Naming Practice

Naming is the one part of this course where being fast matters, because every later question hides a name or a formula inside it. If you have to stop and think about what $\mathrm{Fe}{}_2(\mathrm{SO}{}_4){}_3$ is called, you will lose the thread of the stoichiometry question it was buried in.

Keep Polyatomic Ions and Naming Rules at a Glance beside you for the first pass and then close them.

1. Name these compounds: (a) $\mathrm{KBr}$ (b) $\mathrm{CaO}$ (c) $\mathrm{Li}{}_2\mathrm{S}$ (d) $\mathrm{MgCl}{}_2$ (e) $\mathrm{Na}{}_3\mathrm{N}$

Answer 1

All five are a metal from the first two groups with a non-metal, so all five are ionic and none of them takes a prefix or a Roman numeral. The pattern is metal name, then non-metal stem with -ide.

(a) potassium bromide (b) calcium oxide (c) lithium sulfide (d) magnesium chloride (e) sodium nitride

Notice what the subscripts do not do: they never appear in the name. $\mathrm{MgCl}{}_2$ is not “magnesium dichloride”. The 2 is not a choice somebody made — it is forced, because magnesium forms $\mathrm{Mg}{}^{2+}$ and chloride is $\mathrm{Cl}{}^{-}$, and two chlorides are needed to balance one magnesium ion. Saying the 2 out loud would be repeating information the name already contains.

2. Name these: (a) $\mathrm{FeCl}{}_2$ (b) $\mathrm{FeCl}{}_3$ (c) $\mathrm{CuO}$ (d) $\mathrm{Cu}{}_2\mathrm{O}$ (e) $\mathrm{PbO}{}_2$

Answer 2

These metals have more than one possible charge, so the name has to say which one is in use. That is the entire job of the Roman numeral, and it always gives the charge on the metal ion — never the number of atoms.

Work each one out from the anion, which is the part whose charge you know.

(a) Two chlorides at $1-$ each is $2-$ altogether, so the single iron must be $2+$. Iron(II) chloride.

(b) Three chlorides is $3-$, so iron is $3+$. Iron(III) chloride.

(c) One oxide at $2-$, one copper, so copper is $2+$. Copper(II) oxide.

(d) One oxide at $2-$ shared between two coppers, so each copper is $1+$. Copper(I) oxide. This is the one people get backwards: the subscript 2 is on the copper, and the Roman numeral is I.

(e) Two oxides is $4-$, one lead, so lead is $4+$. Lead(IV) oxide.

The reliable method is always the same: the anion’s charge is fixed, so count the total negative charge and divide it among the metal ions.

3. Write the formula for: (a) sodium carbonate (b) ammonium sulfate (c) calcium nitrate (d) iron(III) sulfate (e) magnesium hydroxide

Answer 3

Polyatomic ions travel as a unit. When you need more than one of them, the whole ion goes in brackets and the subscript goes outside.

(a) $\mathrm{Na}{}^{+}$ and $\mathrm{CO}{}_3{}^{2-}$ — two sodiums per carbonate: $\mathrm{Na}{}_2\mathrm{CO}{}_3$

(b) $\mathrm{NH}{}_4{}^{+}$ and $\mathrm{SO}{}_4{}^{2-}$ — two ammoniums, so brackets: $(\mathrm{NH}{}_4){}_2\mathrm{SO}{}_4$

(c) $\mathrm{Ca}{}^{2+}$ and $\mathrm{NO}{}_3{}^{-}$ — two nitrates, so brackets: $\mathrm{Ca}(\mathrm{NO}{}_3{)}_2$

(d) $\mathrm{Fe}{}^{3+}$ and $\mathrm{SO}{}_4{}^{2-}$ — the lowest whole numbers that balance $3+$ against $2-$ are two irons and three sulfates: $\mathrm{Fe}{}_2(\mathrm{SO}{}_4){}_3$

(e) $\mathrm{Mg}{}^{2+}$ and $\mathrm{OH}{}^{-}$ — two hydroxides, so brackets: $\mathrm{Mg}(\mathrm{OH}{)}_2$

Writing $\mathrm{MgOH}{}_2$ instead of $\mathrm{Mg}(\mathrm{OH}{)}_2$ changes the meaning completely: it says one oxygen and two hydrogens, which is not two hydroxide ions. The brackets are not decoration.

4. Name these: (a) $\mathrm{CO}$ (b) $\mathrm{CO}{}_2$ (c) $\mathrm{N}{}_2\mathrm{O}{}_4$ (d) $\mathrm{PCl}{}_5$ (e) $\mathrm{SF}{}_6$. Then explain why these names use prefixes when the names in question 1 did not.

Answer 4

(a) carbon monoxide (b) carbon dioxide (c) dinitrogen tetroxide (d) phosphorus pentachloride (e) sulfur hexafluoride

The convention drops the prefix mono- on the first element only, which is why (a) and (b) both start with plain “carbon”.

Why the prefixes are needed here. In question 1 the ratio was forced. Sodium is always $\mathrm{Na}{}^{+}$ and chloride is always $\mathrm{Cl}{}^{-}$, so “sodium chloride” can only mean a one-to-one ratio — there is nothing left to specify.

Two non-metals share electrons instead of transferring them, and there is no fixed charge doing the forcing. Carbon and oxygen combine in more than one ratio, and both compounds are real, common, and completely different: one is what you exhale and the other kills you in a closed garage. “Carbon oxide” would not distinguish them. The prefix carries information that nothing else in the name can supply.

5. Name these acids: (a) $\mathrm{HCl}(\mathrm{aq})$ (b) $\mathrm{H}{}_2\mathrm{SO}{}_4(\mathrm{aq})$ (c) $\mathrm{HNO}{}_3(\mathrm{aq})$ (d) $\mathrm{CH}{}_3\mathrm{COOH}(\mathrm{aq})$

Answer 5

(a) hydrochloric acid. A hydrogen with a simple anion takes hydro- on the front and -ic acid on the end.

(b) sulfuric acid. The anion is sulfate, ending in -ate, which becomes -ic acid. No hydro-, because the anion contains oxygen.

(c) nitric acid. Nitrate becomes nitric acid, same rule as (b).

(d) acetic acid — also correctly called ethanoic acid. This is the acid in vinegar, and you will meet it again in Titrating an Acid.

The rule worth memorising is the pair: -ate becomes -ic acid, and -ite becomes -ous acid. So $\mathrm{H}{}_2\mathrm{SO}{}_3$, built from sulfite, is sulfurous acid.

The $\text{(aq)}$ matters. These are names for the substances dissolved in water. Pure hydrogen chloride is a gas, and it is not called hydrochloric acid.

6. Write the formula for: (a) ammonium phosphate (b) copper(II) nitrate (c) sodium hydrogen carbonate (d) barium sulfate (e) potassium permanganate

Answer 6

(a) $\mathrm{NH}{}_4{}^{+}$ and $\mathrm{PO}{}_4{}^{3-}$ — three ammoniums balance one phosphate: $(\mathrm{NH}{}_4){}_3\mathrm{PO}{}_4$

(b) $\mathrm{Cu}{}^{2+}$ (the Roman numeral told you) and $\mathrm{NO}{}_3{}^{-}$ — two nitrates: $\mathrm{Cu}(\mathrm{NO}{}_3{)}_2$

(c) $\mathrm{Na}{}^{+}$ and $\mathrm{HCO}{}_3{}^{-}$ — one each: $\mathrm{NaHCO}{}_3$. This is baking soda, and the older name “sodium bicarbonate” is still on the box.

(d) $\mathrm{Ba}{}^{2+}$ and $\mathrm{SO}{}_4{}^{2-}$ — the charges already cancel one to one: $\mathrm{BaSO}{}_4$

(e) $\mathrm{K}{}^{+}$ and $\mathrm{MnO}{}_4{}^{-}$ — one each: $\mathrm{KMnO}{}_4$

Two habits to build from this set. First, always write the charges down before you write the formula; the formula falls out of them. Second, no brackets when you only need one of the polyatomic ion — $(\mathrm{BaSO}{}_4)$ and $\mathrm{Ba}(\mathrm{SO}{}_4)$ are both wrong-looking ways to write something correct, and the plain $\mathrm{BaSO}{}_4$ is the one to use.

7. Magnesium nitride has the formula $\mathrm{Mg}{}_3\mathrm{N}{}_2$. Show where those subscripts come from. Then explain why the name is not “magnesium(II) nitride” and not “trimagnesium dinitride”.

Answer 7

Where the subscripts come from. Magnesium is in the second group and forms $\mathrm{Mg}{}^{2+}$. Nitrogen gains three electrons to form the nitride ion, $\mathrm{N}{}^{3-}$. A neutral compound needs the total positive charge to equal the total negative charge, so you need the lowest whole numbers $x$ and $y$ with

$2x=3y$

The smallest whole-number solution is $x=3$ and $y=2$: three magnesium ions at $2+$ give $6+$, and two nitride ions at $3-$ give $6-$. Hence $\mathrm{Mg}{}_3\mathrm{N}{}_2$.

Why not “magnesium(II) nitride”. A Roman numeral is only used when the metal has more than one possible charge and the name would otherwise be ambiguous. Magnesium has exactly one, so the numeral would add nothing. Writing it is not a small stylistic slip — it implies to the reader that there is a magnesium(I) or a magnesium(III) somewhere, and there is not.

Why not “trimagnesium dinitride”. Prefixes are for compounds of two non-metals, where the ratio genuinely needs stating. Here the ratio is fixed by the charges, so “magnesium nitride” already tells a chemist the formula. The name and the formula carry the same information by two different routes, and that is the whole design of the system.

8. A student hands in this list. Every line has something wrong with it. Say what, and give the correct version. (a) $\mathrm{FeSO}{}_4$ is iron sulfate. (b) $\mathrm{Ca}(\mathrm{OH}{)}_2$ is calcium dihydroxide. (c) $\mathrm{N}{}_2\mathrm{O}$ is nitrogen oxide. (d) Sodium chloride is $\mathrm{NaCl}{}_2$. (e) $\mathrm{P}{}_4\mathrm{O}{}_{10}$ is phosphorus(V) oxide, because each phosphorus is $5+$.

Answer 8

(a) Missing the Roman numeral. Iron has more than one possible charge, so “iron sulfate” is ambiguous — it could be this compound or $\mathrm{Fe}{}_2(\mathrm{SO}{}_4){}_3$. Sulfate is $2-$ and there is one iron, so the iron is $2+$: iron(II) sulfate.

(b) Prefixes do not belong on an ionic compound. The 2 is forced by the charges: $\mathrm{Ca}{}^{2+}$ needs two $\mathrm{OH}{}^{-}$. Saying “di” adds nothing and signals the wrong kind of compound. It is calcium hydroxide.

(c) Prefixes do belong here, and this is exactly the case that shows why. Nitrogen and oxygen form a whole family of compounds — $\mathrm{NO}$, $\mathrm{NO}{}_2$, $\mathrm{N}{}_2\mathrm{O}$, $\mathrm{N}{}_2\mathrm{O}{}_4$ — and “nitrogen oxide” cannot pick one out. This one is dinitrogen monoxide.

(d) The formula itself is wrong, not the name. Sodium is $\mathrm{Na}{}^{+}$ and chloride is $\mathrm{Cl}{}^{-}$, so one of each balances and the formula is $\mathrm{NaCl}$. $\mathrm{NaCl}{}_2$ would carry a net charge of $1-$, which is not a neutral compound. Worth noticing that this error is invisible if you write formulas by pattern-matching and impossible if you write the charges down first.

(e) The name is acceptable and the reasoning is confused. The compound is commonly named from its empirical formula $\mathrm{P}{}_2\mathrm{O}{}_5$, as phosphorus(V) oxide or as diphosphorus pentoxide. But phosphorus and oxygen are both non-metals — there is no $\mathrm{P}{}^{5+}$ ion sitting in a lattice here. The bonds are shared, not transferred, and the V records a combining capacity rather than an ionic charge. Getting the right name from the wrong picture is the most dangerous kind of correct answer, because nothing marks it wrong until a question depends on the picture.

Reference: Naming and Formulas, Polyatomic Ions, and Naming Rules at a Glance. Where naming stops being bookkeeping and starts predicting behaviour: Ionic and Covalent Bonding.

Curriculum connection

B2.1

use appropriate terminology related to chemical trends and chemical bonding, including, but not limited to: atomic radius, effective nuclear charge, electronegativity, ionization energy, and electron affinity [C]

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B2.7

write chemical formulae of binary and polyatomic compounds, including those with multiple valences, and name the compounds using the International Union of Pure and Applied Chemistry (IUPAC) nomenclature system [AI, C]

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