IUPAC Naming of Inorganic Substances for HSC Chemistry
Learn how to name common ionic, molecular, acidic, and hydrated inorganic substances from their formulae using IUPAC conventions. Includes decision rules, worked examples, and common naming mistakes.
A formula such as \(\ce{FeCl3}\) looks simple enough. Iron, chlorine, three chlorines. So should it be “iron trichloride”?
No. That name would use the wrong naming system.
The useful skill in inorganic nomenclature is not memorising hundreds of compound names. It is recognising what kind of substance the formula represents, then choosing the correct naming rules.
Try this prediction before reading on:
Which formula would you expect to use prefixes such as di- or tri- when naming it: \(\ce{FeCl3}\) or \(\ce{NCl3}\)?
The answer is \(\ce{NCl3}\). Both nitrogen and chlorine are non-metals, so this is a molecular compound. \(\ce{FeCl3}\), however, contains a metal and a non-metal, so it is ionic. Ionic compounds follow a different set of rules.
That first decision solves most HSC naming questions.
01Start by deciding what kind of compound you have
When you are given a formula, look at the elements and ions before trying to name anything.
A useful first-pass decision rule is:
| What you see in the formula | Likely type | Naming system |
|---|---|---|
| Metal + non-metal | Ionic | Cation name + anion ending in -ide |
| Metal + polyatomic ion | Ionic | Cation name + polyatomic ion name |
| \(\ce{NH4+}\) + anion | Ionic | Ammonium + anion name |
| Non-metal + non-metal | Molecular | Prefixes such as mono-, di-, tri- |
| Acid formula in aqueous solution | Acid | Acid naming convention |
| Ionic compound followed by \(\ce{.nH2O}\) | Hydrate | Ionic name + prefix + hydrate |
This is a model, not an absolute description of every inorganic substance ever discovered. For HSC Chemistry, though, it covers the common compounds you are expected to recognise and name.
Quick check
Name the type of compound, not the compound itself:
- \(\ce{MgBr2}\)
- \(\ce{SO3}\)
- \(\ce{Na2CO3}\)
Answers
- \(\ce{MgBr2}\) is ionic because magnesium is a metal and bromine is a non-metal.
- \(\ce{SO3}\) is molecular because sulfur and oxygen are both non-metals.
- \(\ce{Na2CO3}\) is ionic because it contains the metal ion \(\mathrm{Na}^{+}\) and the polyatomic carbonate ion \(\ce{CO3^2-}\).
Notice that \(\ce{Na2CO3}\) contains several non-metal atoms, but that does not make it molecular. The carbonate atoms belong together as one polyatomic ion.
02Naming ionic compounds
An ionic compound is made from positively charged ions called cations and negatively charged ions called anions.
Imagine two people entering a very formal relationship. The formula tells you how many of each ion are needed to keep the whole compound electrically neutral. The name, however, usually does not announce those numbers.
That is why \(\ce{MgCl2}\) is magnesium chloride, not “magnesium dichloride”.
The analogy breaks because ions are obviously not dating each other. The useful part is simply this: the ratio matters when constructing the formula, but ionic names normally identify the ions rather than counting them with prefixes.
Rule 1: Name the cation first
For a simple metal ion, use the element’s name unchanged.
Examples:
- \(\mathrm{Na}^{+}\): sodium
- \(\mathrm{Mg}^{2+}\): magnesium
- \(\mathrm{Al}^{3+}\): aluminium
Rule 2: Simple non-metal anions end in -ide
A single-element negative ion usually changes its ending to -ide.
| Element | Anion name |
|---|---|
| chlorine | chloride |
| bromine | bromide |
| iodine | iodide |
| oxygen | oxide |
| sulfur | sulfide |
| nitrogen | nitride |
| phosphorus | phosphide |
So:
\[
\ce{NaCl}
\]
is sodium chloride, and
\[
\ce{MgO}
\]
is magnesium oxide.
Worked example: Name \(\ce{CaF2}\)
Step 1Identify the compound type
Calcium is a metal and fluorine is a non-metal, so \(\ce{CaF2}\) is ionic.
Step 2Name the cation
\(\mathrm{Ca}^{2+}\) is the calcium ion.
Step 3Name the anion
Fluorine forms fluoride, \(\mathrm{F}^{-}\).
Step 4Combine the names
The compound is calcium fluoride.
The subscript 2 tells us that two fluoride ions are required to balance one \(\mathrm{Ca}^{2+}\) ion. It does not make the name “calcium difluoride”.
Check your understanding
Name \(\ce{Al2O3}\).
Answer
Aluminium is a metal and oxygen is a non-metal, so the compound is ionic.
The ions are \(\mathrm{Al}^{3+}\) and \(\mathrm{O}^{2-}\).
Therefore:
aluminium oxide
Not aluminium trioxide, and not dialuminium trioxide.
03What changes when the metal can have different charges?
Here is where ionic naming becomes more interesting.
Iron can form both \(\mathrm{Fe}^{2+}\) and \(\mathrm{Fe}^{3+}\). Copper commonly forms \(\mathrm{Cu}^{+}\) and \(\mathrm{Cu}^{2+}\).
If someone simply says “iron chloride”, we do not know which iron ion is present.
IUPAC naming solves this by placing the metal’s oxidation state in Roman numerals.
For example:
- \(\ce{FeCl2}\): iron(II) chloride
- \(\ce{FeCl3}\): iron(III) chloride
The Roman numeral is not the number of metal atoms. It tells you the oxidation state of the metal.
Worked example: Name \(\ce{Cu2O}\)
Step 1Recognise an ionic compound
Copper is a metal and oxygen is a non-metal.
Step 2Use the known charge on oxide
Each oxide ion is \(\mathrm{O}^{2-}\).
There is one oxide ion, giving a total negative charge of \(-2\).
Step 3Work out the copper charge
There are two copper atoms. Together they must contribute \(+2\) so that the compound is neutral.
Therefore each copper ion has charge \(+1\):
\[
2(+1) + (-2) = 0
\]
Step 4Name the compound
Copper is in oxidation state \(+1\), so the compound is:
copper(I) oxide
Try one
Name \(\ce{Fe2O3}\).
Answer
Each oxygen is \(\mathrm{O}^{2-}\).
Three oxide ions give:
\[
3(-2)=-6
\]
The two iron ions must therefore provide \(+6\) altogether:
\[
2x=+6
\]
\[
x=+3
\]
Each iron is in oxidation state \(+3\).
The name is iron(III) oxide.
A common mistake is to call it iron(II) oxide because there are two iron atoms. The Roman numeral comes from the charge or oxidation state, not the subscript.
04Ionic compounds containing polyatomic ions
Some ions contain several atoms bonded together but behave as one charged unit. These are polyatomic ions.
You should recognise the common ones because their names usually stay unchanged inside ionic compound names.
| Formula | Name |
|---|---|
| \(\ce{NH4+}\) | ammonium |
| \(\ce{OH-}\) | hydroxide |
| \(\ce{NO3-}\) | nitrate |
| \(\ce{CO3^2-}\) | carbonate |
| \(\ce{SO4^2-}\) | sulfate |
| \(\ce{PO4^3-}\) | phosphate |
| \(\ce{HCO3-}\) | hydrogen carbonate |
For example:
\[
\ce{Na2SO4}
\]
contains sodium ions and sulfate ions, so its name is sodium sulfate.
Do not turn sulfate into “sulfide”. Sulfide refers specifically to the monatomic ion \(\mathrm{S}^{2-}\).
Compare:
- \(\ce{Na2S}\): sodium sulfide
- \(\ce{Na2SO4}\): sodium sulfate
The oxygen atoms make a rather large difference.
Worked example: Name \(\ce{Fe(NO3)3}\)
Step 1Identify the ions
\(\ce{NO3-}\) is nitrate.
There are three nitrate ions.
Step 2Calculate their total charge
\[
3(-1)=-3
\]
Step 3Determine the iron oxidation state
The iron ion must have charge \(+3\) to make the compound neutral.
Step 4Name the compound
The name is iron(III) nitrate.
The parentheses in the formula tell us that the entire nitrate ion occurs three times. They do not appear in the written name.
Quick check
Name \(\ce{Ca(OH)2}\).
Answer
\(\mathrm{Ca}^{2+}\) is calcium and \(\ce{OH-}\) is hydroxide.
The name is calcium hydroxide.
You do not call it calcium dihydroxide in standard ionic naming.
05Naming molecular compounds
Now return to our original pair:
\[
\ce{FeCl3} \qquad \ce{NCl3}
\]
The first is ionic. The second contains two non-metals, so we treat it as a molecular compound.
Here, prefixes do matter because different ratios of the same elements can produce different substances.
For example:
- \(\ce{CO}\): carbon monoxide
- \(\ce{CO2}\): carbon dioxide
Without the prefixes, both would just be called “carbon oxide”, which is not specific enough.
The common numerical prefixes
| Number | Prefix |
|---|---|
| 1 | mono- |
| 2 | di- |
| 3 | tri- |
| 4 | tetra- |
| 5 | penta- |
| 6 | hexa- |
| 7 | hepta- |
| 8 | octa- |
| 9 | nona- |
| 10 | deca- |
The first element keeps its element name. The second element normally ends in -ide.
The prefix mono- is generally omitted from the first element.
So:
\[
\ce{CO}
\]
is carbon monoxide, not monocarbon monoxide.
Worked example: Name \(\ce{N2O4}\)
Step 1Identify the compound type
Nitrogen and oxygen are both non-metals, so this is molecular.
Step 2Count the first element
There are two nitrogen atoms.
Two gives the prefix di-.
So the first part is dinitrogen.
Step 3Count and rename the second element
There are four oxygen atoms.
Four gives tetra-, and oxygen becomes oxide.
The expected combination is tetra + oxide. In conventional spelling, one vowel is dropped, giving tetroxide.
Step 4Combine the name
\(\ce{N2O4}\) is dinitrogen tetroxide.
Try one
Name \(\ce{PCl5}\).
Answer
Both phosphorus and chlorine are non-metals.
There is one phosphorus atom, so no mono- prefix is needed on the first element.
There are five chlorine atoms, so use penta- and change chlorine to chloride.
The name is phosphorus pentachloride.
06The most tempting mistake: using prefixes for ionic compounds
Suppose you see:
\[
\ce{CaCl2}
\]
A student might think, “There are two chlorines, so this must be calcium dichloride.”
That feels reasonable because the formula visibly contains a 2.
But the 2 in an ionic formula exists because of charge balance. Calcium forms \(\mathrm{Ca}^{2+}\), while chloride is \(\mathrm{Cl}^{-}\). Two chloride ions are needed:
\[
(+2)+2(-1)=0
\]
So the correct name is calcium chloride.
Now compare:
\[
\ce{SCl2}
\]
Sulfur and chlorine are both non-metals. Charge balancing between ions is not the naming principle here. We need to specify how many atoms occur in the molecule.
So \(\ce{SCl2}\) is sulfur dichloride.
A useful exam decision rule is therefore:
Do not count atoms until you have decided whether the compound is ionic or molecular.
07Naming common acids
Acids need an extra layer of care because the physical state can matter.
For example, \(\ce{HCl}\) as a molecular gas is hydrogen chloride.
When dissolved in water and written as \(\ce{HCl(aq)}\), it is commonly named hydrochloric acid.
That distinction is useful because the name is telling you something about how the substance is being treated chemically, not just which atoms are present.
Common HSC examples include:
| Formula | Name |
|---|---|
| \(\ce{HCl(aq)}\) | hydrochloric acid |
| \(\ce{HNO3(aq)}\) | nitric acid |
| \(\ce{H2SO4(aq)}\) | sulfuric acid |
| \(\ce{H3PO4(aq)}\) | phosphoric acid |
| \(\ce{H2CO3(aq)}\) | carbonic acid |
There is a useful pattern between several polyatomic ions and their acids:
- nitrate \(\ce{NO3-}\) becomes nitric acid \(\ce{HNO3}\)
- sulfate \(\ce{SO4^2-}\) becomes sulfuric acid \(\ce{H2SO4}\)
- phosphate \(\ce{PO4^3-}\) becomes phosphoric acid \(\ce{H3PO4}\)
You should still learn the common acid names rather than trying to invent a name from a half-remembered suffix rule.
Check your understanding
What is the difference between the names of \(\ce{HCl(g)}\) and \(\ce{HCl(aq)}\)?
Answer
\(\ce{HCl(g)}\) is hydrogen chloride.
\(\ce{HCl(aq)}\) is hydrochloric acid.
The aqueous state tells us that hydrogen chloride is dissolved in water and being described as an acid.
08Naming hydrates
Some ionic crystals contain a fixed number of water molecules within their crystal structure. These compounds are called hydrates.
Their formula may look like:
\[
\ce{CuSO4.5H2O}
\]
Treat the dot as saying, in effect, “this ionic compound is associated with this many waters of crystallisation”.
First name the ionic compound normally:
\[
\ce{CuSO4}
\]
Sulfate has charge \(-2\), so copper must be \(+2\). This gives copper(II) sulfate.
There are five water molecules. The prefix for five is penta-.
Therefore:
copper(II) sulfate pentahydrate
Try one
Name \(\ce{MgSO4.7H2O}\).
Answer
\(\ce{MgSO4}\) is magnesium sulfate.
Seven water molecules gives the prefix hepta-.
The full name is magnesium sulfate heptahydrate.
09A reliable formula-to-name method
When an unfamiliar formula appears in an HSC question, use this order rather than guessing from appearance.
Look for a metal, ammonium ion, or obvious polyatomic ion.
If present, you are probably dealing with an ionic substance.If both elements are non-metals, consider molecular naming.
This is where numerical prefixes usually matter.For an ionic compound, identify the ions before naming anything.
Keep recognised polyatomic ion names intact.Ask whether the metal can have more than one oxidation state.
If so, calculate its oxidation state and show it using a Roman numeral.For a molecular compound, count the atoms.
Apply numerical prefixes and change the second element ending to -ide.Check for special information such as \(\ce{(aq)}\) or \(\ce{.nH2O}\).
You may be dealing with an acid or hydrate.
This approach is much more dependable than trying to remember each compound as a separate vocabulary item.
10Mixed practice: decide the naming system first
Question 1
Name \(\ce{K2S}\).
Answer
Potassium is a metal, so this is ionic.
\(\mathrm{K}^{+}\) is potassium and \(\mathrm{S}^{2-}\) is sulfide.
Potassium sulfide
Question 2
Name \(\ce{SF6}\).
Answer
Sulfur and fluorine are both non-metals, so this is molecular.
There is one sulfur atom and six fluorine atoms.
Six gives hexa-.
Sulfur hexafluoride
Question 3
Name \(\ce{CuCl2}\).
Answer
This is ionic.
Each chloride ion is \(\mathrm{Cl}^{-}\). Two chloride ions give total charge \(-2\), so copper must be \(\mathrm{Cu}^{2+}\).
Copper(II) chloride
Question 4
Name \(\ce{(NH4)2CO3}\).
Answer
This contains the polyatomic ions ammonium, \(\ce{NH4+}\), and carbonate, \(\ce{CO3^2-}\).
Ammonium carbonate
The 2 outside the brackets tells us that two ammonium ions are required. It does not create the name “diammonium carbonate”.
Question 5
Name \(\ce{N2O5}\).
Answer
Both elements are non-metals, so use molecular prefixes.
Two nitrogen atoms gives di-.
Five oxygen atoms gives penta-, with the conventional form pentoxide.
Dinitrogen pentoxide
11The naming clues worth memorising
You do not need to treat every formula as a fresh mystery. A few clues do most of the work.
| Clue | What it tells you |
|---|---|
| Metal at the front | Usually ionic |
| \(\ce{NH4+}\) present | Ionic ammonium compound |
| Familiar polyatomic ion | Keep that ion’s name |
| Two non-metals only | Usually molecular, so consider prefixes |
| Variable-charge metal | Work out the oxidation state |
| \(\ce{(aq)}\) with a common acid formula | Acid naming may apply |
| \(\ce{.nH2O}\) | Hydrate |
Once you can classify the substance correctly, the actual naming becomes fairly mechanical.
This also matters beyond nomenclature. The formula tells you what particles are present, which helps when you later calculate quantities such as percentage composition by mass. It also helps you distinguish pure compounds from systems such as homogeneous mixtures, where several substances may be present even though the sample looks uniform.
The next useful step is to reverse the process: take an IUPAC name, identify the ions or atoms involved, and construct the correct formula while preserving charge neutrality.