Why is "Giving the electron a relative mass of 1" a common mistake in Atoms and Molecules?

Why it happens: Students remember 'proton 1, neutron 1' and carry the pattern over to the electron. How to avoid it: The electron's relative mass is **1/1840** (or 'negligible' / ≈ 0). It is about 1840 times lighter than a proton — that is why almost all the mass is in the nucleus. Source: Edexcel Chemistry examiner reports — recurring error

Why is "Writing charges without the + or − sign (e.g. '1' instead of '+1')" a common mistake in Atoms and Molecules?

Why it happens: Students focus on the number and forget the sign matters. How to avoid it: Always include the sign: proton **+1**, electron **−1**, neutron **0**. A charge written as '1' for an electron is treated as wrong. Source: Edexcel Chemistry examiner reports

Why is "Saying electrons are found in the nucleus" a common mistake in Atoms and Molecules?

Why it happens: Diagrams pack the particles close together, so students lose track of where the electrons sit. How to avoid it: Only **protons and neutrons** are in the nucleus. Electrons are in **shells (energy levels)** around the nucleus. Source: Edexcel Chemistry examiner reports

Why is "Explaining a neutral atom by just saying 'the charges balance'" a common mistake in Atoms and Molecules?

Why it happens: It sounds correct but does not name the particles. How to avoid it: State that the **number of protons equals the number of electrons**, so the +1 and −1 charges **cancel**. Naming the particles is what earns the marks. Source: Edexcel Chemistry examiner reports

Why is "Assuming every molecule is a compound" a common mistake in Atoms and Molecules?

Why it happens: Most familiar molecules (H₂O, CO₂) happen to be compounds. How to avoid it: A molecule is just **atoms bonded together** — it can be an element (O₂, N₂) or a compound (H₂O). A compound must contain **different** elements. Source: Edexcel Chemistry examiner reports

Why is "Using 'atom' and 'element' as if they mean the same thing" a common mistake in Atoms and Molecules?

Why it happens: Both refer to a single type of particle, so the distinction blurs. How to avoid it: An **atom** is a single particle; an **element** is a substance made of only one type of atom. Say 'an atom of oxygen' but 'oxygen is an element'. Source: Edexcel Chemistry examiner reports

Atomic structureEdexcel IGCSE Science(Double Award)-Chemistry (4WSD0-1C)

Atoms and Molecules

Work through the notes, try the practice questions, then take the quiz. The report tells you exactly what to revise next.

Previous Next

Learn this Topic

Start with the slides for the quick version, then go deeper with the full study notes.

Detailed Notes

Full prose, callouts and a recap — built for A* mastery, not just a quick scan.

Take these study notes with you

Download a branded PDF — full prose, callouts, recap and memorise list for Atoms and Molecules, ready to print or save offline.

Atoms and Molecules — Edexcel International GCSE Science (Double Award) Chemistry (4WSD0-1C) Study Notes

The nuclear model of the atom: protons, neutrons and electrons — their relative masses and charges, and where they sit. Why atoms are neutral, and the difference between an atom, a molecule, an element and a compound. Covers Double Award Chemistry spec 1.14–1.15, assessed on Chemistry Paper 1 (4WSD0/1C).

What you’ll learn

Mapped to the Edexcel IGCSE 4WSD0-1C syllabus (2026 onwards).

1.14 — Recall the relative masses and relative charges of protons, neutrons and electrons.
1.15 — Understand that atoms have a small, dense nucleus of protons and neutrons surrounded by electrons in shells.

The three subatomic particles (spec 1.14)

Learn the relative mass and relative charge of the proton, neutron and electron.

Every atom is built from just three subatomic particles: the proton, the neutron and the electron. The exam will ask you to recall their relative masses and relative charges — these are values you simply have to know.

We use relative values because the real masses and charges are tiny. We compare everything to the proton: a proton is given a relative mass of 1 and a relative charge of +1.

Particle	Relative mass	Relative charge	Where it is found
Proton	1	+1	In the nucleus
Neutron	1	0	In the nucleus
Electron	1/1840 (negligible)	−1	In shells around the nucleus

Mass. A proton and a neutron have (almost) the same mass — both relative mass 1. An electron is about 1840 times lighter, so its mass is treated as negligible (≈ 0). This is why nearly all the mass of an atom is concentrated in the nucleus.

Charge. The proton is positive (+1) and the electron is negative (−1) — equal and opposite. The neutron is neutral (0), exactly as its name suggests.

Proton: relative mass 1, relative charge +1, in the nucleus.
Neutron: relative mass 1, relative charge 0, in the nucleus.
Electron: relative mass 1/1840 (≈ 0), relative charge −1, in shells.
Protons and neutrons hold almost all the mass; electrons are negligible.

See the full worked example for atoms and molecules →

The nuclear model of the atom (spec 1.15)

Tiny dense nucleus of protons and neutrons, electrons in shells, mostly empty space.

Atoms have a nuclear structure. At the very centre is a tiny, dense nucleus that contains the protons and neutrons. The electrons move around the nucleus in shells (energy levels).

A tiny dense nucleus of protons and neutrons; electrons in shells; most of the atom is empty space.

Three things the mark scheme rewards:

the nucleus contains protons and neutrons;
the electrons are in shells around the nucleus;
most of the atom is empty space.

Size. The nucleus is incredibly small — about 1/10,000 of the diameter of the whole atom. If the atom were the size of a sports stadium, the nucleus would be the size of a pea at the centre. The rest is empty space through which the electrons move.

The nucleus is positive. Because the nucleus holds the protons (+1 each) and the neutrons (0), the nucleus has an overall positive charge. The negative electrons are attracted to it, which keeps them held around the nucleus.

Nucleus = protons + neutrons; tiny but holds almost all the mass.
Electrons orbit the nucleus in shells (energy levels).
Most of the atom is empty space.
The nucleus is positively charged; electrons are attracted to it.

See the full worked example for atoms and molecules →

Why atoms are neutral

Equal numbers of protons (+) and electrons (−) make the overall charge zero.

An atom has no overall charge — it is neutral. This is because an atom always contains an equal number of protons and electrons.

Each proton has a charge of +1.
Each electron has a charge of −1.
The neutron is neutral (0) and does not affect the charge.

When the number of protons equals the number of electrons, the total positive charge exactly cancels the total negative charge, so the atom is neutral overall.

Example — sodium. A sodium atom has 11 protons (charge +11 in total) and 11 electrons (charge −11 in total). These cancel: +11 + (−11) = 0. The atom is neutral.

Mark-scheme tip. To explain why an atom is neutral, you must say "the number of protons equals the number of electrons" AND that "the positive and negative charges cancel / are equal and opposite". Just saying "they balance" without naming protons and electrons rarely scores full marks.

Atoms are neutral overall.
Number of protons = number of electrons.
Each proton is +1; each electron is −1, so the charges cancel.
Neutrons (0) do not change the overall charge.

See the full worked example for atoms and molecules →

Atom, molecule, element and compound

Atom = smallest particle of an element; molecule = two or more atoms bonded together.

These four words appear constantly in chemistry, and examiners love to test whether you can tell them apart.

An atom is the smallest particle of an element that can exist. For example, a single atom of oxygen (O).
A molecule is two or more atoms chemically bonded (joined) together. For example, an oxygen molecule (O₂) or a water molecule (H₂O).
An element is a substance made of only one type of atom (it cannot be broken down into anything simpler chemically), e.g. oxygen, sodium, carbon.
A compound is a substance made of two or more different elements chemically bonded together in fixed proportions, e.g. water (H₂O) or carbon dioxide (CO₂).

A molecule can be an element OR a compound:

O₂ is a molecule of an element — two atoms of the same element joined together.
H₂O is a molecule of a compound — atoms of different elements (hydrogen and oxygen) joined together.

Term	Meaning	Example
Atom	smallest particle of an element	O, Na, He
Molecule	2+ atoms chemically bonded	O₂, H₂O, CO₂
Element	one type of atom only	oxygen, carbon, iron
Compound	2+ different elements bonded	H₂O, CO₂, NaCl

Key idea. "Molecule" tells you the atoms are bonded together. "Compound" tells you the atoms are of different elements. A water molecule is both a molecule and a compound; an oxygen molecule (O₂) is a molecule but not a compound.

Atom = smallest particle of an element.
Molecule = two or more atoms chemically bonded together.
Element = one type of atom; compound = two or more different elements bonded.
O₂ = molecule of an element; H₂O = molecule of a compound.

See the full worked example for atoms and molecules →

How it’s examined

Double Award Chemistry is assessed on one untiered paper — Chemistry Paper 1 (4WSD0/1C), 2 hours, 110 marks, 33.3% of the Double Award (calculator allowed). Atoms and molecules appears as recall and explain marks: state the relative mass and relative charge of a named particle (often in a table); say where each particle is found; explain why an atom has no overall charge (protons = electrons); and classify substances as atom/molecule/element/compound. The most common lost marks are mixing up the electron's charge and mass, and giving a vague 'they balance' answer instead of 'protons equal electrons so the charges cancel'.

Sources: Pearson Edexcel International GCSE Science (Double Award) 4SD0 specification — Issue 4 (valid for 2026 onwards); Pearson Edexcel International GCSE Chemistry 4CH1 specification — Issue 4 (shared Chemistry Paper 1 content); Pearson Edexcel 4SD0 / 4CH1 examiner reports 2022–2024. Last reviewed 2026-06-07.

Master this Topic

Worked examples, formulae, definitions and the mistakes examiners flag — everything you need to push from a pass to an A*.

Take this whole topic with you

Download a branded revision sheet — worked examples, formulae, definitions and common mistakes for Atoms and Molecules, ready to print or save as PDF.

Step-by-step worked examples — Atoms and Molecules

Step-by-step solutions to past-paper-style questions on atoms and molecules, written exactly the way a tutor would explain them at the board.

1Relative mass and charge of the particles (Getting started)
Getting started• 4WSD0/1C style — short answer• atomic-structure, spec-1.14
Question
State the relative charge of: (a) a proton; (b) a neutron; (c) an electron. (3 marks)
Step-by-step solution
1. Step 1
  (a) A proton has a relative charge of +1 (it is positive).
2. Step 2
  (b) A neutron has a relative charge of 0 (it is neutral).
3. Step 3
  (c) An electron has a relative charge of −1 (it is negative).
Answer
(a) +1; (b) 0; (c) −1.
Examiner tip
Write the sign every time: '+1', '0', '−1'. Writing just '1' for the proton or electron (without the + or −) loses the mark because the charge is not fully stated.
2Where each particle is found (Getting started)
Getting started• 4WSD0/1C style — short answer• nuclear-model, spec-1.15
Question
State where each of the following is found in an atom: (a) protons; (b) neutrons; (c) electrons. (3 marks)
Step-by-step solution
1. Step 1
  (a) Protons are found in the nucleus.
2. Step 2
  (b) Neutrons are found in the nucleus.
3. Step 3
  (c) Electrons are found in shells (energy levels) around the nucleus.
Answer
Protons and neutrons are in the nucleus; electrons are in shells around the nucleus.
Examiner tip
'In shells' or 'in energy levels' or 'orbiting the nucleus' all score for the electron. Saying electrons are 'in the nucleus' is a guaranteed lost mark.
3Explain why an atom is neutral (Building confidence)
Building confidence• 4WSD0/1C style — structured• nuclear-model, spec-1.14, explain
Question
Explain, in terms of subatomic particles, why an atom has no overall charge. (2 marks)
Step-by-step solution
1. Step 1
  Equal numbers (1). An atom contains an equal number of protons and electrons.
2. Step 2
  Charges cancel (1). Each proton is +1 and each electron is −1, so the positive and negative charges cancel out — the atom is neutral overall.
3. Step 3
  Avoid vagueness. 'The charges balance' on its own is too vague — name protons and electrons and state they are equal.
Answer
An atom has equal numbers of protons (+1 each) and electrons (−1 each), so the total positive charge cancels the total negative charge, giving no overall charge.
Examiner tip
Two marks: (1) protons = electrons, (2) the +/− charges are equal and opposite so they cancel. The neutron is irrelevant here because its charge is 0.
4Complete a particle table (Building confidence)
Building confidence• 4WSD0/1C style — structured• atomic-structure, spec-1.14
Question
Copy and complete the table for the three subatomic particles, giving the relative mass and relative charge of each. (4 marks)

Particle Relative mass Relative charge
Proton ? ?
Neutron ? ?
Electron ? ?
Step-by-step solution
1. Step 1
  Proton: relative mass 1, relative charge +1.
2. Step 2
  Neutron: relative mass 1, relative charge 0.
3. Step 3
  Electron: relative mass 1/1840 (or 'negligible' / ≈ 0), relative charge −1.
4. Step 4
  Check the electron mass. Either '1/1840' or 'negligible' is accepted — writing '1' for the electron mass is wrong.
Answer
Proton: mass 1, charge +1. Neutron: mass 1, charge 0. Electron: mass 1/1840 (negligible), charge −1.
Examiner tip
The most common slip is giving the electron a relative mass of 1 (it should be 1/1840 or 'negligible'). Marks are usually split: masses worth marks, charges worth marks.
5Classify substances as element or compound (Stretch)
Stretch• 4WSD0/1C style — extended• element-compound, molecule, reasoning
Question
For each of the following, state whether it is an element or a compound, and whether it exists as molecules: (a) oxygen, O₂; (b) water, H₂O; (c) carbon, C; (d) carbon dioxide, CO₂. (4 marks)
Step-by-step solution
1. Step 1
  (a) O₂ — only one type of atom (oxygen), so it is an element; two atoms are bonded together, so it exists as molecules.
2. Step 2
  (b) H₂O — two different elements (hydrogen and oxygen) bonded, so it is a compound; the atoms are bonded together, so it exists as molecules.
3. Step 3
  (c) C — only one type of atom, so it is an element; single carbon atoms (not bonded in pairs), so not molecules.
4. Step 4
  (d) CO₂ — two different elements (carbon and oxygen) bonded, so it is a compound; the atoms are bonded together, so it exists as molecules.
Answer
(a) element, molecules; (b) compound, molecules; (c) element, not molecules; (d) compound, molecules.
Examiner tip
Key discriminator: a molecule can be an element (O₂) OR a compound (H₂O). 'Molecule' = atoms bonded; 'compound' = different elements bonded. Students often wrongly assume every molecule must be a compound.
6Explain why mass is in the nucleus (Stretch)
Stretch• 4WSD0/1C style — extended• nuclear-model, spec-1.14, spec-1.15, explain
Question
An atom's nucleus contains its protons and neutrons. Explain why almost all the mass of an atom is found in the nucleus, and why the atom is mostly empty space. (4 marks)
Step-by-step solution
1. Step 1
  Mass of nucleus (1). Protons and neutrons each have a relative mass of 1 and they are all held in the nucleus.
2. Step 2
  Electron mass (1). Electrons have a relative mass of only 1/1840, which is negligible — so they add almost nothing to the total mass.
3. Step 3
  Conclusion on mass (1). Therefore almost all the mass is concentrated in the nucleus (the protons + neutrons).
4. Step 4
  Empty space (1). The nucleus is extremely small (about 1/10,000 of the atom's diameter); the electrons occupy shells far from it, so most of the atom's volume is empty space.
Answer
Protons and neutrons (relative mass 1 each) are in the nucleus, while electrons have a negligible relative mass of 1/1840 — so almost all the mass is in the nucleus. The nucleus is tiny compared with the whole atom and the electrons are spread out in shells, so most of the atom is empty space.
Examiner tip
Full marks need the comparison of masses (1 vs 1/1840) AND the size point (nucleus tiny relative to the atom). Saying 'electrons are light' without the value or 'negligible' usually scores only one of the available marks.

Particle	Relative mass	Relative charge
Proton	?	?
Neutron	?	?
Electron	?	?

Model Answers — Atoms and Molecules

High-scoring sample answers for atoms and molecules on the Pearson Edexcel IGCSE Chemistry Paper 1 (4WSD0/1C) paper, with examiner-style notes mapping each response to the mark scheme and assessment objectives.

Question 1
4WSD0/1C style4 marks
Give the relative mass and relative charge of a proton and of an electron. (4 marks)
Model answer
Proton: relative mass 1, relative charge +1.

Electron: relative mass 1/1840 (negligible / ≈ 0), relative charge −1.
Why this scores
1 mark each for the four values. Watch the electron's mass (1/1840, not 1) and remember to include the sign on each charge.
Question 2
4WSD0/1C style2 marks
Name the part of the atom where the protons and neutrons are found, and state where the electrons are found. (2 marks)
Model answer
Protons and neutrons are found in the nucleus. (1)

Electrons are found in shells (energy levels) around the nucleus. (1)
Why this scores
'Energy levels', 'orbits' or 'shells' are all accepted for the electrons. Placing electrons 'in the nucleus' scores zero.
Question 3
4WSD0/1C style4 marks
An atom has no overall charge. Explain why, referring to the particles in the atom and their charges. (4 marks)
Model answer
An atom contains protons (relative charge +1) and electrons (relative charge −1). (1)

The number of protons equals the number of electrons in an atom. (1)

So the total positive charge from the protons exactly cancels the total negative charge from the electrons. (1)

The neutrons have a charge of 0, so they do not affect the overall charge. The atom is therefore neutral. (1)
Why this scores
Marks for: proton +1 and electron −1; protons = electrons; charges cancel; neutron is 0 / atom neutral. The single most important phrase is 'number of protons equals the number of electrons'.
Question 4
4WSD0/1C style4 marks
Explain the difference between an atom and a molecule. Use oxygen as an example in your answer. (4 marks)
Model answer
An atom is the smallest particle of an element that can exist. (1)

A molecule is two or more atoms chemically bonded (joined) together. (1)

A single oxygen atom is written O. (1)

An oxygen molecule is written O₂ — it is two oxygen atoms bonded together. (1)
Why this scores
The discriminator is 'chemically bonded together' for the molecule. A correct example (O vs O₂) is needed for the last marks — naming the bond is not required at this level.
Question 5
4WSD0/1C style — extended6 marks
Describe the structure of an atom. In your answer refer to the nucleus, the particles it contains, the electrons, the relative masses and charges of the particles, and why most of the atom is empty space. (6 marks)
Model answer
An atom has a small, dense, positively charged nucleus at its centre. (1)

The nucleus contains protons (relative mass 1, charge +1) and neutrons (relative mass 1, charge 0). (1)

The electrons (relative mass 1/1840, charge −1) move around the nucleus in shells (energy levels). (1)

Almost all the mass of the atom is in the nucleus, because the electrons have a negligible mass (1/1840). (1)

The atom is neutral because the number of protons equals the number of electrons, so the charges cancel. (1)

The nucleus is tiny compared with the whole atom (about 1/10,000 of its diameter), so most of the atom is empty space. (1)
Why this scores
A full-band answer hits all three structural keywords (nucleus = protons + neutrons; electrons in shells; mostly empty space) AND the relative masses/charges. Vague answers that omit the masses or the 'empty space' point cap themselves below the top band.
Question 6
4WSD0/1C style — extended8 marks
A sample contains the substances oxygen (O₂), water (H₂O), helium (He) and carbon dioxide (CO₂). Classify each substance as an element or a compound, and state whether it exists as molecules. Explain how you can tell whether a substance is a compound. (8 marks)
Model answer
Oxygen, O₂ — only one type of atom (oxygen), so it is an element; two atoms bonded, so it exists as molecules. (1 + 1)

Water, H₂O — two different elements (hydrogen and oxygen) bonded, so it is a compound; the atoms are bonded, so it exists as molecules. (1 + 1)

Helium, He — only one type of atom, so it is an element; single atoms (not bonded), so it does not exist as molecules. (1 + 1)

Carbon dioxide, CO₂ — two different elements (carbon and oxygen) bonded, so it is a compound and exists as molecules. (1)

How to tell a compound: it is made of two or more different elements chemically bonded together in fixed proportions; an element contains only one type of atom. (1)
Why this scores
Strong answers keep 'molecule' (atoms bonded) and 'compound' (different elements bonded) separate. The classic trap is calling O₂ a compound — it is a molecule of an element. Helium exists as single, unbonded atoms.

Key Definitions and Keywords — Atoms and Molecules

Definitions to memorise and the exact keywords mark schemes credit for atoms and molecules answers — sharpened from recent examiner reports for the 2026 Pearson Edexcel IGCSE Chemistry Paper 1 (4WSD0/1C) sitting.

Atom
Examiner keyword
The smallest particle of an element that can exist. Atoms are made of protons, neutrons and electrons.
Proton
Examiner keyword
A positively charged subatomic particle found in the nucleus. Relative mass 1, relative charge +1.
Neutron
Examiner keyword
A neutral subatomic particle found in the nucleus. Relative mass 1, relative charge 0.
Electron
Examiner keyword
A negatively charged subatomic particle found in shells around the nucleus. Relative mass 1/1840 (negligible), relative charge −1.
Nucleus
Examiner keyword
The small, dense, positively charged centre of an atom that contains the protons and neutrons. It holds almost all the atom's mass.
Shell (energy level)
A region around the nucleus in which electrons are found and move around the nucleus.
Molecule
Examiner keyword
Two or more atoms chemically bonded together. A molecule can be of an element (e.g. O₂) or a compound (e.g. H₂O).
Element
Examiner keyword
A substance made of only one type of atom, which cannot be broken down into simpler substances by chemical means.
Compound
Examiner keyword
A substance made of two or more different elements chemically bonded together in fixed proportions, e.g. H₂O or CO₂.
Relative charge
The charge of a subatomic particle compared with the proton: proton +1, neutron 0, electron −1.
Relative mass
The mass of a subatomic particle compared with the proton: proton 1, neutron 1, electron 1/1840 (negligible).

Common Mistakes and Misconceptions — Atoms and Molecules

The traps other students keep falling into on atoms and molecules questions — taken from recent Pearson Edexcel IGCSE Chemistry Paper 1 (4WSD0/1C) examiner reports and mark schemes — and how to avoid them.

✕Giving the electron a relative mass of 1
Edexcel Chemistry examiner reports — recurring error
Why it happens
Students remember 'proton 1, neutron 1' and carry the pattern over to the electron.
How to avoid it
The electron's relative mass is 1/1840 (or 'negligible' / ≈ 0). It is about 1840 times lighter than a proton — that is why almost all the mass is in the nucleus.
✕Writing charges without the + or − sign (e.g. '1' instead of '+1')
Edexcel Chemistry examiner reports
Why it happens
Students focus on the number and forget the sign matters.
How to avoid it
Always include the sign: proton +1, electron −1, neutron 0. A charge written as '1' for an electron is treated as wrong.
✕Saying electrons are found in the nucleus
Edexcel Chemistry examiner reports
Why it happens
Diagrams pack the particles close together, so students lose track of where the electrons sit.
How to avoid it
Only protons and neutrons are in the nucleus. Electrons are in shells (energy levels) around the nucleus.
✕Explaining a neutral atom by just saying 'the charges balance'
Edexcel Chemistry examiner reports
Why it happens
It sounds correct but does not name the particles.
How to avoid it
State that the number of protons equals the number of electrons, so the +1 and −1 charges cancel. Naming the particles is what earns the marks.
✕Assuming every molecule is a compound
Edexcel Chemistry examiner reports
Why it happens
Most familiar molecules (H₂O, CO₂) happen to be compounds.
How to avoid it
A molecule is just atoms bonded together — it can be an element (O₂, N₂) or a compound (H₂O). A compound must contain different elements.
✕Using 'atom' and 'element' as if they mean the same thing
Edexcel Chemistry examiner reports
Why it happens
Both refer to a single type of particle, so the distinction blurs.
How to avoid it
An atom is a single particle; an element is a substance made of only one type of atom. Say 'an atom of oxygen' but 'oxygen is an element'.

Past paper style quiz

Get a report showing which sub-topics you've nailed and which ones still need work.

Atoms and Molecules

Detailed Notes

What you’ll learn

How it’s examined

1Relative mass and charge of the particles (Getting started)

2Where each particle is found (Getting started)

3Explain why an atom is neutral (Building confidence)

4Complete a particle table (Building confidence)

5Classify substances as element or compound (Stretch)

6Explain why mass is in the nucleus (Stretch)

Atom

Proton

Neutron

Electron

Nucleus

Shell (energy level)

Molecule

Element

Compound

Relative charge

Relative mass

✕Giving the electron a relative mass of 1

✕Writing charges without the + or − sign (e.g. '1' instead of '+1')

✕Saying electrons are found in the nucleus

✕Explaining a neutral atom by just saying 'the charges balance'

✕Assuming every molecule is a compound

✕Using 'atom' and 'element' as if they mean the same thing

Past paper style quiz