Formulae of elements, compounds and ions; word equations
Every substance in chemistry has a formula, and every reaction can be written as an equation.
This unit sets out that language and the ideas underneath it: what atoms are made of, how the periodic table is built, and how atoms join by ionic and covalent bonds.
You will use it in every other topic, so it is worth getting secure now.
This topic is examined on both papers.
On Paper 1 it appears in its own questions and inside questions on acids, metals and analysis.
On Paper 2 it is nearly always set inside a question about another topic: the halogens, the alkali metals, fuels, the atmosphere or reaction rates.
So a question about the Earth's early atmosphere may ask for a dot and cross diagram, and a question about group 1 may ask about isotopes.
The worked examples in this unit use contexts from both papers.
Atoms, elements and compounds
Everything is made of atoms.
An element contains only one kind of atom, and each element has a symbol of one or two letters: H for hydrogen, Na for sodium, Cl for chlorine.
The first letter is always a capital and any second letter is always small, so Co is cobalt but CO is carbon monoxide, a compound of carbon and oxygen.
Some elements are made of separate atoms.
Helium, neon and argon are like this, and so are metals such as iron and copper, so their formulae are just their symbols: He, Fe, Cu.
Other elements are made of molecules: small groups of atoms joined together.
Seven common elements exist as molecules of two atoms, called diatomic molecules.
Their formulae must always show the 2:
element
formula
hydrogen
H₂
nitrogen
N₂
oxygen
O₂
fluorine
F₂
chlorine
Cl₂
bromine
Br₂
iodine
I₂
A compound contains atoms of two or more elements chemically joined.
Its formula shows which elements are present and how many atoms of each there are.
The diagram below shows the difference at particle level.
In the first box each particle is a single helium atom, so the formula is He.
In the second box each particle is two chlorine atoms joined, so the formula is Cl₂, not Cl.
In the third box each particle contains two different kinds of atom, so water is a compound, H₂O.
Reading a formula
The small number written low after a symbol (a subscript) tells you how many atoms of that element there are.
It belongs only to the symbol just before it.
No number means one.
- CO₂: one carbon atom and two oxygen atoms. The name "dioxide" means two oxygen atoms.
- H₂SO₄: two hydrogen, one sulfur and four oxygen atoms, seven atoms in all.
- Ca(OH)₂: the 2 after the bracket multiplies everything inside it, so one calcium, two oxygen and two hydrogen atoms.
- (NH₄)₂CO₃: two N, eight H, one C and three O, fourteen atoms in all.
A formula is a fixed recipe.
Changing a subscript changes the substance: H₂O is water, but H₂O₂ is hydrogen peroxide, a different compound with different properties.
This is why you never change a subscript when you balance an equation (next section).
Formulae you need to recall
You need to recall the formulae of elements, simple compounds and ions.
They are not printed anywhere in the paper, so learn them the way you would learn spellings.
These are the ones that come up most across the course.
compound
formula
compound
formula
water
H₂O
hydrochloric acid
HCl
carbon dioxide
CO₂
sulfuric acid
H₂SO₄
carbon monoxide
CO
nitric acid
HNO₃
methane
CH₄
sodium hydroxide
NaOH
ammonia
NH₃
sodium chloride
NaCl
sulfur dioxide
SO₂
calcium carbonate
CaCO₃
An ion is a charged particle.
Its formula always shows the charge, written small and high after the symbol: Na⁺, Mg²⁺, Cl⁻, SO₄²⁻.
A formula without its charge is the formula of an atom or a molecule, not of an ion.
positive ions
formula
negative ions
formula
hydrogen
H⁺
chloride
Cl⁻
sodium
Na⁺
bromide
Br⁻
potassium
K⁺
iodide
I⁻
silver
Ag⁺
hydroxide
OH⁻
ammonium
NH₄⁺
nitrate
NO₃⁻
magnesium
Mg²⁺
oxide
O²⁻
calcium
Ca²⁺
sulfide
S²⁻
copper(II)
Cu²⁺
sulfate
SO₄²⁻
iron(II)
Fe²⁺
carbonate
CO₃²⁻
iron(III)
Fe³⁺
aluminium
Al³⁺
You do not have to learn every simple ion as a separate fact.
The charge on an ion of a group 1, 2, 6 or 7 element follows from its position in the periodic table, as you will see in the section on forming ions.
The ions made of several atoms (ammonium, hydroxide, nitrate, sulfate, carbonate) do have to be learned.
Word equations
A chemical reaction rearranges atoms into new substances.
A word equation names every substance that reacts (the reactants) on the left and every new substance made (the products) on the right, with one arrow between them.
magnesium + oxygen → magnesium oxide
zinc + hydrochloric acid → zinc chloride + hydrogen
calcium carbonate → calcium oxide + carbon dioxide
methane + oxygen → carbon dioxide + water
Four habits make a word equation score:
1. Write the full name of each substance: "sulfuric acid", not "sulfuric" or "acid".
2. Use "+" between substances and one arrow, "→", between the sides. An equals sign or two arrows is not a word equation.
3. An element keeps its element name ("chlorine", "oxygen"). In a compound, the second element's name changes ending: chlorine becomes chloride, oxygen becomes oxide, sulfur becomes sulfide. A metal burning in oxygen always gives the metal oxide.
4. List every product. When an acid reacts with a carbonate, there are three products: a salt, water and carbon dioxide.
Word equations are a quick check on your chemistry before you write symbols.
If the names do not balance in meaning, for example magnesium appears on the left but not on the right, the symbol equation cannot balance either.
Atoms are never created or destroyed in a reaction; they are only rearranged.
- Element: a substance made of only one type of atom.
- Compound: a substance made of atoms of two or more different elements chemically joined together.
- Molecule: two or more atoms joined together by covalent bonds.
- Ion: an atom or group of atoms with a positive or negative charge.
- Reactants and products: the substances that react, written on the left of an equation, and the new substances made, written on the right.
A student adds a piece of iron to dilute sulfuric acid.
Bubbles of hydrogen are given off and a pale green solution of iron(II) sulfate forms.
Write the word equation for the reaction.
(2)
- iron + sulfuric acid (1)
- → iron(II) sulfate + hydrogen (1)
Answer: iron + sulfuric acid → iron(II) sulfate + hydrogen.
"Iron + acid" does not score the first mark, because the acid must be named in full.
This question is about the group 7 elements.
When bromine vapour passes over hot iron wool, a brown solid, iron(III) bromide, forms.
Complete the word equation for this reaction.
(1)
iron + ............
→ ............
- iron + bromine → iron(III) bromide (1)
Answer: bromine on the left and iron(III) bromide on the right, both needed for the mark.
"Bromide" on the left is wrong, because the reactant is the element bromine; "iron bromine" on the right is wrong, because the product is a compound.
Ammonium phosphate, (NH₄)₃PO₄, is used in some fertilisers.
Calculate the total number of atoms shown in the formula (NH₄)₃PO₄.
(2)
1. Inside the bracket, NH₄ has 1 N and 4 H. The 3 after the bracket gives 3 N and 12 H.
2. PO₄ has 1 P and 4 O.
- N 3 and H 12 (1)
- total 3 + 12 + 1 + 4 = 20 (1)
Answer: 20 atoms.
An answer of 9 forgets that the 3 multiplies the whole bracket.
Which of these is the formula of oxygen gas?
- A ☐ O
- B ☐ O₂
- C ☐ O²⁻
- D ☐ 2O
(1)
Answer: B O₂ is the only correct answer.
A is a single oxygen atom, C is the oxide ion, and D is two separate oxygen atoms, not a molecule.
When a question asks for a word equation, write one: names only, one arrow, every product listed.
Words such as "hydrogen bromide" or "potassium chloride" need both parts; writing "potassium" on its own as a product, or a formula among the words, loses the mark.
- Single atoms for diatomic elements. It is tempting to write Cl or O because one symbol looks simpler, but chlorine, oxygen, hydrogen, nitrogen and the other halogens exist as molecules of two atoms: Cl₂, O₂, H₂, N₂, Br₂, F₂, I₂.
- The wrong product of burning. A metal burning in air gives the metal oxide (zinc gives zinc oxide). Carbon dioxide comes only from something that contains carbon.
- Chlorine for chloride. The element is chlorine; the ion and its compounds are chloride. The same goes for bromine and bromide, oxygen and oxide.
- Splitting a compound into its parts. "Sodium carbonate" is one substance. It does not react as "sodium" and "carbonate" separately, and it is not written as two products.
- Two answers on one line. "Chlorine or chloride" includes a wrong answer, so it scores 0. Write the one you mean.
- Capitals and subscripts. Mg, not MG or mg; CO₂ with the 2 written small and low. A formula with the wrong case can be a different substance, and the mark is lost.
Check your understanding
1. This question is about formulae.
(a) Which is the formula of sulfuric acid?
- A ☐ HSO₄
- B ☐ H₂SO₃
- C ☐ H₂SO₄
- D ☐ H₂S
(1)
(b) State the number of atoms of each element in one formula unit of calcium nitrate, Ca(NO₃)₂. (2)
calcium ............
nitrogen ............
oxygen ............
Total [3]
2. Draw one straight line from each name to its formula. (3)
name
formula
ammonia
CO
carbon monoxide
NH₃
nitrogen
N₂
NO₂
Total [3]
3. A student burns a piece of magnesium ribbon in air. A white powder forms.
(a) Name the white powder. (1)
(b) Write the word equation for the reaction. (2)
(c) The student says the mass of the white powder is greater than the mass of the magnesium ribbon. Explain why. (2)
Total [5]
4. This question is about metals that react with water. Calcium is in group 2. When calcium is added to water, bubbles of hydrogen form and a solution of calcium hydroxide is made.
(a) Write the word equation for the reaction. (2)
(b) Give the formula of the ion in calcium hydroxide solution that makes it alkaline. (1)
Total [3]
5. A student writes the formula of chlorine gas as Cl and the formula of chlorine in sodium chloride as Cl₂.
Explain what is wrong with each formula.
(2)
Total [2]
6. Copper carbonate reacts with dilute hydrochloric acid.
Complete the word equation.
(2)
copper carbonate + hydrochloric acid → copper chloride + ............
- ............
Total [2]
Answers
1. (a) C H₂SO₄ is the only correct answer (1). A has one hydrogen too few; B is sulfurous acid, with three oxygen atoms; D is hydrogen sulfide, with no oxygen.
(b) calcium 1, nitrogen 2 and oxygen 6: all three right (2); two right (1). The 2 after the bracket doubles the N and the O in NO₃.
2. ammonia to NH₃ (1); carbon monoxide to CO (1); nitrogen to N₂ (1). Two lines from one name score 0 for that name. NO₂ (nitrogen dioxide) is not used.
3. (a) magnesium oxide (1). reject magnesium oxygen.
(b) magnesium + oxygen (1) → magnesium oxide (1). "Air" in place of oxygen does not score the first mark.
(c) oxygen atoms from the air join to the magnesium (1); so the product contains the mass of the magnesium plus the mass of the oxygen that combined with it (1).
4. (a) calcium + water (1) → calcium hydroxide + hydrogen (1).
(b) OH⁻ (1). The charge must be shown; "OH" alone is not the formula of an ion.
5. chlorine gas is made of diatomic molecules, so its formula is Cl₂ (1); in sodium chloride chlorine is present as chloride ions, one for each sodium ion, so it is Cl (in NaCl) and not Cl₂ (1).
6. water (1); carbon dioxide (1). Either order. "Hydrogen" scores 0: a carbonate and an acid give carbon dioxide, not hydrogen.