the polyatomic ion as a single, complete unit with a specific name that must be memorized.

Naming Rules

The naming process:

1. The cation (metal) is named first, using the full name of the element.
2. The anion (polyatomic ion) is named second, using its specific memorized name (e.g., “sulfate,” “nitrate,” “hydroxide”).

Note that the name of the polyatomic ion is used exactly “as-is”. The ending is not changed to “–ide” (unless the ion’s name already ends in “–ide,” such as hydroxide or cyanide).

For example, for the compound NaNO₃, the cation is Na⁺ (sodium ion) and the polyatomic anion is NO₃⁻ (nitrate). Combining them gives the name sodium nitrate.

Examples:

• Ca₃(PO₄)₂: calcium phosphate
• MgCO₃: magnesium carbonate
• MgSO₄: magnesium sulfate
• NaOH: sodium hydroxide

Multivalent metal + polyatomic ion

This category combines the two skills we have just learned: naming multivalent metals with the Stock system and recognizing polyatomic ions. The compound consists of a multivalent metal cation bonded to a polyatomic anion.

The naming process remains the same, but instead of using a simple nonmetal anion to deduce the metal’s charge, we now use the known charge of the entire polyatomic group.

Naming Rules

The naming process follows these steps:

1. The cation (metal) is named first, using the full name of the element.
2. The charge of the metal cation is written as a Roman numeral in parentheses immediately following the metal’s name.
3. The anion (polyatomic ion) is named second, using its specific memorized name.

NoteDeducing the Roman Numeral with Polyatomic Ions

The logic of charge balancing is the same, but you must use the charge of the entire polyatomic ion.

To name Fe(NO₃)₃:

1. Start with the anion: We have memorized that the nitrate ion (NO₃⁻) has a charge of 1−.
2. Calculate total negative charge: The formula shows there are three nitrate ions, so the total negative charge is 3 × (1−) = 3−.
3. Balance the charge: To make the compound neutral, the total positive charge from the iron cation(s) must be 3+.
4. Find the charge per cation: Since there is only one iron atom, its charge must be 3+.
5. Write the name: The cation is iron(III). The full name is iron(III) nitrate.

Examples:

• Fe(NO₃)₃: iron(III) nitrate
• CuSO₄: copper(II) sulfate
• CoCO₃: cobalt(II) carbonate
• Ni(OH)₂: nickel(II) hydroxide

Polyatomic cation + nonmetal

So far, all the cations we have seen have been monatomic metal ions. However, a polyatomic ion can also carry a positive charge and act as the cation in an ionic compound. The most common of these by far is the ammonium ion (NH₄⁺).

When a polyatomic cation is bonded to a monatomic nonmetal anion, the naming convention remains consistent with the patterns we have already learned.

Naming Rules

The naming process:

1. The polyatomic cation is named first, using its specific memorized name (e.g., “ammonium”).
2. The nonmetal anion is named second, by taking the root of the element’s name and changing the suffix to –ide.

For example, for the compound NH₄Cl, the cation is NH₄⁺ (ammonium) and the anion is Cl⁻ (chloride). Combining them gives the name ammonium chloride.

Examples:

• NH₄Cl: ammonium chloride
• (NH₄)₂S: ammonium sulfide

Polyatomic cation + polyatomic anion

Another type of ionic compound occurs when both the cation and the anion are polyatomic ions. The formulas can look long, but the naming is simple.

This structure typically involves the ammonium ion (NH₄⁺) acting as the cation, bonded to one of the polyatomic anions we have learned previously.

Naming Rules

The naming process:

1. The polyatomic cation is named first, using its specific memorized name.
2. The polyatomic anion is named second, using its specific memorized name.

No prefixes are used, and no suffixes are changed. You simply state the name of each ion in order.

For example, for the compound (NH₄)₂SO₄, the cation is NH₄⁺ (ammonium) and the anion is SO₄²⁻ (sulfate). Combining them gives the name ammonium sulfate.

Examples:

• (NH₄)₂CO₃: ammonium carbonate
• (NH₄)₂SO₄: ammonium sulfate

Hydrates

Many ionic compounds, when crystallized from an aqueous solution, incorporate a specific number of water molecules into their solid crystal lattice structure. These compounds are called hydrates (or ionic hydrates), and the incorporated water is referred to as the water of hydration. If this water is removed, typically by heating, the remaining ionic compound is said to be anhydrous (without water).

Naming Rules

The nomenclature for hydrates builds directly on the ionic naming rules, with one final step to account for the water molecules.

1. Name the ionic compound part of the formula first, following the standard rules for invariant or multivalent metals.
2. For the water part, add a second word consisting of a Greek prefix to denote the number of water molecules, attached to the root word hydrate.

In the chemical formula, the ionic compound and the water of hydration are separated by a centered dot (·).

NoteGreek Prefixes for Hydrates

To specify the number of water molecules, we use a system of Greek prefixes. You will see these same prefixes again when learning to name covalent molecular compounds. The table below lists the prefixes you will need.

For example, a compound with six water molecules, like CoCl₂ · 6 H₂O, is named cobalt(II) chloride hexahydrate.

The prefix hemi- is used to denote one-half (0.5). For example, CaSO₄ · 0.5 H₂O is named calcium sulfate hemihydrate.

Examples:

• CaSO₄ · 0.5 H₂O: calcium sulfate hemihydrate
• Ba(OH)₂ · 8 H₂O: barium hydroxide octahydrate
• CoCl₂ · 6 H₂O: cobalt(II) chloride hexahydrate
• FeCl₂ · 4 H₂O: iron(II) chloride tetrahydrate
• FeCl₃ · 6 H₂O: iron(III) chloride hexahydrate

Writing Formulas from Names

So far, we have focused on one direction: reading a chemical formula and producing the correct name. You must also be able to do the reverse: start with a compound’s name and write its correct chemical formula.

To write a formula from a name:

1. Identify the ions. The cation is always listed first in the name.
2. Determine the charges. For invariant-charge metals, the charge is predictable. For multivalent metals, the Roman numeral tells you the charge. For polyatomic ions, recall the memorized charge.
3. Balance the charges. Use subscripts so the total positive charge equals the total negative charge. Use the lowest whole-number ratio.

The Criss-Cross Method

The criss-cross method is a quick way to balance charges. Take the numerical value of each ion’s charge and make it the subscript of the other ion.

Example: Aluminum Oxide

Write the formula for aluminum oxide.

• The cation is aluminum (Al³⁺) and the anion is oxide (O²⁻).
• Criss-cross: aluminum’s charge (3) becomes oxygen’s subscript; oxygen’s charge (2) becomes aluminum’s subscript.
• Result: Al₂O₃
• Check: (2 × 3+) + (3 × 2−) = 0

WarningRemember to Simplify

The criss-cross method sometimes produces subscripts that can be reduced. Always simplify to the lowest ratio.

For lead(II) oxide: Pb²⁺ and O²⁻ gives Pb₂O₂, which simplifies to PbO.

For tin(IV) oxide: Sn⁴⁺ and O²⁻ gives Sn₂O₄, which simplifies to SnO₂.

Formulas with Polyatomic Ions

When a polyatomic ion requires a subscript greater than 1, enclose the entire ion in parentheses before adding the subscript.

Example: Calcium Phosphate

Write the formula for calcium phosphate.

• The cation is calcium (Ca²⁺) and the anion is phosphate (PO₄³⁻).
• Criss-cross: calcium’s charge (2) becomes phosphate’s subscript; phosphate’s charge (3) becomes calcium’s subscript.
• Since phosphate needs a subscript of 2, enclose it in parentheses: Ca₃(PO₄)₂
• Check: (3 × 2+) + (2 × 3−) = 0

Summary Examples

The table below shows examples of writing formulas from names.