Chemistry · Lesson

Naming common compounds and recognising chemical identifiers

Connect simple chemical names to formulas, then distinguish a name from a database identifier and from evidence about a real sample.

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By the end, you can

Name selected simple ionic and molecular compounds.
Interpret a Roman numeral in a variable-metal name.
Distinguish names, formulas and substance identifiers.

Names make descriptions easier to exchange

A useful chemical name lets another reader identify the intended chemical composition or structure. Naming conventions make that exchange more reliable across documents and laboratories. They are a language with several systems and familiar retained names, rather than one short rule that uniquely names every substance. Water is a widely used retained name; the formula H₂O conveys its molecular composition.

This lesson introduces a bounded set of common inorganic examples. More complex salts, coordination compounds and organic structures require additional rules in later chapters. When a name is unfamiliar, consult an appropriate nomenclature source and the formula or structure instead of inventing a rule from one example.

Name the cation, then the anion

For a simple ionic compound, name the cation first and the anion second. NaCl is sodium chloride; CaCl₂ is calcium chloride. A simple monatomic anion often uses an “-ide” ending, as chloride does. Familiar polyatomic-ion names are retained: Ca(OH)₂ is calcium hydroxide and Ca₃(PO₄)₂ is calcium phosphate.

Some metal names need their oxidation state stated. FeCl₂ is iron(II) chloride and FeCl₃ is iron(III) chloride. With chloride assigned −1, neutrality gives iron +2 and +3 respectively. The Roman numeral refers to the iron oxidation state in these compounds; it is not a prefix counting how many iron atoms appear. Formal oxidation-state assignment is developed later and is not generally a measured local charge.

A small, bounded naming dictionary

Formula	Name	Information highlighted
NaCl	Sodium chloride	Cation then anion
Ca(OH)₂	Calcium hydroxide	Retained polyatomic-ion name
FeCl₂	Iron(II) chloride	Iron oxidation state +2
FeCl₃	Iron(III) chloride	Iron oxidation state +3
CO₂	Carbon dioxide	Two oxygen atoms per molecule

Examples of introductory compositional names. The Roman numeral distinguishes iron oxidation state; prefixes in the molecular examples count oxygen atoms.

Prefixes can count composition

In selected simple binary molecular names, numerical prefixes specify atom counts. CO is carbon monoxide and CO₂ is carbon dioxide. “Di-” indicates two oxygen atoms in the latter name. The first element commonly omits “mono-” when there is one atom. Learn the convention with its scope rather than assuming every chemical name uses prefixes in the same way.

A compositional name need not state connectivity or three-dimensional structure. IUPAC permits more than one systematic name for many substances and retains a number of traditional names. Compare formula and structural information when the question needs more than composition. Different accepted names need not mean different substances.

Identifiers help link records

A CAS Registry Number is an assigned identifier used to connect substance records despite differences in names. Its hyphenated digits are not atom counts or a code that you should interpret as a chemical formula. InChI is a structure-based identifier developed through international IUPAC collaboration, with representation conventions for connecting chemical data.

A name, formula or database identifier does not independently establish the contents, purity or safety of a bottle. Check the actual sample information and relevant evidence. Likewise, structure identifiers have defined representation scope; they should not be assumed to distinguish every possible material, mixture or structural detail without considering those conventions.

Worked examples

Why the iron names differ

Why do FeCl₂ and FeCl₃ have different Roman numerals?

Assign chloride oxidation state −1 in each compound.
For FeCl₂, two chlorides contribute −2, requiring iron +2 for a neutral formula.
For FeCl₃, three chlorides contribute −3, requiring iron +3.

The names are iron(II) chloride and iron(III) chloride. Each formula has one iron atom; the Roman numeral does not count iron atoms.

Try it yourself

Name CaCl₂ in the introductory ionic convention.
Show the answer and reasoning
Calcium chloride.
The cation name comes first; chloride names the anion.
Name Ca(OH)₂.
Show the answer and reasoning
Calcium hydroxide.
The familiar OH⁻ group retains the hydroxide name.
Does iron(III) chloride mean a molecule containing three iron atoms?
Show the answer and reasoning
No.
III identifies the assigned iron oxidation state +3 in this compound.
Does a listed CAS Registry Number by itself prove a commercial sample is pure or safe?
Show the answer and reasoning
No.
The identifier links a substance record. Evidence about the actual sample and use is still needed.

What to carry forward

Apply naming conventions within their stated scope.
Selected ionic names place the cation before the anion.
Roman numerals in these metal names indicate oxidation state.
Names and identifiers organize information; they do not replace sample evidence.

References

Teaching resources and reference material supporting this lesson’s explanations and examples.