C. Chemistry: Chemical Reactions · Chemical ReactionsNames, Formulas and Composition of Compounds
C3.1 C3.8 C2.1 A1.12About 70 minutes
Learning goals- I can describe the composition of a compound from its formula.
- I can decide whether a binary compound is ionic or molecular and name it correctly.
- I can name and write formulas for compounds of multivalent metals using Roman numerals (the Stock system).
- I can work out a metal ion's charge from a formula.
Words to know
chemical formula
Symbols and subscripts that show which elements are in a compound and in what numbers.
subscript
A small number written low after a symbol; it tells how many atoms or ions of that element there are.
composition
What a compound is made of: which elements, and how many atoms or ions of each.
binary compound
A compound made of exactly two different elements.
multivalent metal
A metal that can form ions with more than one charge, such as iron (Fe²⁺ or Fe³⁺).
Stock system
Naming a multivalent metal's ion with its charge as a Roman numeral in brackets, e.g. iron(III).
formula unit
The simplest ratio of ions in an ionic compound, e.g. one Na⁺ and one Cl⁻ in NaCl.
One atom makes all the difference
Carbon monoxide and carbon dioxide are both made of carbon and oxygen. Their names and formulas differ by just one oxygen atom. Yet carbon dioxide is the gas you breathe out every few seconds, while carbon monoxide is a poisonous gas with no colour and no smell. That is why Ontario homes with fuel-burning furnaces, fireplaces or attached garages must have carbon monoxide alarms.
In chemistry, a name is not just a label. A correct name tells you exactly which atoms are in a compound, and how many. This unit is all about chemical reactions, and before you can write reactions you need to read and write the names and formulas of the substances in them. This lesson reviews what you learned in Grade 9 and adds one important new skill: naming compounds of metals that can form more than one kind of ion.
Remember from Grade 9
In Grade 9 you learned that metals lose electrons to form positive ions (cations) and nonmetals gain electrons to form negative ions (anions). You wrote formulas for binary ionic compounds with the crisscross method (Al₂O₃, MgCl₂) and named them with -ide endings. You also learned that nonmetals share electrons in molecular compounds, named with prefixes (carbon dioxide, dinitrogen monoxide). This lesson builds directly on those skills.
What a chemical formula tells you
A chemical formula uses element symbols and subscripts (small numbers written low and to the right) to show the composition of a compound: which elements it contains and in what numbers.
- In CO₂, there is no subscript after C, which means 1. The subscript 2 belongs to O. So one molecule has 1 carbon atom and 2 oxygen atoms, 3 atoms in total.
- In Al₂O₃, there are 2 aluminium ions for every 3 oxide ions.
- A subscript belongs only to the symbol right before it.
A binary compound contains exactly two different elements, such as NaCl, H₂O, CO₂ or Fe₂O₃. (H₂O₂ is binary too: two elements, even though there are four atoms.)
The name, the formula and the composition always match each other. If you know one, you can work out the other two. That is what expectation C3.1 asks you to do.
Quick review: the two naming systems
1. Binary ionic compounds (metal + nonmetal). Name the metal, then the nonmetal with its ending changed to -ide. No prefixes.
- NaCl: sodium chloride. CaF₂: calcium fluoride. Al₂S₃: aluminium sulfide. K₂O: potassium oxide.
- The formula shows the lowest ratio of ions, chosen so the total charge is zero.
2. Binary molecular compounds (two nonmetals). Use prefixes to show how many atoms of each element: mono (1), di (2), tri (3), tetra (4), penta (5), hexa (6), hepta (7), octa (8), nona (9), deca (10). Never put mono- on the first element.
- CO: carbon monoxide. N₂O₄: dinitrogen tetroxide. PCl₃: phosphorus trichloride. SF₆: sulfur hexafluoride.
- The formula shows the actual atoms in one molecule, so it is never reduced.
How do you decide which system to use? Look for a metal. The flowchart below sums it up.
New: metals with more than one charge
In Grade 9 you mostly used main-group metals such as sodium, magnesium and aluminium. Each of them forms only one kind of ion, so its charge comes straight from its group number.
Many transition metals (the middle block of the periodic table) can form two or more different ions. These are called multivalent metals. Iron can be Fe²⁺ or Fe³⁺. Copper can be Cu⁺ or Cu²⁺. Iron and chlorine can therefore make two different compounds: FeCl₂ and FeCl₃. They have different colours and properties, so they need different names.
Chemists solve this with the Stock system: write the ion's charge as a Roman numeral in brackets right after the metal's name.
- FeCl₂ is iron(II) chloride (said "iron two chloride").
- FeCl₃ is iron(III) chloride ("iron three chloride").
The Roman numeral is the charge of one metal ion, not the number of metal atoms in the formula.
| Metal | Common ions | Stock names |
|---|
| iron | Fe²⁺, Fe³⁺ | iron(II), iron(III) |
| copper | Cu⁺, Cu²⁺ | copper(I), copper(II) |
| lead | Pb²⁺, Pb⁴⁺ | lead(II), lead(IV) |
| tin | Sn²⁺, Sn⁴⁺ | tin(II), tin(IV) |
| chromium | Cr²⁺, Cr³⁺ | chromium(II), chromium(III) |
| cobalt | Co²⁺, Co³⁺ | cobalt(II), cobalt(III) |
| zinc (one charge only) | Zn²⁺ | zinc (no numeral) |
| silver (one charge only) | Ag⁺ | silver (no numeral) |
Working backward: finding the metal's charge from a formula
To name a compound with a multivalent metal, you must work out the metal's charge from the formula.
- Find the total negative charge from the nonmetal ions. (You know these from the group: O²⁻, Cl⁻, S²⁻, N³⁻...)
- The total positive charge must be the same size, because the compound is neutral.
- Divide the total positive charge by the number of metal ions. That is the charge on each metal ion, and the Roman numeral.
Watch out: do not just "un-crisscross" the subscripts. Formulas are reduced, so the subscripts can hide the real charge. Always check with the charges.
Worked exampleExample 1: Name FeCl₃ and Cu₂O
Name each compound: (a) FeCl₃, (b) Cu₂O.
- (a) Iron is multivalent, so it needs a Roman numeral. Chloride is Cl⁻. Three chloride ions give a total charge of 3 × (1−) = 3−.
- The one iron ion must therefore be 3+. Name: iron(III) chloride.
- (b) Copper is multivalent. Oxide is O²⁻, so the total negative charge is 2−.
- There are two copper ions sharing 2+, so each is 2+ ÷ 2 = 1+. Name: copper(I) oxide.
(a) iron(III) chloride; (b) copper(I) oxide. In (b), notice the Roman numeral is I even though the formula has a subscript of 2: the numeral is the charge of each ion, not the number of ions.
Worked exampleExample 2: A trap with a reduced formula
- Lead is multivalent (Pb²⁺ or Pb⁴⁺). Oxide is O²⁻. Two oxide ions give 4− in total.
- There is one lead ion, so it must be 4+.
- If you had "un-crisscrossed" PbO₂, you would wrongly guess Pb²⁺ and O¹⁻. But oxide is never 1−. The real ions are Pb⁴⁺ and O²⁻, and the crisscrossed formula Pb₂O₄ was reduced to PbO₂.
lead(IV) oxide. Always check the nonmetal's real charge before deciding the metal's charge.
Worked exampleExample 3: From name to formula
Write the formula for (a) tin(II) fluoride and (b) iron(III) oxide. Then give the composition of each.
- (a) The Roman numeral II means Sn²⁺. Fluoride is F⁻. Two F⁻ are needed to balance 2+: SnF₂.
- (b) The Roman numeral III means Fe³⁺. Oxide is O²⁻. Crisscross: Fe₂O₃. Check: 2 × (3+) = 6+ and 3 × (2−) = 6−. ✔
- Composition: SnF₂ has 1 tin ion for every 2 fluoride ions. Fe₂O₃ has 2 iron ions for every 3 oxide ions.
(a) SnF₂ (tin(II) fluoride is used in some toothpastes, where it is called stannous fluoride); (b) Fe₂O₃ (a main part of rust).
Worked exampleExample 4: Composition of a molecular compound
Name N₂O₄ and describe its composition. How many atoms are in 5 molecules?
- Both elements are nonmetals, so it is molecular: use prefixes.
- 2 nitrogen atoms → dinitrogen. 4 oxygen atoms → tetra + oxide → tetroxide (drop the a before oxide).
- One molecule has 2 N + 4 O = 6 atoms. Five molecules have 5 × 6 = 30 atoms (10 N and 20 O).
Dinitrogen tetroxide; each molecule has 2 nitrogen and 4 oxygen atoms; 5 molecules contain 30 atoms.
Hands-on activityChemical name detective
You need: packages and labels from around your home (toothpaste, multivitamin bottle, sunscreen, salt, a bag of road salt or ice melter if you have one), paper and a pencil. Safety: only read labels; do not open or taste any product. Leave ice melter, cleaners and medicine in their containers.
- Search the ingredient lists for names that look like binary compounds or ions. Look for words ending in -ide and for Roman numerals or the older names ferrous, ferric, cupric, cuprous, stannous.
- For each one you find, write the name, then try to write the formula. Use a periodic table (one is easy to find online from a trusted source).
- Decide whether each compound is ionic or molecular.
- Older names use -ous for the lower charge and -ic for the higher charge: ferrous = iron(II), ferric = iron(III), cuprous = copper(I), cupric = copper(II), stannous = tin(II). Rewrite any older names you find using the Stock system.
What to look for: common finds are sodium fluoride (NaF) or stannous fluoride (SnF₂) in toothpaste; zinc oxide (ZnO) and titanium dioxide (TiO₂) in sunscreen; potassium iodide (KI) on iodized salt; sodium chloride (NaCl) or calcium chloride (CaCl₂) on ice melter; cupric oxide (CuO, copper(II) oxide) or zinc oxide in a multivitamin. All of these except titanium dioxide are named as ionic compounds. Titanium dioxide uses a prefix name by tradition, even though titanium is a metal; its Stock name would be titanium(IV) oxide.
Just for funPlumbers are named after lead. The symbol Pb comes from the Latin word plumbum, and ancient Roman water pipes were often made of lead. (Today we know lead is toxic, so modern plumbing uses copper and plastic.)
Your turn
1Give the composition of each formula: (a) H₂S, (b) CaCl₂, (c) P₂O₅.
(a) 2 hydrogen atoms and 1 sulfur atom per molecule; (b) 1 calcium ion for every 2 chloride ions; (c) 2 phosphorus atoms and 5 oxygen atoms per molecule.
2Name: (a) KBr, (b) CS₂, (c) MgO.
(a) potassium bromide (ionic, no prefixes); (b) carbon disulfide (molecular); (c) magnesium oxide (ionic).
3Why does CuCl₂ need a Roman numeral in its name, but CaCl₂ does not?
Copper is multivalent (Cu⁺ or Cu²⁺), so the name must say which ion is present: copper(II) chloride. Calcium only forms Ca²⁺, so calcium chloride is enough.
FeO: oxide is 2−, so one iron is 2+ → iron(II) oxide. Fe₂O₃: three oxides give 6−, so two irons share 6+, each 3+ → iron(III) oxide.
5Write the formula for copper(II) sulfide.
Cu²⁺ and S²⁻: charges already balance 1 : 1 → CuS.
6Write the formula for dinitrogen pentoxide and say how many atoms are in one molecule.
7Is SO₃ ionic or molecular? Name it.
Sulfur and oxygen are both nonmetals → molecular: sulfur trioxide.
More practice: warm-up
1What does the subscript 2 in H₂O tell you?
There are 2 hydrogen atoms in each molecule (the 2 belongs to H).
2What does "binary" mean in "binary compound"?
The compound contains exactly two different elements.
3What does the Roman numeral in iron(III) tell you?
Each iron ion has a charge of 3+.
4What number does the prefix tetra- stand for?
More practice: on your own
Four Cl⁻ give 4−, so the one tin ion is 4+ → tin(IV) chloride.
2Write the formula for lead(II) bromide.
Pb²⁺ and Br⁻: two bromide ions balance 2+ → PbBr₂.
3Write the formula for chromium(III) oxide.
Cr³⁺ and O²⁻: crisscross → Cr₂O₃. Check: 2 × 3+ = 6+; 3 × 2− = 6−. ✔
4A student names CO₂ "carbon oxide". What is wrong, and what is the correct name?
CO₂ is molecular (two nonmetals), so the name needs prefixes to show the number of oxygen atoms. Correct: carbon dioxide. "Carbon oxide" could mean CO or CO₂.
5A student names CuO "copper oxide". Explain what is missing and fix it.
Copper is multivalent, so the name must give the charge. Oxide is 2−, so copper is 2+: copper(II) oxide.
Challenge
1Two compounds of manganese and oxygen are MnO and MnO₂. Name both, and explain how the same two elements can make two compounds.
MnO: one O²⁻ (2−) → Mn²⁺ → manganese(II) oxide. MnO₂: two O²⁻ (4−) → Mn⁴⁺ → manganese(IV) oxide. Manganese is multivalent: it can lose different numbers of electrons, so it forms ions with different charges, and each ion combines with oxide in a different ratio.
2A compound of chromium and chlorine has 1 chromium ion for every 3 chloride ions. Write its formula and Stock name. How many ions are in 4 formula units?
Formula CrCl₃. Three Cl⁻ give 3−, so chromium is 3+: chromium(III) chloride. Each formula unit has 1 + 3 = 4 ions, so 4 formula units contain 16 ions (4 Cr³⁺ and 12 Cl⁻).
3Iron(III) oxide and iron(II) oxide both contain only iron and oxygen. In which compound is the ratio of oxygen to iron greater? Explain using the formulas.
Iron(II) oxide is FeO: 1 O per Fe. Iron(III) oxide is Fe₂O₃: 3 O per 2 Fe = 1.5 O per Fe. So iron(III) oxide has the greater oxygen-to-iron ratio, because each Fe³⁺ ion needs more negative charge to balance it.
Quiz
1. What is the composition of one molecule of P₂O₅?
2. Which name is correct for FeCl₂?
3. What is the formula of copper(I) oxide?
4. Which compound should be named using prefixes?
6. Why does zinc oxide (ZnO) not need a Roman numeral?
Big idea: expectation C3.1 asks students to connect a compound's formula, composition and name, and C3.8 asks them to write formulas of simple ionic and molecular compounds using the periodic table. Students met binary ionic and molecular naming in Grade 9; the new skill here is the Stock system (Roman numerals) for multivalent metals such as iron, copper, lead and tin.
Common difficulties:
- Thinking the Roman numeral counts atoms. In Cu₂O the numeral is I, not II: it is the charge of each ion.
- "Un-crisscrossing" reduced formulas. PbO₂ is lead(IV), not lead(II). Always start from the nonmetal's known charge.
- Using prefixes on ionic names ("iron trichloride") or forgetting them on molecular names ("carbon oxide").
- Forgetting the -ide ending on the nonmetal.
How to help: make flash cards with a formula on one side and the name on the other, and quiz in both directions. Ask your teen to "prove it" by saying the charges out loud: "Three chlorides is three minus, so iron must be three plus." The label hunt is a good way to show that this chemistry is printed on things in your home. Keep all products closed during the activity.