Why practice names so much?
On the AP exam and in English-language textbooks, substances usually appear by name only: a question says “a solution of iron(III) chloride,” not FeCl₃. When you read that name, you need to write FeCl₃ at once and know that the iron is +3. Lesson 1.5.4 covered the basics: name the cation first, end monatomic anions in -ide, and add a Roman numeral for metals like iron. This chapter works through the whole English naming system, starting here with binary ionic compounds, made of one metal and one nonmetal.
Naming cations
Metals with a fixed charge: the cation’s name is just the element name.
| Cations | Name | Chinese name |
|---|---|---|
| Group 1: Li⁺, Na⁺, K⁺ | lithium, sodium, potassium | 锂离子, 钠离子, 钾离子 |
| Group 2: Mg²⁺, Ca²⁺, Ba²⁺ | magnesium, calcium, barium | 镁离子, 钙离子, 钡离子 |
| Al³⁺, Zn²⁺, Ag⁺ | aluminum, zinc, silver | 铝离子, 锌离子, 银离子 |
Metals with more than one common charge: most transition metals, plus tin and lead, form more than one cation. The name is the element name followed immediately by the ion’s charge as a Roman numeral in parentheses. This is the Stock systemStock 命名法; iron(II) is read “iron two.”
| Ion | Stock name | Older name | Chinese name |
|---|---|---|---|
| Fe²⁺ | iron(II) | ferrous | 亚铁离子 |
| Fe³⁺ | iron(III) | ferric | 铁离子 |
| Cu⁺ | copper(I) | cuprous | 亚铜离子 |
| Cu²⁺ | copper(II) | cupric | 铜离子 |
| Sn²⁺ | tin(II) | stannous | 亚锡离子 |
| Sn⁴⁺ | tin(IV) | stannic | 锡离子 |
| Pb²⁺ | lead(II) | plumbous | 铅离子 |
The older names use Latin roots, with -ous for the lower charge and -ic for the higher. Textbooks and AP now use Stock names, but you may still see the old ones on reagent bottles. Chinese names mark the lower of two charges with 亚. One odd case: mercury’s 1+ ion is two mercury atoms bonded together, Hg₂²⁺, called mercury(I).
Naming monatomic anions
A monatomic anion is named by changing the end of the element name to -ide. Its charge follows from its group (lesson 2.2.6):
| Group | Anions | Names | Chinese names |
|---|---|---|---|
| 17 | F⁻, Cl⁻, Br⁻, I⁻ | fluoride, chloride, bromide, iodide | 氟离子, 氯离子, 溴离子, 碘离子 |
| 16 | O²⁻, S²⁻ | oxide, sulfide | 氧离子, 硫离子 |
| 15 | N³⁻, P³⁻ | nitride, phosphide | 氮离子, 磷离子 |
From formula to name
The name of a binary ionic compound is the cation name followed by the anion name. For a metal whose charge can vary, first work out its charge from the formula:
- Total anion charge. The anion’s charge is fixed; multiply it by the number of anions.
- Metal ion charge. The compound is neutral, so the total positive charge equals the total negative charge; divide by the number of metal atoms.
- Write the name. Put that charge as a Roman numeral in parentheses.
For Fe₂O₃: three O²⁻ ions carry 6− in all, so two iron ions carry 6+, each Fe³⁺. The name is iron(III) oxide (Chinese 氧化铁).
Going from a name to a formula uses the method of lesson 1.5.3: write both ions with their charges, then cross and reduce so the charges cancel.
Choose names for formulas, then switch to choosing formulas for names; every answer comes with the reasoning:
Example 1: Two copper oxides
Name CuO and Cu₂O in English and Chinese.
-
The idea. Copper commonly forms both +1 and +2 ions, so the name must state the copper ion’s charge. Find it from the fact that the compound is neutral; oxide is O²⁻.
-
CuO: let the copper ion’s charge be x.
It’s Cu²⁺: copper(II) oxide (氧化铜, a black solid).
-
Cu₂O: two copper ions share the 2− of O²⁻.
It’s Cu⁺: copper(I) oxide (氧化亚铜, a brick-red solid).
-
A common mistake. Seeing two Cu atoms in Cu₂O and writing copper(II). The Roman numeral gives the charge of one ion, not the number of atoms.
Example 2: From names to formulas
Write the formulas of lead(II) iodide and chromium(III) oxide.
-
Lead(II) iodide: write the ions first. The (II) means the lead ion is Pb²⁺; iodide is I⁻.
-
Make the charges add to zero. One Pb²⁺ needs two I⁻:
It’s a bright yellow precipitate (碘化铅 in Chinese).
-
Chromium(III) oxide. Cr³⁺ and O²⁻; the least common multiple of 3 and 2 is 6, so two Cr³⁺ and three O²⁻:
-
The method. Crossing the charges (each ion’s charge becomes the other’s subscript) is a shortcut; reduce to the simplest whole-number ratio afterward and check that the charges add to zero.
- FeCl₃
Three Cl⁻ make 3−, so the ion is Fe³⁺: iron(III) chloride.
- CuO
One O²⁻ is 2−, so the ion is Cu²⁺: copper(II) oxide.
- Cu₂O
The 2− of O²⁻ is shared by two copper ions, so each is Cu⁺: copper(I) oxide.
- SnO₂
Two O²⁻ make 4−, so the ion is Sn⁴⁺: tin(IV) oxide.
- PbI₂
Two I⁻ make 2−, so the ion is Pb²⁺: lead(II) iodide.
- Cr₂O₃
Three O²⁻ make 6−, shared by two chromium ions, so each is Cr³⁺: chromium(III) oxide.
- MnCl₂
Two Cl⁻ make 2−, so the ion is Mn²⁺: manganese(II) chloride.
- SnCl₄
Four Cl⁻ make 4−, so the ion is Sn⁴⁺: tin(IV) chloride.
- CuCl
One Cl⁻ is 1−, so the ion is Cu⁺: copper(I) chloride.
Common mistakes
Mistake 1: “The Roman numeral is the number of metal atoms.” The Roman numeral is the charge of one metal ion. Fe₂O₃ has two iron atoms, but its name is iron(III) oxide; SnO₂ has one tin atom, but its name is tin(IV) oxide.
Mistake 2: “Every metal gets a Roman numeral.” Only metals whose charge can vary do. NaCl is sodium chloride, never sodium(I) chloride. Zn²⁺ and Ag⁺ have fixed charges too, so ZnO is simply zinc oxide.
Mistake 3: “Ionic compounds use prefixes like di- and tri-.” Fe₂O₃ is not diiron trioxide, and CaCl₂ is not calcium dichloride. In an ionic compound the charges fix the numbers of ions, so the name doesn’t state them. Prefixes are only for molecular compounds of two nonmetals (lesson 2.5.3).
Quick check
What is the name of Cu₂O?
The 2− of O²⁻ is shared by two copper ions, so each copper ion is 1+: copper(I) oxide. Copper's charge varies, so the Roman numeral is required, and ionic compounds don't use prefixes.
What is the formula of tin(IV) chloride?
Tin(IV) is Sn⁴⁺, so it takes four Cl⁻ to balance it: SnCl₄.
Summary
- A binary ionic compound is named cation first, then anion; monatomic anions end in -ide.
- Metals in groups 1 and 2, plus Al, Zn, and Ag, have fixed charges and no Roman numeral. Metals whose charge can vary (Fe, Cu, Sn, Pb, Cr, Mn, and others) show the ion’s charge as a Roman numeral in parentheses.
- From formula to name, use the total anion charge to find the metal ion’s charge. From name to formula, write both ions and balance the charges.
On to AP (for later)
AP questions often give only a substance’s name, such as “solid iron(III) oxide reacts with aluminum,” and expect you to write Fe₂O₃ and Al. AP U1.8 also asks you to predict the charges of main-group ions from their position on the periodic table, which is exactly what you need to write anion formulas.