Naming ionic compounds: positive ions, negative ions and Roman numerals
By the end of this lesson you will be able to
- State what a chemical formula shows and outline the rule that an ionic name gives the positive ion first and the negative ion second.
- Deduce the charge on the ion of a main-group element from its group, and name positive ions and -ide negative ions.
- Construct the formula of an ionic compound from its name by balancing charges, and name an ionic compound from its formula.
- Explain why metals such as iron, copper, tin and lead need a Roman numeral, and determine that numeral from a formula.
- Calculate the relative formula mass and the percentage of a metal in a compound named in this way, and interpret conductivity data as evidence for ions.
You met this before
- Electrons fill shells 2, 8, 8; for Groups 1, 2 and 13–17 the number of outer electrons is the last digit of the group number (sodium 2,8,1 is in Group 1; chlorine 2,8,7 is in Group 17).
- In an ionic bond a metal atom loses its outer electrons to become a positive ion and a non-metal atom gains electrons to complete its outer shell as a negative ion; both then have a noble-gas arrangement.
- An ionic compound is a giant lattice of oppositely charged ions held by electrostatic attraction. It conducts electricity when molten or dissolved, because the ions can then move, but not as a solid.
- A formula uses element symbols (capital letter, then lower case) with subscripts that count the atom or ion written just before them: CaCl₂ has one calcium for every two chlorides. No subscript means one.
- The oxidation state of a simple ion is its charge: +2 for Mg²⁺, −1 for Cl⁻ (Unit 4, Lesson 1). Relative formula mass is the sum of the relative atomic masses in the formula (Unit 2).
Magnesium is 2,8,2 and chlorine is 2,8,7. Write the symbols and charges of the two ions they form, and hence the formula of magnesium chloride.
Reveal
Magnesium loses its two outer electrons: Mg²⁺ (now 2,8). Chlorine gains one electron: Cl⁻ (now 2,8,8).
One Mg²⁺ carries +2, so two Cl⁻ ions (−1 each) are needed to cancel it: MgCl₂. That charge-balancing step is the heart of this lesson.
Revise it in full: Year 3 · Atomic structure and bonding · Lesson 3 — Ionic bonding: metals give, non-metals take, and the lattice holds · Year 3 · Energy and chemical change · Lesson 2 — Writing the change down: formulas, symbol equations and balancing · Year 3 · Atomic structure and bonding · Lesson 2 — Electronic configuration
The puzzle
On a pharmacy shelf in Francistown sits a box of tablets labelled “ferrous sulfate 200 mg”, prescribed for anaemia. In an electronics workshop down the road a technician pours “ferric chloride” into a tray, and it eats the unwanted copper off a circuit board.
Both are compounds of iron. One is swallowed every day; the other is corrosive enough to dissolve a metal. A chemist in Mumbai, Lima or Oslo writes them as iron(II) sulfate and iron(III) chloride, FeSO₄ and FeCl₃. Why does one metal need two names — and how does a small Roman numeral carry that difference to anyone in the world?
Two brown bottles are labelled FeCl₂ and FeCl₃. Each chloride ion carries a charge of −1. Predict the charge on the iron ion in each bottle, and say why a label reading just “iron chloride” could be dangerous in a laboratory.
Compare your answer
FeCl₂: two chlorides give −2, so the iron is Fe²⁺. FeCl₃: three chlorides give −3, so the iron is Fe³⁺. The compound as a whole must be neutral.
“Iron chloride” could be either substance. They have different colours, different reactions and different hazards (iron(III) chloride is the corrosive etchant), so a name that fits two substances is not a name at all. The Roman numeral — iron(II) chloride, iron(III) chloride — removes the doubt.
The science — structured notes
1.1 One substance, one name: what a formula and a name must do
Chemistry has more than a hundred million known substances. A name is only useful if it points to exactly one of them and anyone, anywhere, can turn it back into the formula. The rules that make this possible are agreed internationally by the International Union of Pure and Applied Chemistry (IUPAC), founded in 1919.
For an ionic compound the rule is short: name the positive ion first, then the negative ion. Sodium chloride, calcium oxide, aluminium bromide. The formula follows the same order: NaCl, CaO, AlBr₃.
| Part of the formula Al₂O₃ | What it tells you |
|---|---|
| Al, O | The elements present: aluminium and oxygen. A symbol has one capital letter, sometimes followed by one lower-case letter (Co is cobalt; CO is carbon monoxide). |
| Subscript ₂ after Al | Two aluminium ions in each formula unit. |
| Subscript ₃ after O | Three oxide ions in each formula unit. |
| No charges written | The compound is neutral overall: 2 × (+3) and 3 × (−2) cancel. |
| Metal first | The positive ion is written and named first. |
An ionic compound is a giant lattice, not a molecule, so its formula is a ratio: the simplest whole-number ratio of the ions. The group Al₂O₃ is called a formula unit. A grain of aluminium oxide contains billions of billions of them, stacked without any boundary between one “unit” and the next.
1.2 Positive ions: metals keep their own name
A positive ion (a cation) is named after its element: Na⁺ is the sodium ion, Ca²⁺ the calcium ion. For the main-group metals the charge follows from the group, because the atom loses all its outer electrons to reach the arrangement of the noble gas before it.
| Group | Outer electrons | Ion charge | Examples (name and symbol) |
|---|---|---|---|
| 1 | 1 | +1 | lithium Li⁺, sodium Na⁺, potassium K⁺ |
| 2 | 2 | +2 | magnesium Mg²⁺, calcium Ca²⁺, barium Ba²⁺ |
| 13 | 3 | +3 | aluminium Al³⁺ |
| Fixed d-block ions | — | one charge only | zinc Zn²⁺, silver Ag⁺ |
| Not a metal, but positive | — | +1 | hydrogen H⁺; ammonium NH₄⁺ (a group of atoms — Lesson 2) |
Write the symbol and charge of the ion formed by each of: potassium, barium, aluminium, lithium, strontium (Group 2). Then name each ion.
Reveal answer
K⁺ potassium ion · Ba²⁺ barium ion · Al³⁺ aluminium ion · Li⁺ lithium ion · Sr²⁺ strontium ion.
The charge equals the number of outer electrons lost, which for these metals is set by the group. The name of a simple positive ion is just the name of the element.
1.3 Negative ions: the -ide ending
A simple negative ion (an anion) is named by taking the stem of the non-metal and adding -ide. The charge is the number of electrons gained to reach eight in the outer shell: Group 17 gains one, Group 16 two, Group 15 three.
| Element | Group | Ion | Name of ion |
|---|---|---|---|
| fluorine, chlorine, bromine, iodine | 17 | F⁻, Cl⁻, Br⁻, I⁻ | fluoride, chloride, bromide, iodide |
| oxygen, sulfur | 16 | O²⁻, S²⁻ | oxide, sulfide |
| nitrogen, phosphorus | 15 | N³⁻, P³⁻ | nitride, phosphide |
| hydrogen (with a reactive metal) | 1 | H⁻ | hydride (as in sodium hydride, NaH) |
So a compound of two elements, one metal and one non-metal, always ends in -ide: potassium iodide, calcium sulfide, lithium nitride. The -ide tells a reader that nothing but those two elements is present.
“The chloride ion is a chlorine atom that has lost one electron, which is why it has a charge of −1.”
Reveal answer
False. Losing a negative electron would leave a positive charge. A chloride ion is a chlorine atom that has gained one electron: 17 protons and 18 electrons, charge −1, arrangement 2,8,8.
Non-metals gain electrons (negative ions); metals lose them (positive ions).
1.4 From name to formula: balance the charges
Every ionic compound is neutral, so the total positive charge must equal the total negative charge. That one rule turns any name into a formula. Work in four lines:
- Write the two ions with their charges.
- Find the lowest number that both charges divide into (the lowest common multiple).
- Work out how many of each ion make that total.
- Write the numbers as subscripts; leave out any 1; do not write the charges in the final formula.
Write the formula of aluminium oxide, the alumina refined from bauxite and the white film that protects every aluminium window frame.
Ions. Al³⁺ and O²⁻.
Lowest common multiple of 3 and 2. 6
Positive side. 6 ÷ 3 = 2 aluminium ions, carrying +6
Negative side. 6 ÷ 2 = 3 oxide ions, carrying −6
Formula. Al₂O₃
Check. 2 × (+3) + 3 × (−2) = 0
Lithium is the only Group 1 metal that reacts with the nitrogen of the air at room temperature, which is one reason lithium metal for batteries is handled under argon, not just dry air. Name the product and write its formula.
Name. Metal first, then nitrogen with -ide: lithium nitride.
Ions. Li⁺ (Group 1) and N³⁻ (Group 15 gains three).
Lowest common multiple of 1 and 3. 3
Numbers. 3 Li⁺ carry +3; 1 N³⁻ carries −3
Formula. Li₃N
Check. 3 × (+1) + 1 × (−3) = 0
Write the formulas of: calcium bromide, potassium sulfide, aluminium chloride, magnesium nitride, barium oxide, sodium hydride.
Reveal answer
CaBr₂ (Ca²⁺ needs two Br⁻) · K₂S (two K⁺ for one S²⁻) · AlCl₃ (one Al³⁺ for three Cl⁻) · Mg₃N₂ (lowest common multiple of 2 and 3 is 6: three Mg²⁺ and two N³⁻) · BaO (+2 and −2 already cancel) · NaH (Na⁺ and H⁻).
A student writes: “magnesium chloride MgCl · aluminium oxide Al₃O₂ · calcium oxide Ca₂O₂ · sodium sulfide NaS₂”. Find the error in each and correct it.
Reveal answer
MgCl leaves +1 unbalanced: Mg²⁺ needs two Cl⁻, so MgCl₂.
Al₃O₂ has the numbers swapped the wrong way: 3 × (+3) + 2 × (−2) = +5. Correct: Al₂O₃.
Ca₂O₂ balances but is not the simplest ratio: CaO.
NaS₂ has one +1 against −4: sodium sulfide needs two Na⁺ for one S²⁻, so Na₂S.
1.5 Metals with more than one ion: the Roman numeral
Main-group metals form one ion each. Many transition metals, and tin and lead, can lose different numbers of electrons and so form more than one ion. Iron forms Fe²⁺ and Fe³⁺; copper forms Cu⁺ and Cu²⁺. The name must say which.
The modern system, proposed by the German chemist Alfred Stock in 1919, puts the charge — the oxidation state — of the metal ion in Roman numerals in brackets straight after the metal’s name, with no space: iron(II), iron(III), copper(I), copper(II). Read iron(II) aloud as “iron two”.
| Metal | Ions | Systematic names | Older names you will still see | Example compounds |
|---|---|---|---|---|
| iron | Fe²⁺, Fe³⁺ | iron(II), iron(III) | ferrous, ferric | FeSO₄ iron(II) sulfate; Fe₂O₃ iron(III) oxide |
| copper | Cu⁺, Cu²⁺ | copper(I), copper(II) | cuprous, cupric | Cu₂O copper(I) oxide (red); CuO copper(II) oxide (black) |
| tin | Sn²⁺, Sn⁴⁺ | tin(II), tin(IV) | stannous, stannic | SnF₂ tin(II) fluoride, in some toothpastes; SnO₂ tin(IV) oxide, the ore cassiterite |
| lead | Pb²⁺, Pb⁴⁺ | lead(II), lead(IV) | plumbous, plumbic | PbI₂ lead(II) iodide; PbO₂ lead(IV) oxide, in car batteries |
| chromium, manganese | Cr³⁺; Mn²⁺ (and others) | chromium(III), manganese(II) | chromic, manganous | Cr₂O₃ chromium(III) oxide, a green pigment |
The older names used -ous for the lower charge and -ic for the higher. They survive on medicine boxes (“ferrous sulfate”) and in industry (“ferric chloride”), but they do not say the charge, and they fail for metals with three or more ions. In your answers always use the numeral.
Do not add a numeral where there is no choice: sodium chloride, not sodium(I) chloride; zinc oxide and silver nitrate, not zinc(II) oxide or silver(I) nitrate. The numeral answers a question — “which ion?” — and is left out when there is only one answer.
To name a compound whose metal can have more than one charge, work backwards from the negative ions, whose charges you know.
Name the red-brown iron ore haematite, Fe₂O₃, and the dark brown solid PbO₂ in the positive plate of a car battery.
Fe₂O₃, negative charge. 3 × (−2) = −6
Positive charge needed. +6, shared by 2 iron ions
Charge on each iron. +6 ÷ 2 = +3
Name. iron(III) oxide
PbO₂, negative charge. 2 × (−2) = −4, carried by one lead ion
Name. lead(IV) oxide
Name each compound: FeCl₂, CuCl₂, Cu₂O, SnO₂, PbBr₂, Cr₂O₃, ZnCl₂.
Reveal answer
iron(II) chloride · copper(II) chloride · copper(I) oxide (−2 shared by two coppers, so +1 each) · tin(IV) oxide · lead(II) bromide · chromium(III) oxide (−6 shared by two) · zinc chloride — no numeral, because zinc only ever forms Zn²⁺.
Explain why sodium never needs a Roman numeral but iron does, referring to electrons.
Reveal answer
Sodium (2,8,1) has one outer electron; losing it gives the stable 2,8 arrangement, and removing a second electron from the full inner shell would need far more energy than any reaction supplies. So sodium only ever forms Na⁺ and its name is unambiguous.
Iron’s outer electrons lie in two sub-shells of similar energy, so losing two (Fe²⁺) or three (Fe³⁺) are both possible in ordinary reactions. Because two different ions — and two different sets of compounds — exist, the name must state the charge.
1.6 From name to numbers: formula mass and percentage of metal
Once the formula is right, every calculation follows from it. The relative formula mass, Mr, is the sum of the relative atomic masses of everything in the formula. The percentage by mass of an element is its share of that total.
percentage of element = (number of atoms of element) × ArMr × 100
A mining company can buy haematite, iron(III) oxide, or magnetite, Fe₃O₄. Compare the percentage of iron.
Haematite, Fe₂O₃. Mr = 2 × 55.8 + 3 × 16.0
Mr = 111.6 + 48.0
Mr = 159.6
% Fe = 111.6 ÷ 159.6 × 100
% Fe = 69.9 %
Magnetite, Fe₃O₄. Mr = 3 × 55.8 + 4 × 16.0
Mr = 167.4 + 64.0
Mr = 231.4
% Fe = 167.4 ÷ 231.4 × 100
% Fe = 72.3 % — magnetite is richer per tonne of pure ore.
What if the tablet contained the hydrated salt iron(II) sulfate-7-water, FeSO₄·7H₂O (Mr 277.9), instead of dry FeSO₄? Calculate the mass of iron in a 200 mg tablet and comment.
Reveal answer
mass of Fe = 200 mg × 55.8 ÷ 277.9
mass of Fe = 40.2 mg
Only about 55 % of the iron in the same mass of dry salt (73 mg), because seven water molecules come with every FeSO₄. That is why pharmacists quote elemental iron, and why the full name — including the water — must be on the label.
Magnetite, Fe₃O₄, is sometimes named iron(II,III) oxide. Use charge balance to show that it contains both kinds of iron ion, find how many of each are in one formula unit, and suggest why the name has two numerals.
Reveal answer
Negative charge = 4 × (−2) = −8, so the three iron ions carry +8 in total.
+8 ÷ 3 is not a whole number, so the irons cannot all have the same charge.
Try x Fe²⁺ and (3 − x) Fe³⁺: 2x + 3(3 − x) = 8, so x = 1.
One Fe²⁺ and two Fe³⁺: 2 + 3 + 3 = +8. Magnetite is FeO·Fe₂O₃ in one lattice — a mixed oxide — so its name lists both oxidation states. (The numerals list the charges present, not their numbers.)
Try it — is an ionic name a promise of ions?
If a name like “sodium chloride” means Na⁺ and Cl⁻ ions, the solution should carry a current, and more dissolved salt should mean more ions and a larger current. A compound with no ions — sugar, a covalent molecule — should carry almost none, whatever you do.
In the lab, two graphite electrodes 3 cm apart dip into 100 cm³ of liquid, connected to a 6 V supply and a milliammeter. You choose how many grams of solute to dissolve (0–10 g per 100 cm³), which solute — sodium chloride, copper(II) sulfate crystals (the lab calls them “copper sulfate”: they are CuSO₄·5H₂O, Lesson 2) or sugar — and whether the solute is dissolved or dry.
Suppose your mean currents for sodium chloride were: 0 g → 0.1 mA; 2 g → 46.2; 4 g → 88.4; 6 g → 126.8; 8 g → 161.4; 10 g → 192.1 mA. Sugar at 4 g gave 0.1 mA and dry salt gave 0.0 mA.
(a) Divide current by concentration at 2, 6 and 10 g and state whether the current is directly proportional to concentration.
(b) Explain the readings for sugar and for dry salt using the names of the substances.
(c) Suggest why 4 g of copper(II) sulfate crystals gives a much smaller current than 4 g of sodium chloride.
Reveal answer
(a) Current ÷ concentration:
at 2 g: 46.2 ÷ 2 = 23.1 mA per g
at 6 g: 126.8 ÷ 6 = 21.1 mA per g
at 10 g: 192.1 ÷ 10 = 19.2 mA per g
The ratio falls steadily, so the current rises with concentration but is not directly proportional: it is close to proportional at low concentration and bends over as the ions crowd and slow one another.
(b) Sugar (sucrose, C₁₂H₂₂O₁₁) is a covalent compound with no metal and no -ide ion in its name; it dissolves as whole neutral molecules, so almost nothing carries charge (0.1 mA is the water itself). Dry sodium chloride does contain Na⁺ and Cl⁻ ions, but they are locked in the lattice and cannot move to the electrodes, so the current is zero.
(c) Per gram there are fewer ions: 4 g of NaCl is 4 ÷ 58.5 = 0.068 mol, giving 0.137 mol of ions; 4 g of CuSO₄·5H₂O is 4 ÷ 249.6 = 0.016 mol, giving 0.032 mol of ions — about four times fewer. Much of that crystal mass is water of crystallisation, and the larger Cu²⁺ and SO₄²⁻ ions also move more slowly.
Common mistakes to avoid
- Naming the element instead of the ion — “sodium chlorine”, “magnesium oxygen”. The negative ion ends in -ide.
- Wrong order — “chloride sodium” or ClNa. The positive ion comes first in both the name and the formula.
- Unbalanced formulas — MgCl, AlO. Check that the total charge is zero before you write the answer.
- Not simplifying — Mg₂O₂ or Ca₂O₂. Ionic formulas are the lowest whole-number ratio.
- Copying the numeral into the formula — writing iron(III) oxide as Fe₃O. The numeral is the charge on each ion.
- Adding numerals where none is needed — sodium(I) chloride, zinc(II) oxide. Use a numeral only when the metal has more than one common ion.
- Writing charges in the final formula — Na⁺Cl⁻ is a description of the ions, not the formula; write NaCl.
- Symbol case — CO is carbon monoxide; Co is cobalt. One capital letter per symbol.
Study smart: how this is examined
Study routine. Today: make a card of ion charges and -ide names, then write the formulas of twenty random metal–non-metal pairs with the zero-charge check. In two days: cover the names in the Roman-numeral table and name each example compound from its formula, working back from the negative ion.
In a week: run the lab as a full investigation including the proportionality test, then do the Think task and the quiz without notes.
How we found out — from “butter of antimony” to iron(III)
Before 1787 a chemist might buy “oil of vitriol”, “spirit of salt”, “flowers of zinc”, “sugar of lead” or “butter of antimony”. The names described how a substance looked, tasted or was made — and said nothing about what it was made of. Many substances had several names, and some names covered several substances.
In 1787 four French chemists — Louis-Bernard Guyton de Morveau, Antoine Lavoisier, Claude-Louis Berthollet and Antoine-François Fourcroy — published the Méthode de nomenclature chimique. Its principle was that a name should state the composition. Compounds of oxygen became oxides; the salts of each acid got a family name. “Flowers of zinc” became zinc oxide.
The idea was controversial because it was built on Lavoisier’s new theory of oxygen; to accept the names was almost to accept the theory. Within a generation it had won. In 1813–14 the Swedish chemist Jöns Jacob Berzelius added the letter symbols we still use — Fe from the Latin ferrum, Cu from cuprum, Pb from plumbum.
The -ous and -ic endings of the nineteenth century (ferrous, ferric) could only tell “lower” from “higher”. In 1919 Alfred Stock, a German chemist famous for his work on boron hydrides, proposed writing the charge as a Roman numeral: iron(II), iron(III). The same year chemists founded IUPAC, which later wrote Stock’s notation into its international rules.
Today IUPAC’s inorganic rules (the “Red Book”, 2005 edition) and organic rules (the “Blue Book”, 2013 recommendations) run to hundreds of pages. But the principle is still the 1787 one: the name tells you the composition, and one name means one substance.
The 1787 reformers renamed “flowers of zinc” as zinc oxide. Explain why the new name was more useful to a chemist than the old one, using an idea from this lesson.
Reveal answer
“Flowers of zinc” describes the fluffy white appearance of the solid formed when zinc burns, but gives no composition, so a chemist could not predict its formula, mass or reactions from it.
“Zinc oxide” says it contains only zinc and oxygen (the -ide ending), with zinc as the positive ion. From Zn²⁺ and O²⁻ a chemist can write ZnO, calculate Mr = 81.4 and predict that it will react like other metal oxides — the name carries information.
Where it is used now
- Medicines. Iron(II) sulfate, iron(II) fumarate and iron(II) gluconate treat iron-deficiency anaemia, which the World Health Organization counts among the most common nutritional disorders worldwide. The numeral decides the dose of elemental iron.
- Water treatment. Iron(III) chloride and aluminium sulfate are added to raw water as coagulants: their highly charged Fe³⁺ and Al³⁺ ions help tiny suspended particles clump together so they can settle and be filtered.
- Mining in southern Africa. The Kalahari Copper Belt in north-western Botswana is mined for copper sulfide ores such as chalcocite, copper(I) sulfide, Cu₂S, and chalcopyrite, CuFeS₂. Knowing the formula tells a metallurgist how much copper each tonne can yield.
- Pigments. Iron(III) oxide gives red ochre and the colour of most red bricks; chromium(III) oxide is a stable green; titanium(IV) oxide, TiO₂, is the white in almost all paint.
- Batteries. A lead–acid car battery works by converting lead and lead(IV) oxide into lead(II) sulfate — the two numerals are the whole story of charging and discharging.
- Dental care. Tin(II) fluoride, SnF₂, is used in some toothpastes; sodium fluoride, NaF, in many others — two fluorides, one with a numeral and one without.
Science now — and the scientist you need to be
What is happening in 2026
Names now travel through machines as much as through people. Free databases such as PubChem, run by the US National Library of Medicine, link every name, formula and structure of a substance with its hazards and uses. The IUPAC International Chemical Identifier (InChI), first released in 2005, gives each structure a unique text code that computers can compare.
Open-source software such as OPSIN, from the University of Cambridge, turns systematic names into structures automatically, which is how millions of names in patents and papers are read by search engines.
Safety labels follow the Globally Harmonized System (GHS), adopted in many countries, which requires the product identifier — the name — alongside the hazard pictograms. AI assistants can now draft names and formulas in seconds, but they still make charge-balance errors (writing “Fe₃O” for iron(III) oxide is a classic).
The chemist of the next decade must be able to check a machine’s name as fast as the machine writes it.
The mindshift
What you need to be able to do
Turn any metal–non-metal name into a balanced formula, and any such formula into a name with the right numeral; calculate the formula mass and the percentage of the metal; and explain in terms of electrons why some metals need a numeral and others do not.
See an example
A multivitamin label lists “zinc oxide” (ZnO: Zn²⁺ and O²⁻, no numeral because zinc has only one ion) and “ferrous fumarate” (an iron(II) salt of fumaric acid — the older name hides the numeral).
Correct AI answers: Cu₂O (two Cu⁺ for one O²⁻), FeCl₃ (one Fe³⁺ for three Cl⁻), PbO₂ (one Pb⁴⁺ for two O²⁻). If an assistant writes CuO for copper(I) oxide, it has named copper(II) oxide instead.
Key terms
| Cation | A positive ion, formed when an atom loses electrons; named after its element (sodium ion, Na⁺). |
| Anion | A negative ion, formed when an atom gains electrons; a simple anion is named with -ide (oxide, O²⁻). |
| Formula unit | The simplest whole-number ratio of ions in an ionic lattice, e.g. Al₂O₃. |
| Subscript | The small number after a symbol giving how many of that atom or ion are in the formula; 1 is not written. |
| Oxidation state | The charge an atom would have if all its bonds were ionic; for a simple ion it equals the charge. |
| Stock notation | A Roman numeral in brackets after a metal’s name giving its oxidation state, e.g. iron(III). |
| Variable-charge metal | A metal that forms more than one common ion, such as iron (Fe²⁺, Fe³⁺) or copper (Cu⁺, Cu²⁺). |
| Relative formula mass (Mr) | The sum of the relative atomic masses of all the atoms or ions in a formula unit. |
| Percentage by mass | The mass of an element in a compound as a percentage of the compound’s Mr. |
| IUPAC | The International Union of Pure and Applied Chemistry, founded in 1919, which agrees the naming rules. |
Think like a scientist
Criterion A A water-treatment works near a river stores four chemicals in tanks labelled with formulas only: FeCl₃, Al₂(SO₄)₃ (you will name the sulfate part in Lesson 2; here it is “aluminium sulfate”), CaO and NaOCl. A visiting engineer asks you to check the paperwork, which lists the first tank as “iron chloride” and the third as “calcium oxygen”.
(a) Give the correct systematic name of FeCl₃ and of CaO, and explain the error in each name on the paperwork. [4]
(b) State the charge on the metal ion in FeCl₃, showing how you worked it out. [2]
(c) Write the formula of the other chloride of iron and name it. [2]
(d) Calculate the percentage by mass of iron in FeCl₃ (Ar: Fe 55.8, Cl 35.5). [3]
(e) Explain how iron and chlorine atoms form the ions in FeCl₃ and what holds the solid together. [4]
Model answer
(a) FeCl₃ is iron(III) chloride. “Iron chloride” is ambiguous because iron forms two chlorides, FeCl₂ and FeCl₃; the name must include the numeral (III) to say which [2].
CaO is calcium oxide. “Calcium oxygen” names the element oxygen, not the oxide ion O²⁻ present in the compound; a two-element ionic compound ends in -ide [2].
(b) Each chloride is −1, so three give −3. The compound is neutral, so the iron ion is +3, Fe³⁺ [2].
(c) FeCl₂ [1], iron(II) chloride [1].
(d) Mr = 55.8 + 3 × 35.5
Mr = 55.8 + 106.5
Mr = 162.3 [1]
% Fe = 55.8 ÷ 162.3 × 100 [1]
% Fe = 34.4 % [1]
(e) Each iron atom loses three electrons to form an Fe³⁺ ion [1]; each chlorine atom gains one electron to form a Cl⁻ ion with a full outer shell, 2,8,8 [1]; three chlorine atoms are needed for every iron atom, so the ratio is 1 : 3 [1]; the oppositely charged ions attract one another by strong electrostatic forces in every direction, forming a giant ionic lattice [1].
Probe deeper — exam-style questions
Attempt every question before opening the answer. Marks in brackets show what a full answer is worth. Ar: H 1.0, N 14.0, O 16.0, Na 23.0, Mg 24.3, Al 27.0, S 32.1, Cl 35.5, K 39.1, Ca 40.1, Fe 55.8, Cu 63.5, Br 79.9, Pb 207.2.
Select the correct formula of calcium nitride: A CaN · B Ca₂N₃ · C Ca₃N₂ · D Ca₃N.
Reveal answer
C, Ca₃N₂: three Ca²⁺ (+6) balance two N³⁻ (−6) [1].
Name: K₂O, CuBr₂, PbO. Then write the formula of: aluminium sulfide, iron(III) bromide, copper(I) chloride.
Reveal answer
potassium oxide; copper(II) bromide; lead(II) oxide [1 for all three names, with the two numerals].
Al₂S₃; FeBr₃ [1]; CuCl [1].
State why the name “copper oxide” is not acceptable as the name of a black powder with formula CuO, and give the correct name.
Reveal answer
Copper forms two oxides, Cu₂O and CuO, so “copper oxide” does not identify one substance [1]. CuO: O²⁻ is −2, so copper is +2 — copper(II) oxide [1].
Calculate the relative formula mass of aluminium oxide and the percentage by mass of aluminium in it.
Reveal answer
Al₂O₃ [1]
Mr = 2 × 27.0 + 3 × 16.0
Mr = 102.0 [1]
% Al = 54.0 ÷ 102.0 × 100
% Al = 52.9 % [1]
Explain, in terms of electrons, how sodium and sulfur form sodium sulfide, and deduce its formula.
Reveal answer
Each sodium atom (2,8,1) loses one electron to form Na⁺ (2,8) [1]; each sulfur atom (2,8,6) gains two electrons to form S²⁻ (2,8,8) [1]; two sodium atoms are needed for each sulfur atom, so the formula is Na₂S, and the ions are held by electrostatic attraction [1].
A green solid has the formula Cr₂O₃ and a toxic dark red solid has the formula CrO₃. Name both.
Reveal answer
Cr₂O₃: −6 shared by two chromiums, so +3 each — chromium(III) oxide [1].
CrO₃: −6 on one chromium, so +6 — chromium(VI) oxide [1]. (The numeral can be larger than III; chromium(VI) compounds are carcinogenic, so the numeral is a safety warning too.)
7. A 2.00 g sample of an oxide of copper is reduced to copper metal, which has a mass of 1.78 g. Determine the formula of the oxide and give its systematic name.
Reveal answer
mass of oxygen = 2.00 − 1.78 = 0.22 g [1]
moles of Cu = 1.78 ÷ 63.5 = 0.0280 mol
moles of O = 0.22 ÷ 16.0 = 0.0138 mol [1]
ratio Cu : O = 0.0280 ÷ 0.0138 : 1 = 2.03 : 1 ≈ 2 : 1
Formula Cu₂O [1]. O²⁻ is −2, shared by two coppers, so each is +1: copper(I) oxide [1].
8. A laboratory has a bottle of anhydrous iron(II) chloride and a bottle of anhydrous iron(III) chloride, and the labels have fallen off. Both are 10.0 g samples. A technician dissolves each and adds excess silver nitrate, which precipitates all the chloride as silver chloride, AgCl (Mr 143.4). Calculate the mass of silver chloride each bottle would give, and explain how this distinguishes them.
Reveal answer
FeCl₂: Mr = 55.8 + 71.0 = 126.8.
moles FeCl₂ = 10.0 ÷ 126.8 = 0.0789 mol
moles Cl⁻ = 2 × 0.0789 = 0.158 mol
mass AgCl = 0.158 × 143.4 = 22.6 g [1]
FeCl₃: Mr = 55.8 + 106.5 = 162.3.
moles FeCl₃ = 10.0 ÷ 162.3 = 0.0616 mol
moles Cl⁻ = 3 × 0.0616 = 0.185 mol
mass AgCl = 0.185 × 143.4 = 26.5 g [1]
Per gram, iron(III) chloride contains a larger fraction of chlorine (65.6 % against 56.0 %), so it gives about 4 g more precipitate [1]; the difference is far larger than the precision of a balance, so weighing the dried precipitate identifies each bottle — the numeral in the name predicts a measurable result [1].
Summary
- An ionic name gives the positive ion first and the negative ion second; a two-element compound ends in -ide (chloride, oxide, sulfide, nitride).
- Main-group ion charges follow the group: +1, +2, +3 for Groups 1, 2, 13; −3, −2, −1 for Groups 15, 16, 17.
- Formula from name: balance the charges using the lowest common multiple, simplify, and write no charges in the formula.
- Metals with more than one ion (iron, copper, tin, lead, chromium) carry a Roman numeral — the charge on each ion, found from the formula by working back from the negative ions.
- The formula leads to the numbers: Mr, percentage of metal, and the dose of an element in a medicine or the yield of an ore.
Next time → Polyatomic ions: sulfate, nitrate, carbonate, hydroxide, ammonium and phosphate — ions made of several atoms that travel as one, brackets in formulas, and the water hidden in a crystal.