Atoms, elements, ions, molecules, compounds and the two kinds of formula
This unit is Topic 1, and it is examined on the Unit 1 paper, WCH11 (Structure, Bonding and Introduction to Organic Chemistry).
Ideas from this topic are applied in every other unit, and in the 2024 and 2025 papers it carried more marks than any other Unit 1 topic: about a quarter of the paper.
The practical side (gas volumes, formulae by experiment, test-tube reactions) is examined again on the Unit 3 practical paper, WCH13.
This first section fixes the vocabulary that every later calculation uses.
The idea: what are we counting?
Chemistry is about rearranging particles.
Before you can count them (the mole, in the next section), you need to name them precisely.
A jar of "salt" and a jar of "sugar" look alike, but one is a lattice of ions and the other is a crowd of separate molecules, and that difference decides how each melts, dissolves, conducts and reacts.
There are seven terms to know: atom, element, ion, molecule, compound, empirical formula and molecular formula.
The first five describe particles and substances; the last two describe how we write a substance down.
Particles: atoms, ions and molecules
- An atom is the basic unit. Every atom has a nucleus (protons and neutrons) with electrons around it; Topic 2 takes this apart.
- An ion forms when an atom, or a group of atoms, gains or loses electrons. Sodium atoms lose one electron to become Na⁺; chlorine atoms gain one to become Cl⁻. A group of atoms can carry a charge as a whole: the sulfate ion SO₄²⁻ and the ammonium ion NH₄⁺.
- A molecule is a group of atoms held together by covalent bonds. It can be two atoms of the same element (O₂, Cl₂) or atoms of different elements (H₂O, CO₂).
Substances: elements and compounds
- An element contains only one type of atom (all with the same atomic number). Some elements exist as single atoms (He, Ar), some as molecules (H₂, N₂, O₂, the halogens F₂, Cl₂, Br₂, I₂, and also P₄ and S₈), and some as giant structures (metals, diamond).
- A compound contains two or more elements chemically combined, in a fixed ratio. It may be made of molecules (water) or of ions (sodium chloride).
Why the distinction matters: a mixture of iron filings and sulfur can be separated with a magnet, because each element keeps its own properties.
Heat them and they combine to form the compound iron(II) sulfide, FeS, which has new properties and a fixed composition.
The seven elements that are diatomic at room temperature (H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂) must be written as molecules in every equation.
Writing "Cl" for chlorine gas is one of the commonest ways to lose both marks for an equation.
Two kinds of formula
A molecular formula tells you exactly how many atoms of each element are in one molecule.
Ethane is C₂H₆: two carbon atoms and six hydrogen atoms in every molecule.
An empirical formula is the simplest whole number ratio.
For ethane, 2 : 6 simplifies to 1 : 3, so the empirical formula is CH₃.
You find it by dividing every subscript by the largest number that goes into all of them.
Different compounds can share an empirical formula.
The figure below shows three molecules whose molecular formulae are all different but whose empirical formula is the same, CH₂O.
Count the atoms in each: A has 1 C, 2 H, 1 O; B has 2 C, 4 H, 2 O; C has 3 C, 6 H, 3 O. Each ratio simplifies to 1 : 2 : 1.
Glucose, C₆H₁₂O₆, belongs to the same family.
So an empirical formula alone never identifies a compound: you also need the relative molecular mass (the method is in the empirical and molecular formulae section of this unit).
Formulae of giant structures
Ionic compounds and other giant structures have no molecules, so they have no molecular formula.
Their formula is the simplest ratio of ions or atoms: NaCl means one Na⁺ for every Cl⁻ in a lattice of billions of ions, and SiO₂ means one silicon atom for every two oxygen atoms in a giant covalent network.
The formula of an ionic compound is fixed by charge: the total positive charge must equal the total negative charge.
Ion
Formula
Ion
Formula
ammonium
NH₄⁺
hydroxide
OH⁻
hydrogen
H⁺
nitrate
NO₃⁻
copper(II)
Cu²⁺
hydrogencarbonate
HCO₃⁻
iron(II)
Fe²⁺
carbonate
CO₃²⁻
iron(III)
Fe³⁺
sulfate
SO₄²⁻
silver
Ag⁺
oxide
O²⁻
zinc
Zn²⁺
sulfide
S²⁻
aluminium
Al³⁺
phosphate
PO₄³⁻
The table above lists ions that Topic 1 questions use without giving their charges, so learn them.
Group 1 metals form 1+ ions, Group 2 form 2+ ions, Group 7 form 1− ions and Group 6 form 2− ions; a Roman numeral in a name gives the charge on a metal ion, so copper(II) is Cu²⁺.
Where this leads
Every calculation in the rest of this unit starts from a correct formula: a wrong formula gives a wrong relative formula mass, and every later step inherits the error.
In Topic 3 you will see why sodium chloride forms a lattice and water forms molecules, and in Topic 4 the same empirical and molecular formula ideas are used to describe organic compounds.
- In the 2024 and 2025 WCH11 mark schemes, each scoring point was worth one mark, shown as "(1)" at the end of the line. Points were called M1, M2 in the guidance only; no letter codes were printed.
- A 2-mark equation gave (1) for correct formulae, balanced, and (1) for state symbols. The state symbol mark needed the first mark, and state symbols were only marked when the question asked for them.
- In calculations, an earlier wrong value used correctly later still earned the later marks (TE, ecf). The same slip in linked parts was penalised once only.
- When a question set no number of significant figures, the schemes ignored s.f. except an answer to 1 s.f.; when it asked for 2 or 3 s.f., the final mark needed them.
- Extra wrong chemistry beside a right answer cancelled the mark (the list principle). WCH11 set no asterisked 6-mark question and no levels marking.
Learn these word for word: in definitions the marks go on the key words.
- Atom: the smallest part of an element that can take part in a chemical reaction.
- Element: a substance made of only one type of atom (all the atoms have the same atomic number).
- Ion: an atom or group of atoms that carries an electrical charge, because it has lost or gained electrons.
- Molecule: a group of two or more atoms held together by covalent bonds.
- Compound: a substance made of two or more elements chemically combined.
- Empirical formula: the simplest whole number ratio of atoms of each element in a compound.
- Molecular formula: the actual number of atoms of each element in one molecule of a compound.
Identify each of the following as an atom, an ion or a molecule, and state whether the substance it comes from is an element or a compound: Ar, N₂, NH₃, NO₃⁻, Mg²⁺.
(5)
1. Ar: an atom; argon is an element (one type of atom, existing as single atoms) (1)
2. N₂: a molecule; nitrogen is an element (two atoms of one element joined covalently) (1)
3. NH₃: a molecule; ammonia is a compound (two elements combined) (1)
4. NO₃⁻: an ion made of a group of atoms; it is part of a compound such as potassium nitrate (1)
5. Mg²⁺: an ion; it is part of a compound such as magnesium oxide (1)
Answer: atoms: Ar; molecules: N₂ (element), NH₃ (compound); ions: NO₃⁻ and Mg²⁺ (both found in compounds).
Notice that a molecule can be an element (N₂) and an ion can contain several atoms (NO₃⁻).
Give the empirical formula of each compound: butane C₄H₁₀, hydrogen peroxide H₂O₂, benzene C₆H₆, propanone C₃H₆O. (4)
1. C₄H₁₀: divide 4 : 10 by 2, giving C₂H₅ (1)
2. H₂O₂: divide 2 : 2 by 2, giving HO (1)
3. C₆H₆: divide 6 : 6 by 6, giving CH (1)
4. C₃H₆O: 3 : 6 : 1 has no common factor, so the empirical formula is C₃H₆O, the same as the molecular formula (1)
Answer: C₂H₅, HO, CH, C₃H₆O. A compound whose subscripts share no common factor has the same empirical and molecular formula.
Write the formulae of aluminium sulfate, calcium nitrate and ammonium carbonate.
(3)
1. Aluminium sulfate: Al³⁺ and SO₄²⁻. The lowest common multiple of 3 and 2 is 6, so 2Al³⁺ (+6) balance 3SO₄²⁻ (−6): Al₂(SO₄)₃ (1)
2. Calcium nitrate: Ca²⁺ needs two NO₃⁻: Ca(NO₃)₂ (1)
3. Ammonium carbonate: CO₃²⁻ needs two NH₄⁺: (NH₄)₂CO₃ (1)
Answer: Al₂(SO₄)₃, Ca(NO₃)₂, (NH₄)₂CO₃.
Brackets go round a group ion whenever there is more than one of it; CaNO₃₂ is not a formula.
When a question asks for an equation, check every formula before you balance: the 2024 report on WCH11 noted that a wrong formula (a halogen written as single atoms, or the wrong charge on a copper ion) cost both equation marks.
Balancing can never rescue a wrong formula, because changing a subscript changes the substance.
- Calling an ionic compound "molecules". It is tempting because both have formulae, but NaCl is a lattice of ions with no separate molecules, so it has an empirical formula only. Use "formula unit" for one NaCl.
- Writing halogens and other diatomic elements as atoms. Chlorine gas is Cl₂, not Cl; hydrogen is H₂. Learn the seven diatomic elements.
- Getting the charge on a transition metal ion wrong. The Roman numeral gives it: copper(II) sulfate contains Cu²⁺, so its formula is CuSO₄, not Cu₂SO₄.
- Simplifying a formula that cannot be simplified. Ethanol, C₂H₆O, already has no common factor; its empirical formula is C₂H₆O, not CH₃O.
Check your understanding
1. Which of these compounds has the same empirical formula and molecular formula? (1)
- A C₂H₆
- B H₂O₂
- C C₃H₈
- D C₆H₆ Total [1]
2. Which substance is made of ions? (1)
- A ammonia
- B magnesium oxide
- C carbon dioxide
- D sulfur Total [1]
3. Complete the table by writing the formula of each compound. (4)
Compound
Formula
potassium sulfate
iron(III) hydroxide
zinc nitrate
ammonium phosphate
Total [4]
4. A student writes: "Sodium chloride has the molecular formula NaCl, because there is one sodium for every chlorine." Explain what is wrong with this statement. (2) Total [2]
5. Compound Q contains only carbon and hydrogen. Each molecule of Q contains eight carbon atoms and eighteen hydrogen atoms.
(a) Give the molecular formula of Q. (1)
(b) Give the empirical formula of Q. (1)
(c) A second hydrocarbon, R, has the empirical formula CH₂ and twice as many carbon atoms per molecule as ethene, C₂H₄. Deduce the molecular formula of R. (1)
(d) State why the empirical formula of R is not enough to identify R. (1)
Total [4]
6. Phosphorus exists as P₄ molecules and sulfur as S₈ molecules. State whether white phosphorus is an element or a compound, and whether it is made of atoms, molecules or ions, giving a reason for each. (2) Total [2]
Answers
1. C (1). C₃H₈ has no common factor in 3 : 8, so both formulae are C₃H₈. A simplifies to CH₃. B simplifies to HO. D simplifies to CH. Each wrong option comes from forgetting to divide through.
2. B (1). Magnesium oxide is Mg²⁺ and O²⁻ ions in a lattice (a metal with a non-metal). A, ammonia, and C, carbon dioxide, are covalent molecules of two non-metals. D, sulfur, is an element made of S₈ molecules.
3. K₂SO₄ (1); Fe(OH)₃ (1); Zn(NO₃)₂ (1); (NH₄)₃PO₄ (1). Brackets are needed round OH, NO₃ and NH₄ because there is more than one of each.
4. Sodium chloride is ionic: a giant lattice of Na⁺ and Cl⁻ ions with no separate molecules (1). So it has no molecular formula; NaCl is its empirical formula, giving the 1 : 1 ratio of ions (1).
5. (a) C₈H₁₈ (1). (b) C₄H₉ (1), dividing 8 : 18 by 2. (c) Twice the carbon atoms of C₂H₄ is four, so R is C₄H₈ (1), which fits CH₂. (d) Many compounds share the empirical formula CH₂ (for example C₂H₄, C₃H₆ and C₄H₈), so the actual number of atoms in a molecule, or the relative molecular mass, is also needed (1).
6. An element, because it contains only one type of atom (phosphorus atoms) (1). It is made of molecules, because each P₄ unit is four atoms joined by covalent bonds (1).