Atoms, Elements and Compounds

Lessons

Senior 1 · Chemistry

Atoms, Elements and Compounds

5 lessons

Lesson 5 of 5

Radicals, Formulae and Naming Compounds

25 min

Reading the label on a bag of fertiliser

At an agro-dealer's shop in Musanze, one sack is labelled "Ammonium sulfate, $\mathrm{(NH_4)_2SO_4}$ , 21% N" and another "Potassium nitrate, $\mathrm{KNO_3}$ ". A tin of baking powder in a Kigali supermarket lists "sodium hydrogencarbonate, $\mathrm{NaHCO_3}$ ". Each of these names and formulae follows rules that chemists everywhere use. Once you know the rules you can write the formula from the name, and the name from the formula, of thousands of substances.

Notice the groups of atoms that keep turning up: $\mathrm{NH_4}$ , $\mathrm{SO_4}$ , $\mathrm{NO_3}$ , $\mathrm{HCO_3}$ . These are radicals.

Definition

Radical: a group of atoms of different elements that carries an electric charge and stays together as one unit in most chemical reactions. It is also called a polyatomic ion. Example: the sulfate radical, $\mathrm{SO_4^{2-}}$ , made of one sulfur and four oxygen atoms, with a charge of $- 2$ .

A radical has a valency just like an ion of a single element. Its valency is the size of its charge. Only one common radical is positive: ammonium.

Table 5.1: Common radicals and their valencies

Radical	Formula and charge	Valency	Where you meet it
Ammonium	$\mathrm{NH_4^+}$	1	ammonium sulfate fertiliser
Hydroxide	$\mathrm{OH^-}$	1	sodium hydroxide in soap-making
Nitrate	$\mathrm{NO_3^-}$	1	potassium nitrate fertiliser
Nitrite	$\mathrm{NO_2^-}$	1	sodium nitrite in cured meat
Hydrogencarbonate	$\mathrm{HCO_3^-}$	1	baking soda, $\mathrm{NaHCO_3}$
Hydrogensulfate	$\mathrm{HSO_4^-}$	1	some toilet cleaners
Carbonate	$\mathrm{CO_3^{2-}}$	2	limestone, $\mathrm{CaCO_3}$
Sulfate	$\mathrm{SO_4^{2-}}$	2	copper(II) sulfate, a fungicide on crops
Sulfite	$\mathrm{SO_3^{2-}}$	2	a preservative in dried fruit
Phosphate	$\mathrm{PO_4^{3-}}$	3	phosphate fertilisers

Some metals have more than one valency. Iron forms $\mathrm{Fe^{2+}}$ and $\mathrm{Fe^{3+}}$ ; copper forms $\mathrm{Cu^{+}}$ and $\mathrm{Cu^{2+}}$ . The name then shows the valency with a Roman numeral in brackets: iron(II), iron(III).

Table 5.2: Valencies of common ions (learn these)

	Positive ions	Negative ions
Valency 1	$\mathrm{H^+}$ , $\mathrm{Li^+}$ , $\mathrm{Na^+}$ , $\mathrm{K^+}$ , $\mathrm{Ag^+}$ , copper(I) $\mathrm{Cu^+}$ , $\mathrm{NH_4^+}$	$\mathrm{F^-}$ , $\mathrm{Cl^-}$ , $\mathrm{Br^-}$ , $\mathrm{I^-}$ , $\mathrm{OH^-}$ , $\mathrm{NO_3^-}$ , $\mathrm{HCO_3^-}$
Valency 2	$\mathrm{Mg^{2+}}$ , $\mathrm{Ca^{2+}}$ , $\mathrm{Zn^{2+}}$ , $\mathrm{Ba^{2+}}$ , $\mathrm{Pb^{2+}}$ , iron(II) $\mathrm{Fe^{2+}}$ , copper(II) $\mathrm{Cu^{2+}}$	$\mathrm{O^{2-}}$ , $\mathrm{S^{2-}}$ , $\mathrm{CO_3^{2-}}$ , $\mathrm{SO_4^{2-}}$ , $\mathrm{SO_3^{2-}}$
Valency 3	$\mathrm{Al^{3+}}$ , iron(III) $\mathrm{Fe^{3+}}$	$\mathrm{N^{3-}}$ , $\mathrm{PO_4^{3-}}$

Writing a formula by swapping valencies

A compound has no overall charge: the positive charges must exactly cancel the negative charges. The quickest way to find the right numbers is to swap the valencies (the "criss-cross" method):

Write the symbol of the positive part first (usually the metal or ammonium), then the negative part.
Write each valency above its symbol.
Swap them: each valency becomes the subscript of the other symbol.
Divide both subscripts by any common factor, and leave out a subscript of 1.
If a radical needs a subscript bigger than 1, put the radical in brackets.

Figure 5.1: Swapping valencies. Each valency becomes the subscript of the other part. The check underneath shows that the charges add up to zero.

Worked example

Problem. Write the formula of aluminium oxide, which forms the tough skin on aluminium cooking pots.

Solution. Aluminium has valency 3 and oxygen has valency 2. Swap them: $\mathrm{Al_2O_3}$ . There is no common factor of 2 and 3, so the formula is $\mathrm{Al_2O_3}$ .

Check: $2 \times (+3) + 3 \times (-2) = +6 - 6 = 0$ .

Worked example

Problem. Write the formulae of (a) magnesium oxide, (b) calcium hydroxide (slaked lime), (c) iron(III) sulfate.

Solution.

(a) Mg valency 2, O valency 2. Swapping gives $\mathrm{Mg_2O_2}$ ; divide both by 2: $\mathrm{MgO}$ .
(b) Ca valency 2, OH valency 1. Swapping gives Ca with 1 and OH with 2. The hydroxide radical needs a subscript 2, so it goes in brackets: $\mathrm{Ca(OH)_2}$ . Check: $(+2) + 2 \times (-1) = 0$ .
(c) Iron(III) means valency 3; sulfate has valency 2. Swapping: $\mathrm{Fe_2(SO_4)_3}$ . Check: $2 \times (+3) + 3 \times (-2) = 0$ .

Naming compounds

The positive part comes first and keeps its name: sodium, calcium, ammonium.
A single non-metal ion ends in -ide: chloride, oxide, sulfide, nitride. Two radicals also end in -ide: hydroxide, $\mathrm{OH^-}$ , and cyanide, $\mathrm{CN^-}$ .
A radical containing oxygen ends in -ate (sulfate, nitrate, carbonate, phosphate); the radical with less oxygen ends in -ite (sulfite $\mathrm{SO_3^{2-}}$ , nitrite $\mathrm{NO_2^-}$ ).
A metal with more than one valency shows it in Roman numerals: $\mathrm{FeCl_2}$ is iron(II) chloride and $\mathrm{FeCl_3}$ is iron(III) chloride.
Two non-metals joined together use number prefixes: mono- (1), di- (2), tri- (3), tetra- (4), penta- (5). "Mono" is left off the first element: $\mathrm{CO}$ is carbon monoxide, $\mathrm{CO_2}$ carbon dioxide, $\mathrm{SO_3}$ sulfur trioxide, $\mathrm{CCl_4}$ carbon tetrachloride.

Table 5.3: Name endings and prefixes

Ending or prefix	Meaning	Examples
-ide	a single non-metal (or OH, CN)	sodium chloride $\mathrm{NaCl}$ ; zinc oxide $\mathrm{ZnO}$ ; potassium hydroxide $\mathrm{KOH}$
-ate	a radical with oxygen	calcium carbonate $\mathrm{CaCO_3}$ ; sodium sulfate $\mathrm{Na_2SO_4}$
-ite	the same radical with less oxygen	sodium sulfite $\mathrm{Na_2SO_3}$ ; potassium nitrite $\mathrm{KNO_2}$
hydrogen-	a radical that still has one H	sodium hydrogencarbonate $\mathrm{NaHCO_3}$
(II), (III)	the valency of the metal	copper(II) sulfate $\mathrm{CuSO_4}$ ; iron(III) oxide $\mathrm{Fe_2O_3}$
di-, tri-, tetra-, penta-	numbers of atoms, for two non-metals	nitrogen dioxide $\mathrm{NO_2}$ ; diphosphorus pentoxide $\mathrm{P_2O_5}$

Worked example

Problem. Name these compounds: (a) $\mathrm{KNO_3}$ , (b) $\mathrm{NH_4Cl}$ , (c) $\mathrm{FeSO_4}$ , (d) $\mathrm{Na_2CO_3}$ , (e) $\mathrm{N_2O}$ .

Solution.

(a) K is potassium; $\mathrm{NO_3}$ is nitrate: potassium nitrate.
(b) $\mathrm{NH_4}$ is ammonium; Cl alone is chloride: ammonium chloride.
(c) Sulfate has valency 2, so this iron has valency 2: iron(II) sulfate.
(d) Sodium and carbonate: sodium carbonate.
(e) Two non-metals: 2 N and 1 O: dinitrogen monoxide ("mono" is kept on the second element; its common name is nitrous oxide, or "laughing gas", used as an anaesthetic).

Warning

Never change a radical. Calcium hydroxide is $\mathrm{Ca(OH)_2}$ , not $\mathrm{CaOH_2}$ (which would mean one O and two H) and not $\mathrm{CaO_2H_2}$ . Brackets are used only when a radical is needed more than once; a single element never gets brackets: magnesium chloride is $\mathrm{MgCl_2}$ , not $\mathrm{Mg(Cl)_2}$ .

Tip

Check every formula you write by adding up the charges. If they do not come to zero, the formula is wrong.

Wayne Breslyn (Dr. B.): The Criss-Cross Method for Writing Chemical Formulas (6 min). He writes charges where we write valencies; the method is the same. Watch for the step where he simplifies the subscripts.

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Practice

What is a radical? Give two examples with their charges.
Write the formulae of: (a) sodium chloride, (b) calcium oxide, (c) aluminium chloride, (d) potassium sulfate.
Write the formulae of: (a) ammonium nitrate, (b) magnesium hydroxide, (c) calcium phosphate, (d) copper(II) nitrate.
Name: (a) $\mathrm{MgCO_3}$ , (b) $\mathrm{ZnS}$ , (c) $\mathrm{Cu_2O}$ , (d) $\mathrm{SO_2}$ , (e) $\mathrm{KHCO_3}$ .
Explain the difference between sodium sulfate and sodium sulfite.
A student writes aluminium hydroxide as $\mathrm{AlOH_3}$ . What is wrong? Write it correctly.
Iron rust contains iron(III) oxide. Write its formula and show that the charges balance.

Show answersHide answers

A charged group of atoms of different elements that acts as one unit. Examples: $\mathrm{SO_4^{2-}}$ , $\mathrm{NH_4^+}$ , $\mathrm{NO_3^-}$ , $\mathrm{OH^-}$ .
(a) $\mathrm{NaCl}$ (b) $\mathrm{CaO}$ (c) $\mathrm{AlCl_3}$ (d) $\mathrm{K_2SO_4}$ .
(a) $\mathrm{NH_4NO_3}$ (b) $\mathrm{Mg(OH)_2}$ (c) $\mathrm{Ca_3(PO_4)_2}$ (d) $\mathrm{Cu(NO_3)_2}$ .
(a) magnesium carbonate (b) zinc sulfide (c) copper(I) oxide (d) sulfur dioxide (e) potassium hydrogencarbonate.
Sodium sulfate, $\mathrm{Na_2SO_4}$ , contains the sulfate radical with 4 oxygen atoms; sodium sulfite, $\mathrm{Na_2SO_3}$ , contains the sulfite radical with only 3.
Without brackets the 3 applies only to H, meaning one O and three H. The hydroxide radical is needed three times: $\mathrm{Al(OH)_3}$ .
$\mathrm{Fe_2O_3}$ : $2 \times (+3) + 3 \times (-2) = +6 - 6 = 0$ .

1Reading the label on a bag of fertiliser

2Writing a formula by swapping valencies

3Naming compounds

Reading the label on a bag of fertiliser

Writing a formula by swapping valencies

Naming compounds