Write the formula and charge
hypochlorite
Worked solution
Answer: ClO -
- Identify the ionsClO- is hypochlorite.
- Where it sits on the ladderhypochlorite 1 Ochlorite 2 Ochlorate 3 Operchlorate 4 O Within one family, -ate is the reference. -ite has one oxygen fewer, hypo...ite one fewer again, and per...ate one more than -ate. The charge stays the same all the way up.
- Write the namehypochlorite, ClO-.
Naming ionic compounds from a formula
Naming an ionic compound is three decisions, taken in order. Find the two ions. Work out whether the metal needs a Roman numeral. Write the cation name then the anion name. Everything that goes wrong in an exam goes wrong at one of those three points, and it is usually the second.
Step one: split the formula into ions
Ionic formulas are written cation first, so the split is almost always at the obvious place. CaCO3 is Ca and CO3. NaNO3 is Na and NO3, not Na and N and O three times.
Brackets make this easier rather than harder. In Fe2(SO4)3 the bracket tells you the second ion is the whole SO4 group and that there are three of them. Without the bracket you would have to know that sulfate exists to read it correctly.
Two cases trip people up. In Na2O2 the second ion is peroxide O22-, not two oxides, and Na2O2 is a genuinely different compound from Na2O. In Hg2Cl2 the cation is mercury(I), a single ion made of two mercury atoms sharing one 2+ charge, so the subscript inside it never cancels against the chlorides.
Step two: decide whether the metal needs a numeral
A Roman numeral says which ion a metal has formed. It is needed exactly when the metal forms more than one.
Group 1, group 2, aluminium, zinc and silver each form a single ion, so a numeral adds nothing. Sodium chloride is sodium chloride, never sodium(I) chloride. Most transition metals form two or more, so iron, copper, lead, tin, chromium, manganese, cobalt and mercury all take one.
Some courses ask for a numeral on every transition metal, including zinc and silver. That is a teaching convention rather than a chemical fact, and the practice tool above accepts silver(I) nitrate while noting that most textbooks drop it.
Step three: read the numeral off the anions
The numeral is not something you remember. It is something you calculate, and the calculation runs backwards from the anion, whose charge is fixed.
Take Fe2(SO4)3. There are three sulfates. Sulfate is 2-, so that is six units of negative charge. The compound is neutral, so the two irons carry six units of positive charge between them, which is 3+ each. The name is iron(III) sulfate.
The same arithmetic on FeSO4 gives one sulfate at 2-, shared over one iron, so iron(II) sulfate. Nothing was memorised. The worked solution under every question in the practice tool shows this calculation with the actual numbers from the question you just answered.
The endings, and what they tell you
An anion ending in -ide is a single element: chloride, oxide, sulfide, nitride. An anion ending in -ate or -ite contains oxygen as well.
So if you see oxygen in the anion and you have written -ide, something is wrong. Sodium chloride is NaCl. Sodium chlorate is NaClO3. They are different compounds and one of them is table salt.
The three exceptions are hydroxide, cyanide and peroxide, which end in -ide despite being polyatomic. Learn those three and the rule is reliable everywhere else.
What an ionic name never contains
Prefixes. There is no such thing as diiron trisulfate. Prefixes such as di-, tri- and tetra- belong to covalent naming, where carbon monoxide and carbon dioxide really are different compounds with the same two elements.
Ionic compounds do not need them, because the charges fix the ratio. Once you have said iron(III) sulfate, there is only one possible formula, so stating the ratio would add nothing.
Where students actually lose marks
Three mistakes account for most of them:
- The numeral is missing. Writing “iron sulfate” is ambiguous, because two different compounds answer to it.
- The numeral is the subscript. In Fe2O3 the 2 is how many irons there are, not the charge on each. The charge is 3+, and the name is iron(III) oxide.
- -ate and -ite swapped. Sodium sulfate and sodium sulfite differ by one oxygen atom and are both real.
The practice tool tells these apart. Getting “you used sulfite where the answer needs sulfate” is a different piece of information from getting a cross, and it points at a different fix.
Where to go next
The reverse skill, going from a name to a formula, is the ionic compound formula writer. If the ion names themselves are the bottleneck, drill those first with the polyatomic ions quiz.
For reference, 33Science publishes the complete list of polyatomic ions, a list of binary compounds and a broader list of chemical compounds with formulas and uses. The underlying reason any of this works is covered in how ionic bonds work.
Common questions
When do I use a Roman numeral?
When the metal forms more than one ion. Calculate it from the anions rather than recalling it: total the negative charge, divide by the number of metal ions, and that is the numeral.
Is it sulphate or sulfate?
Both. Sulfate is the IUPAC spelling and is standard in the United States; sulphate is standard in the United Kingdom and much of the Commonwealth. The practice tool accepts either.
What is the difference between ferric and iron(III)?
Nothing chemically. Ferric and ferrous are the older names for iron(III) and iron(II). Modern courses teach the Roman numeral form, and the tool accepts both.