Difficulty: Introductory. Prerequisites: basic understanding of the periodic table, atomic structure, and electron configuration.
Ionic compounds are made of a positive ion (cation) and a negative ion (anion) held together by electrostatic attraction. To name them, you name the cation first, then the anion. To write the formula, you balance the charges so the compound is electrically neutral. Transition metals need a Roman numeral in the name to show which charge they carry.
Ion
An atom or group of atoms that carries a net electrical charge because it has gained or lost electrons. In simple terms, it is an atom that is no longer electrically neutral.
Cation
A positively charged ion, formed when an atom loses one or more electrons. Think of it as: the "cat" is "paws-itive." Metals typically form cations.
Anion
A negatively charged ion, formed when an atom gains one or more electrons. Think of it as: "a negative ion." Non-metals typically form anions.
Polyatomic ion
An ion made up of two or more atoms covalently bonded together that collectively carry a charge. In simple terms, it is a group of atoms that acts as a single charged unit (e.g. NO₃⁻, SO₄²⁻).
Ionic compound
A compound formed by the electrostatic attraction between cations and anions. The formula reflects the simplest whole-number ratio of ions needed to achieve electrical neutrality.
Oxidation state (charge)
The charge an atom carries in a compound. For main-group metals, it is predictable from their group on the periodic table. For transition metals, it can vary, which is why the charge is specified with a Roman numeral.
Roman numeral notation (Stock notation)
The system used to indicate the charge of a transition metal cation in the name of an ionic compound. For example, Iron(II) means Fe²⁺ and Iron(III) means Fe³⁺.
Main-group metals keep their element name: Na⁺ = sodium, K⁺ = potassium, Ca²⁺ = calcium, Mg²⁺ = magnesium, Ba²⁺ = barium, Sr²⁺ = strontium, Li⁺ = lithium, Al³⁺ = aluminium.
Transition metals require a Roman numeral to indicate charge: Fe²⁺ = iron(II), Fe³⁺ = iron(III), Pb⁴⁺ = lead(IV).
The Roman numeral is placed in parentheses immediately after the metal name, with no space.
Take the root of the element name and add the suffix "-ide": Br⁻ = bromide, O²⁻ = oxide, Cl⁻ = chloride, F⁻ = fluoride.
These appear repeatedly in general chemistry and need to be committed to memory, including their charges.
OH⁻ = hydroxide
NO₃⁻ = nitrate
NO₂⁻ = nitrite
SO₄²⁻ = sulfate
CO₃²⁻ = carbonate
HCO₃⁻ = bicarbonate (hydrogen carbonate)
PO₄³⁻ = phosphate
NH₄⁺ = ammonium (the only common polyatomic cation)
ClO₃⁻ = chlorate
ClO₂⁻ = chlorite
Notice the pattern: "-ate" ions have more oxygen atoms than "-ite" ions. Nitrate (NO₃⁻) vs nitrite (NO₂⁻). Chlorate (ClO₃⁻) vs chlorite (ClO₂⁻). Sulfate (SO₄²⁻) vs sulfite (SO₃²⁻).
The overall charge of an ionic compound must be zero. To write the formula:
Write the cation first, then the anion.
Determine the charge of each ion.
Find the smallest whole-number ratio that balances the charges to zero.
Write the subscripts. If a polyatomic ion needs a subscript greater than 1, enclose it in parentheses.
Worked examples from the source material:
Sodium bromide: Na⁺ and Br⁻, charges already balance 1:1, so NaBr.
Calcium hydroxide: Ca²⁺ and OH⁻, need two OH⁻ to balance Ca²⁺, so Ca(OH)₂.
Aluminium chloride: Al³⁺ and Cl⁻, need three Cl⁻, so AlCl₃.
Iron(II) chloride: Fe²⁺ and Cl⁻, need two Cl⁻, so FeCl₂.
Iron(III) phosphate: Fe³⁺ and PO₄³⁻, charges balance 1:1 (both are magnitude 3), so FePO₄.
Lithium carbonate: Li⁺ and CO₃²⁻, need two Li⁺, so Li₂CO₃.
Iron(III) oxide: Fe³⁺ and O²⁻, cross the charges to get Fe₂O₃.
Lead(IV) carbonate: Pb⁴⁺ and CO₃²⁻, need two CO₃²⁻, so Pb(CO₃)₂.
A shortcut for balancing: take the magnitude of each ion's charge and use it as the subscript for the other ion. Then simplify to the lowest whole-number ratio.
For Fe₂O₃: iron(III) is 3+, oxide is 2−. The 3 becomes O's subscript, the 2 becomes Fe's subscript, giving Fe₂O₃.
Students often think all metals need Roman numerals. They do not. Main-group metals (Groups 1, 2, and aluminium) always have the same charge, so no Roman numeral is needed. Sodium is just "sodium," never "sodium(I)."
Students confuse "-ate" and "-ite" endings and assume they are interchangeable. They are not. The "-ate" ion has one more oxygen than the "-ite" ion with the same central atom. Getting them backwards changes the compound entirely.
Students sometimes write the subscripts without simplifying. If the criss-cross gives Mg₂O₂, that should be reduced to MgO.
Students forget to use parentheses around polyatomic ions when subscripts are needed. Ca(OH)₂ is correct. CaOH₂ is wrong and implies a different formula altogether.
⚠️ Naming polyatomic ions from memory is tested frequently. Expect to be given a formula and asked to produce the name, or vice versa, without a reference table.
⚠️ Writing correct formulas for compounds with transition metals (Roman numeral notation) is a staple exam question. You must determine the charge from the formula or deduce the formula from the name.
⚠️ Parentheses around polyatomic ions are a common mark-losing error. Examiners look for this specifically.
⚠️ The difference between "-ate" and "-ite" (e.g. nitrate vs nitrite, sulfate vs sulfite, chlorate vs chlorite) appears in both naming and formula-writing questions.
True or false: Sodium chloride should be written as NaCl, and sodium needs a Roman numeral in the name. (False. Sodium is a Group 1 metal with only one possible charge, so no Roman numeral is used.)
Fill in the blank: The polyatomic ion NO₃⁻ is called ____. (Nitrate.)
True or false: The formula for calcium hydroxide is CaOH₂. (False. It is Ca(OH)₂, with parentheses around the polyatomic ion.)
Fill in the blank: In Iron(III) oxide, the Roman numeral III tells you that the iron ion has a charge of ____. (3+, i.e. Fe³⁺.)
True or false: NH₄⁺ is a polyatomic anion. (False. It is a polyatomic cation.)
Q: Name the compound FeCl₂.
A: Iron(II) chloride. The two chloride ions (each 1−) tell you the iron must be 2+ to balance.
Q: Write the formula for aluminium chloride.
A: AlCl₃. Aluminium is Al³⁺, chloride is Cl⁻, so you need three chlorides.
Q: What is the formula for magnesium chlorate?
A: Mg(ClO₃)₂. Magnesium is Mg²⁺, chlorate is ClO₃⁻, so two chlorates are required. Parentheses go around the polyatomic ion.
Q: Name the compound Li₂CO₃.
A: Lithium carbonate. Two lithium ions (each 1+) balance one carbonate ion (2−).
Q: Write the formula for lead(IV) carbonate.
A: Pb(CO₃)₂. Lead is Pb⁴⁺, carbonate is CO₃²⁻, so two carbonates balance the 4+ charge.
Q: Write the ions present in ammonium nitrate and give its formula.
A: NH₄⁺ and NO₃⁻. They balance 1:1, so the formula is NH₄NO₃.
Q: What is the difference between nitrate and nitrite?
A: Nitrate is NO₃⁻ (three oxygens). Nitrite is NO₂⁻ (two oxygens). The "-ate" form has one more oxygen than the "-ite" form.
Ionic naming conventions feed directly into naming acids: many acids are simply the hydrogen form of a familiar anion (e.g. sulfate becomes sulfuric acid, nitrate becomes nitric acid). Understanding charges here also underpins balancing chemical equations and stoichiometry later in the course. The concept of polyatomic ions returns when studying solubility rules and precipitation reactions.
Related Terms / Search Tags
Ionic compounds, naming ions, cation, anion, polyatomic ions, Roman numeral notation, Stock notation, transition metal charges, criss-cross method, charge balancing, ion formulas, hydroxide, nitrate, nitrite, sulfate, carbonate, bicarbonate, phosphate, ammonium, chlorate, chlorite, bromide, oxide, general chemistry, Purdue, Module 3, sections 2.5 to 2.7