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Turning a formula into the name on the bottle: two ions in a fixed order, and a roman numeral exactly where it is doing work.
Paper packet. Every task here also exists on screen, where it is checked automatically; answers written on paper are not assessed by Nydus. When you are back at a device, enter your answers there.
By the end of this lesson you will be able to name an ionic compound from its formula: the positive ion first and unchanged, the negative ion second with an -ide ending or with a polyatomic ion's own name, and a roman numeral after the metal if and only if that metal forms more than one kind of ion. You will be able to work out the charge that numeral has to state, by making the positive and negative charges in one formula unit cancel.
You can read a formula as a count of atoms, and you know from the periodic table which elements give electrons away and which take them in. This lesson turns a formula into a name, which is the form every label, every order form and every safety sheet in the world actually uses.
| Term | What it means |
|---|---|
| Ion | An atom or group of atoms carrying a charge. |
| Cation | A positive ion. |
| Anion | A negative ion. |
| Polyatomic ion | Several atoms joined and carrying one charge as a unit, such as sulfate. |
| Systematic name | A name built by rule, so it leads back to one formula. |
| Roman numeral | In a name, the charge on the metal ion. |
Every ionic compound is a positive ion and a negative ion, packed together in whatever ratio makes the charges cancel. Its name is those two ions, named in that order.
1. The positive ion first, unchanged. Sodium stays sodium, magnesium stays magnesium. The one positive ion that is not a metal is ammonium, and it keeps that name too.
2. The negative ion second. If it is a single element, change the ending to -ide: chlorine becomes chloride, oxygen becomes oxide, sulfur becomes sulfide. If it is a polyatomic ion, use its own name: sulfate, nitrate, carbonate, hydroxide.
3. A roman numeral after the metal, if and only if that metal forms more than one ion. Iron forms $\mathrm{Fe^{2+}}$ and $\mathrm{Fe^{3+}}$, so iron chloride is ambiguous and iron(II) chloride is not. Sodium forms only $\mathrm{Na^+}$, so sodium(I) chloride is not more precise — it is just longer.
Notice what is not in the name: the subscripts. Nobody says "iron-one-chlorine-three". They do not need to, because once you know the two ions and their charges the subscripts are forced — and that is the next lesson.
A useful test for the third rule: ask whether removing the numeral would leave a reader guessing. If it would, write it. If it would not, do not.
Another way: picture
Think of the name as an address written the way a mailing address is: the parts in a fixed order that never varies. Iron(II) chloride is metal, charge, non-metal. Swap the parts and it is not a stricter or a looser address, it is not an address at all — which is why "chloride iron" names nothing.
Another way: steps
To name an ionic compound from its formula:
Split the formula. Find the positive ion, usually the metal written first, and the negative ion that follows it, with any bracketed group taken whole.
Name the positive ion. Unchanged: calcium, iron, ammonium.
Decide whether a numeral is needed. Does this metal form more than one ion? Groups 1 and 2, aluminum, zinc and silver do not; iron, copper, lead, tin, chromium and manganese do.
If so, work out the charge. Total negative charge in one formula unit, divided by the number of metal ions.
Name the negative ion. -ide for a single element; its own name for a polyatomic ion.
Check the work. Read your name back and rebuild the formula from it: the two ions, their charges, the ratio that cancels them. Do you get the formula you started with? Is there a numeral on a metal that never needed one, or none on a metal that did? And is the positive ion first?
Naming the positive ion first is a convention, but a fixed one, so that every chemist reads names the same way. A convention only works if nobody bends it.
Working out the metal's charge from the anions is allowed because a compound has no overall charge. The negative charge of the anions is known from their identity, so the positive charge the metal ions carry between them must equal it exactly.
Writing a numeral only for an ambiguous metal is allowed because a name exists to identify one substance. Where a metal has one possible ion, the name already identifies the compound; where it has two, the numeral is the only part of the name that tells them apart.
Keeping a polyatomic ion's own name is allowed because the group stays together through reactions and in the solid. Sulfate is one particle with one charge, so it gets one name, not a list of its atoms. Each step, in other words, either follows a fixed convention or follows from the neutrality of the compound, and nothing in the name has to be remembered by heart.
These travel whole through a reaction and are written whole in a formula, which is why a bracket appears in the formula as soon as more than one of them is needed. Each has a name of its own and never takes an -ide ending.
| Ion | Formula | Charge | Atoms in it |
|---|---|---|---|
| hydroxide | $\mathrm{OH^-}$ | $1-$ | 2 |
| nitrate | $\mathrm{NO_3^-}$ | $1-$ | 4 |
| carbonate | $\mathrm{CO_3^{2-}}$ | $2-$ | 4 |
| sulfate | $\mathrm{SO_4^{2-}}$ | $2-$ | 5 |
| phosphate | $\mathrm{PO_4^{3-}}$ | $3-$ | 5 |
| ammonium | $\mathrm{NH_4^+}$ | $1+$ | 5 |
| hydrogencarbonate | $\mathrm{HCO_3^-}$ | $1-$ | 5 |
Hydroxide is the one that catches people out: it ends in -ide and yet it is a group of two atoms rather than a single element. That is history rather than logic, and it is the only one of these worth simply knowing.
Ammonium is the other oddity, and the useful one: it is the only positive ion in this course that is not a metal. It behaves exactly as a metal ion does — it carries one positive charge, it pairs with any of the negative ions above, and it goes first in the name — which is why ammonium sulfate is a fertilizer written and named on precisely the same rules as magnesium sulfate.
A metal in group 1 has one electron to give away and gives it; a metal in group 2 has two. So sodium is always $\mathrm{Na^+}$, calcium is always $\mathrm{Ca^{2+}}$, and aluminum in group 3 is always $\mathrm{Al^{3+}}$. One possible ion each, so one possible compound with any given anion, so nothing for a numeral to distinguish.
The transition metals are the ones that need it. Their outer shell is not what is changing across the block — the shell beneath it is — so several different numbers of electrons can come off at a similar cost. Iron gives up two or three; copper gives up one or two; chromium gives up two or three.
Zinc is the exception inside the exception. It is in the middle block and still forms only $\mathrm{Zn^{2+}}$, so zinc chloride needs no numeral. That is worth knowing because it shows the rule is about the metal and not about the block: you write a numeral when a particular metal is ambiguous, not when it happens to sit in the middle of the table.
Water treatment plants across the United States buy compounds by name, and the roman numeral on the drum decides whether the right chemistry happens. Pittsburgh's water works, drawing from the Allegheny River, adds iron(III) chloride, $\mathrm{FeCl_3}$, to raw water. The $\mathrm{Fe^{3+}}$ ions react with the water to form a fluffy iron hydroxide that grabs fine clay and bacteria and settles them out of the water before filtering.
Iron(II) chloride, $\mathrm{FeCl_2}$, is a different substance. Its $\mathrm{Fe^{2+}}$ ions do not form the same settling floc nearly as well, and they use up oxygen in the water as they slowly change to $\mathrm{Fe^{3+}}$. A plant that ordered by the name iron chloride alone could receive the wrong one, and the clarifying tanks would not clear.
The arithmetic behind the numeral is the same as this lesson's: three chloride ions at $1-$ each carry $3-$ of charge, so the single iron ion in $\mathrm{FeCl_3}$ must carry $3+$. Plants in Texas use iron(III) sulfate, $\mathrm{Fe_2(SO_4)_3}$, for the same job, and its name is reached the same way: three sulfates at $2-$ give $6-$, shared between two irons, $3+$ each.
Pool services in Florida add copper(II) sulfate, $\mathrm{CuSO_4}$, to kill algae. The numeral matters: copper(I) compounds behave differently, and the $\mathrm{Cu^{2+}}$ ion is the one that is toxic to algae at low doses. The sulfate keeps its own name on the label, and one sulfate at $2-$ against one copper means the copper must be $2+$ — the numeral the bucket carries, worked out from the formula alone.
A numeral is written where nothing is ambiguous. "Sodium(I) chloride", "magnesium(II) oxide", "aluminum(III) oxide". All three metals form exactly one ion, so the numeral carries no information. It is not wrong in the sense of stating a false charge; it is wrong in the sense that a systematic name says exactly what is needed and no more.
A numeral is left off where something is ambiguous. "Iron chloride" names two substances. One of them is a pale green solid and the other is a dark brown one, they react differently, and a bottle labeled that way cannot be used for anything that matters.
The numeral is read as a subscript. In iron(III) chloride the three is the charge on the iron ion, not the number of iron atoms. There is one iron in the formula unit and three chlorides, and the three chlorides are there because the iron carries $3+$.
The ions are named in the wrong order. The positive ion always comes first. "Chloride sodium" is not a loose way of saying sodium chloride; it names nothing.
A polyatomic ion is taken apart. $\mathrm{NH_4Cl}$ is ammonium chloride, not "nitrogen hydride chloride". The five atoms of ammonium carry one charge between them and behave as one thing.
Split the formula.
$\mathrm{Mg(OH)_2}: \text{magnesium, hydroxide}$
Split it into its two ions first.
Name the positive ion.
$\text{magnesium}$
First and unchanged.
Decide on a numeral.
$\text{none: only } \mathrm{Mg^{2+}}$
Group 2 forms one ion.
Name the negative ion.
$\text{hydroxide}$
A polyatomic ion keeps its own name.
Write the name.
$\text{magnesium hydroxide}$
The subscript is forced, so it is not named.
Split the formula.
$\mathrm{Fe_2O_3}: \text{iron, oxide}$
Two irons, three oxides.
Find the total negative charge.
$3 \times 2 = 6$
Three oxides at 2− each.
Share it between the irons.
$6 \div 2 = 3$
Each iron is $\mathrm{Fe^{3+}}$.
Decide on a numeral.
$\text{needed: iron forms 2+ and 3+}$
The numeral tells the two apart.
Write the name.
$\text{iron(III) oxide}$
Metal, charge, non-metal.
Name its sister compound.
$\mathrm{FeO}: 2 \div 1 = 2 \Rightarrow \text{iron(II) oxide}$
The same arithmetic, a different substance.
Split the formula.
$\mathrm{Fe_2(SO_4)_3}: \text{iron, sulfate}$
The bracket holds the sulfate group.
Read the sulfate's charge.
$\mathrm{SO_4^{2-}}$
From the table of polyatomic ions.
Find the total negative charge.
$3 \times 2 = 6$
Three sulfates.
Share it between the irons.
$6 \div 2 = 3$
Each iron is 3+.
Decide on a numeral.
$\text{needed}$
Iron forms two ions.
Name the negative ion.
$\text{sulfate, not sulfide}$
A polyatomic ion keeps its own name.
Write the name.
$\text{iron(III) sulfate}$
Rebuilding the formula from it gives $\mathrm{Fe_2(SO_4)_3}$ back.
Find the negative charge.
$\mathrm{SO_4^{2-}}: 2$
Start from the ion whose charge you know.
Find the copper's charge.
$2 \div 1 = 2 \Rightarrow \mathrm{Cu^{2+}}$
Copper forms two ions, so the numeral is needed.
Write the name.
Match each formula to its systematic name.
| aluminum oxide | zinc chloride | calcium hydroxide | potassium nitrate | |
|---|---|---|---|---|
| $\mathrm{Al_2O_3}$ | ||||
| $\mathrm{ZnCl_2}$ | ||||
| $\mathrm{Ca(OH)_2}$ | ||||
| $\mathrm{KNO_3}$ |
Complete the worked solution: a bottle of manganese oxide holds $\mathrm{MnO_2}$, and each oxide ion carries a charge of two minus. Find the total negative charge in one formula unit, and the charge on the metal ion that the roman numeral must state.
Find the total negative charge.
$(\text{oxide ions}) \times (\text{charge on each}) =$ t
Two oxide ions in one formula unit.
Find the metal ion's charge.
$(\text{total}) \div (\text{metal ions}) =$ c
One metal ion supplies all the positive charge.
Write the name.
$\text{manganese(IV) oxide}$
This metal forms more than one ion, so the numeral is needed.
Which of these is the correct systematic name for $\mathrm{Ca(OH)_2}$?
A bottle is labeled calcium bromide, and its formula is $\mathrm{CaBr_2}$. Complete the sentence with three numbers.
Each calcium ion in $\mathrm{CaBr_2}$ carries a charge of a+, and one formula unit holds calcium and bromide ions in the ratio b to c.
$\mathrm{CuSO_4}$ is the chemical a Florida pool service adds to kill algae. Each sulfate ion carries a charge of $2-$. What charge does each metal ion carry — the number the roman numeral in its name must state?
The answer: a.
A stockroom is being relabeled. Four bottles carry only formulas: two of them are copper compounds and two are chromium compounds. A oxide ion carries a charge of $2-$ and a chloride ion carries a charge of $1-$. For each bottle, work out the charge on the metal ion — which is the number the roman numeral in its name has to state.
| metal ions in one formula unit | negative ions in one formula unit | charge on each metal ion | |
|---|---|---|---|
| $\mathrm{Cu_2O}$ | 2 | 1 | |
| $\mathrm{CuO}$ | 1 | 1 | |
| $\mathrm{CrCl_2}$ | 1 | 2 | |
| $\mathrm{CrCl_3}$ | 1 | 3 |
Lesson test: one question per skill, one attempt each, no hints. Your answers are checked when you submit.
For each of these three compounds, the charge on the negative ion is given. Work out the charge on the positive ion, and say whether the name needs a roman numeral after the metal.
| charge on each negative ion | charge on each positive ion | does the name need a roman numeral? | |
|---|---|---|---|
| $\mathrm{FeCl_3}$ | 1 | ||
| $\mathrm{NH_4Cl}$ | 1 | ||
| $\mathrm{NaOH}$ | 1 |
You can name an ionic compound from its formula, and decide whether the name needs a roman numeral rather than guessing. Say out loud why sodium(I) chloride is a poor name and iron(II) chloride is a good one. Next: the same reasoning run backwards, to write a formula from a name.
15. Your turn: name $\mathrm{CuSO_4}$., step 3
$\text{copper(II) sulfate}$
Sulfate keeps its own name.