What is the basic structure of an atom according to the Edexcel IGCSE Chemistry syllabus?

In the Edexcel IGCSE Chemistry syllabus, an atom is described as consisting of a central nucleus containing protons and neutrons, surrounded by electrons in shells. The nucleus is positively charged due to the protons, while the electrons are negatively charged and orbit the nucleus in defined energy levels or shells.

How do you determine the number of protons, neutrons, and electrons in an atom?

The number of protons in an atom is equal to its atomic number, which is unique for each element. The number of electrons in a neutral atom is also equal to the atomic number. To find the number of neutrons, subtract the atomic number from the mass number (the total number of protons and neutrons). This information is crucial for understanding atomic structure in the Edexcel IGCSE Chemistry curriculum.

What is the significance of electron shells in atomic structure?

Electron shells are important in atomic structure as they determine the arrangement of electrons around the nucleus. Each shell can hold a specific number of electrons, and the distribution of electrons across these shells influences an atom's chemical properties and reactivity. Understanding electron configuration is a key aspect of the Principles of Chemistry in the Edexcel IGCSE syllabus.

How does the concept of isotopes relate to atomic structure?

Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. This results in different mass numbers. Isotopes are significant in atomic structure because they illustrate how variations in neutron numbers can exist without affecting the chemical properties of an element, a concept covered in the Edexcel IGCSE Chemistry course.

Why is understanding atomic structure important in Chemistry?

Understanding atomic structure is fundamental in Chemistry because it explains how elements interact, form compounds, and undergo chemical reactions. It provides insight into the periodic table's organization and helps predict the behavior of elements. This knowledge is essential for mastering the Principles of Chemistry in the Edexcel IGCSE curriculum.

Why is "Saying isotopes have different numbers of protons or electrons" a common mistake in Atomic Structure ?

Why it happens: Confusing isotopes with ions. How to avoid it: Isotopes differ ONLY in the number of NEUTRONS. Same protons (same element), same electrons (same chemistry), different neutrons (different mass). Ions are formed by gaining or losing electrons — a different concept. Source: 4CH1 Examiner Reports 2022-2024

Why is "Putting 18 electrons in the 3rd shell at IGCSE" a common mistake in Atomic Structure ?

Why it happens: Misremembering 2n² shell capacity rule from A-level. How to avoid it: At IGCSE for the FIRST 20 ELEMENTS, the 3rd shell holds only 8 electrons before the 4th shell starts to fill (with K and Ca). Use 2, 8, 8, 2.

Why is "Calculating Ar as a simple average (e.g. (35 + 37)/2 = 36 for chlorine)" a common mistake in Atomic Structure ?

Why it happens: Treating the two isotopes as if they had equal abundance. How to avoid it: Use the WEIGHTED average: Ar = (isotope mass × % abundance) summed, divided by 100. For chlorine: (35 × 75 + 37 × 25)/100 = 35.5, not 36. Source: 4CH1 Examiner Reports 2022-2024

Why is "Treating the electron mass as 1" a common mistake in Atomic Structure ?

Why it happens: Treating all three subatomic particles symmetrically. How to avoid it: Electron relative mass is 1/1840 (effectively zero / negligible). Only protons and neutrons contribute to the mass number. Electrons contribute to charge but not to mass.

Why is "Saying noble gases react readily because they have full outer shells" a common mistake in Atomic Structure ?

Why it happens: Confusing 'full' with 'available to share'. How to avoid it: A full outer shell is a STABLE arrangement — noble gases (He, Ne, Ar, Kr, Xe) are UNREACTIVE/INERT for exactly this reason. Other atoms gain, lose or share electrons to ACHIEVE the noble-gas configuration. Source: 4CH1 Examiner Reports 2022-2024

Principles of ChemistryEdexcel IGCSE Chemistry (4CH1)

Atomic Structure

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Atomic Structure — Pearson Edexcel International GCSE Chemistry 4CH1 Study Notes (2026 onwards, Spec Issue 4)

The nuclear atom: protons, neutrons and electrons; relative mass and charge. Atomic number (Z), mass number (A), and how to read $^A_Z X$ notation. Isotopes and weighted-average relative atomic mass ( $A_r$ ). Electron configuration (2, 8, 8) and how it sets group and period.

What you’ll learn

Mapped to the Cambridge IGCSE 4CH1 syllabus (2026 onwards).

1.14 — Recall the relative masses and relative charges of protons, neutrons and electrons.
1.15 — Know that atoms have a nuclear structure with protons and neutrons in the nucleus and electrons in shells around the nucleus.
1.16 — Know the atomic number is the number of protons; the mass number is the total number of protons and neutrons.
1.17 — Calculate the number of protons, neutrons and electrons in atoms and ions from their atomic number and mass number.
1.18 — Understand that atoms of the same element have the same atomic number.
1.19 — Define an isotope as different atoms of the same element with the same number of protons but different numbers of neutrons.
1.20 — Calculate the relative atomic mass of an element from the abundance of its isotopes.
1.21 — Write the electronic configuration of the first 20 elements in the periodic table (2, 8, 8, 2 rule).

The nuclear atom (spec 1.14, 1.15)

Tiny dense nucleus, electrons in shells, mostly empty space.

Atoms have a nuclear structure: a tiny dense nucleus at the centre containing protons and neutrons, with electrons moving in shells (energy levels) around the nucleus.

Particle	Relative mass	Relative charge	Where it is
Proton	1	+1	Nucleus
Neutron	1	0	Nucleus
Electron	1/1840 (negligible)	-1	Shells around nucleus

Mass. Almost all the mass of the atom is in the nucleus — electrons contribute negligibly because their relative mass is 1/1840. The protons and neutrons share the mass roughly equally.

Charge balance. In a NEUTRAL atom, the number of protons equals the number of electrons, so positive and negative charges cancel. An ion is formed when an atom GAINS or LOSES electrons:

LOSE electrons → positive ion (cation), e.g. Na atom (11p, 11e) → Na⁺ (11p, 10e).
GAIN electrons → negative ion (anion), e.g. Cl atom (17p, 17e) → Cl⁻ (17p, 18e).

Size. The nucleus is roughly 1/10,000 the diameter of the atom — most of the atom is empty space. This was first demonstrated by the Geiger–Marsden (Rutherford) alpha-scattering experiment: most α-particles passed straight through a thin gold foil (showing the atom is mostly empty), but ~1 in 8000 were deflected backwards (showing a tiny, dense, positive nucleus).

Mark-scheme tip. Edexcel mark schemes credit "electrons in shells", "nucleus contains protons + neutrons", and "most of the atom is empty space" as distinct mark-bearing keywords. Use all three.

Nucleus = protons + neutrons; tiny but contains essentially all the mass.
Electrons orbit in shells; mass negligible.
Most of the atom is empty space.
Ions form by gain (anion) or loss (cation) of electrons.

See the full worked example for atomic structure →

Atomic number Z and mass number A (spec 1.16, 1.17)

Z (bottom) = protons; A (top) = protons + neutrons.

Each element is identified by its atomic number Z = number of protons in the nucleus. All atoms of the same element have the SAME Z (this is what makes them the same element).

The mass number A is the total number of nucleons (protons + neutrons): $A = Z + n$ . So the number of neutrons is:

$n = A - Z$

Standard notation. Atoms are written as $^A_Z X$ , with the mass number on top and the atomic number on the bottom of the chemical symbol. For example:

$^{12}_6 \text{C}$ — carbon-12: 6 protons, 6 neutrons, 6 electrons.
$^{16}_8 \text{O}$ — oxygen-16: 8 protons, 8 neutrons, 8 electrons.
$^{23}_{11} \text{Na}$ — sodium-23: 11 protons, 12 neutrons, 11 electrons.
$^{27}_{13} \text{Al}$ — aluminium-27: 13 protons, 14 neutrons, 13 electrons.

Counting electrons in a NEUTRAL atom is easy: electrons = protons = Z. For an ION, adjust:

Cation X^(n+) has Z protons and (Z − n) electrons.
Anion X^(n−) has Z protons and (Z + n) electrons.

For example $^{16}_{\ 8}\text{O}^{2-}$ has 8 protons (oxygen), 8 neutrons (16 − 8) and 10 electrons (8 + 2).

Mark-scheme tip. When you write "number of neutrons = A − Z" explicitly, you earn the method mark — even if your arithmetic afterwards is wrong (ECF). Always show the subtraction line.

Z = protons (bottom number).
A = protons + neutrons (top number).
Neutrons = A − Z.
Neutral atom: electrons = Z. Ion: adjust by the charge.

See the full worked example for atomic structure →

Isotopes (spec 1.19)

Same element, different mass.

Isotopes are atoms of the same element (same atomic number Z) with different numbers of neutrons (and so different mass numbers A).

Common examples:

Hydrogen: protium (¹H), deuterium (²H), tritium (³H) — all have 1 proton; differ by 0, 1, 2 neutrons.
Carbon: ¹²C, ¹³C, ¹⁴C — all have 6 protons; differ by 6, 7, 8 neutrons.
Chlorine: ³⁵Cl, ³⁷Cl — both have 17 protons; differ by 18 or 20 neutrons.
Oxygen: ¹⁶O, ¹⁷O, ¹⁸O — all have 8 protons.

Why do isotopes have the same chemical properties? Chemical reactions are governed by outer-shell electrons. All isotopes of an element have the same number of protons → same number of electrons → same electron configuration → same chemical behaviour.

Why do isotopes have different physical properties? Anything that depends on mass differs between isotopes. Examples:

Density: D₂O is ~11% denser than H₂O.
Boiling point: D₂O boils at 101.4 °C; H₂O boils at 100.0 °C.
Diffusion rate: lighter isotopes diffuse faster (at the same T, $\tfrac{1}{2}mv^2$ is constant → smaller $m$ → larger $v$ ).
Radioactivity: some isotopes (e.g. ¹⁴C, tritium) are radioactive while others are stable.

Real-world applications.

¹⁴C dating archaeological samples (because ¹⁴C decays at a known rate).
¹³C NMR spectroscopy uses the magnetic ¹³C nucleus.
Heavy water (D₂O) was used as a neutron moderator in early nuclear reactors.

Isotopes: same Z, different A.
Same chemistry (same electrons), different physics (different mass).
Examples: ³⁵Cl / ³⁷Cl, ¹²C / ¹³C / ¹⁴C, ¹H / ²H / ³H.

See the full worked example for atomic structure →

Relative atomic mass Ar (spec 1.20)

Weighted average over natural isotopic abundances.

Relative atomic mass ( $A_r$ ) = the weighted-average mass of the atoms of an element compared with 1/12 of the mass of a ¹²C atom. It has no units (it is a ratio).

$A_r = \dfrac{\sum (\text{isotope mass} \times \% \text{ abundance})}{100}$

Because abundances total 100%, the denominator is 100.

Worked example 1 — chlorine. ³⁵Cl is 75.0% abundant; ³⁷Cl is 25.0% abundant.

$A_r(\text{Cl}) = \dfrac{35 \times 75.0 + 37 \times 25.0}{100} = \dfrac{2625 + 925}{100} = \dfrac{3550}{100} = 35.5$

This matches the chlorine Ar on the periodic table.

Worked example 2 — copper. Cu has two stable isotopes: ⁶³Cu and ⁶⁵Cu. The Ar of Cu is 63.5. What is the percentage abundance of each isotope?

Let the abundance of ⁶³Cu be $x$ . Then ⁶⁵Cu = (100 − x):

$\dfrac{63x + 65(100-x)}{100} = 63.5$

$63x + 6500 - 65x = 6350$

$-2x = -150 \Rightarrow x = 75$

So ⁶³Cu = 75% and ⁶⁵Cu = 25%.

Common error. Treating the calculation as a simple average — for chlorine that would give (35 + 37)/2 = 36, which is wrong. Always WEIGHT by abundance.

Why is Ar usually not a whole number? Because most elements are mixtures of isotopes in nature. Chlorine's Ar of 35.5 reflects the 3:1 ratio of ³⁵Cl to ³⁷Cl in naturally occurring chlorine.

$A_r$ is a weighted average — multiply mass × abundance, sum, divide by 100.
No units.
Most Ar values are not whole numbers because of natural isotope mixtures.

See the full worked example for atomic structure →

Electron configuration (spec 1.21)

Fill 2, 8, 8 — group = outer electrons; period = shells used.

Electrons occupy shells (energy levels) around the nucleus, filling the lowest shell first. For the first 20 elements, the maximum capacity of each shell is:

1st shell: 2 electrons.
2nd shell: 8 electrons.
3rd shell: 8 electrons.
4th shell: starts to fill (with 2) for K and Ca.

Writing the configuration.

Element	Z	Configuration
Hydrogen, H	1	1
Helium, He	2	2
Lithium, Li	3	2, 1
Carbon, C	6	2, 4
Oxygen, O	8	2, 6
Neon, Ne	10	2, 8
Sodium, Na	11	2, 8, 1
Magnesium, Mg	12	2, 8, 2
Aluminium, Al	13	2, 8, 3
Silicon, Si	14	2, 8, 4
Phosphorus, P	15	2, 8, 5
Sulfur, S	16	2, 8, 6
Chlorine, Cl	17	2, 8, 7
Argon, Ar	18	2, 8, 8
Potassium, K	19	2, 8, 8, 1
Calcium, Ca	20	2, 8, 8, 2

Link to the periodic table.

Group number = number of electrons in the OUTER shell (Groups 1-7).
- Na (2, 8, 1) → Group 1 (1 outer electron).
- Cl (2, 8, 7) → Group 7 (7 outer electrons).
- Group 0 (noble gases) have a FULL outer shell (2 for He; 8 for Ne, Ar, etc.).
Period number = number of OCCUPIED shells.
- Na (2, 8, 1) → 3 shells → Period 3.
- K (2, 8, 8, 1) → 4 shells → Period 4.

Stability and reactivity.

Atoms with a full outer shell (noble gases) are very stable / unreactive.
Atoms with 1 or 2 outer electrons (Groups 1, 2) easily LOSE them to achieve the previous noble-gas configuration → reactive metals.
Atoms with 6 or 7 outer electrons (Groups 6, 7) easily GAIN electrons to achieve the next noble-gas configuration → reactive non-metals.

This pattern is the key to predicting how elements react — explored in detail in 4CH1 Topic 2 (Inorganic Chemistry) and Topic 1.5 (Ionic Bonding).

Capacities: 2, 8, 8 (for first 20 elements).
Fill from the lowest shell up.
Group = outer electrons; period = shells used.
Full outer shell = stable (noble gas configuration).

See the full worked example for atomic structure →

How it’s examined

Atomic structure appears on every 4CH1 Paper 1 (4CH1/1C). Highest-frequency items: 3-4 mark counting of protons / neutrons / electrons from $^A_Z X$ ; 4-6 mark Ar weighted-average calculation; 4-5 mark electron configuration + linking to group / period; isotope definition + comparison of chemistry vs physics (4-6 marks). Paper 2 (4CH1/2C) sometimes asks for reverse Ar calculations (find abundances given Ar) and ion-counting from compound formulae. Worth 8-12 marks across both papers in a typical session.

Sources: Pearson Edexcel International GCSE Chemistry 4CH1 specification — Issue 4, September 2024 (valid for 2026 onwards); Pearson Edexcel 4CH1 Examiner Reports 2022-2024; 4CH1/1C May/Jun 2024 question paper and mark scheme; 4CH1/1C January 2024 question paper and mark scheme; 4CH1/1CR January 2024 question paper and mark scheme. Last reviewed 2026-05-12.

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Step-by-step worked examples — Atomic Structure

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

1Particles in an atom from $^A_Z X$ notation (4 marks, Paper 1)
Core• Adapted from 4CH1/1C Jan 2024 Q2• spec-1.14, spec-1.15, spec-1.16
Question
An atom is represented as $^{27}_{13}\text{Al}$ . State the number of protons, neutrons and electrons in this neutral atom. Explain how you worked out the number of neutrons. (4 marks)
Step-by-step solution
1. Step 1
  Read Z (the bottom number) = number of protons (1 M). Atomic number Z = 13, so there are 13 protons.
2. Step 2
  Neutral atom → electrons = protons (1 A). A neutral atom has zero net charge, so the number of electrons equals the number of protons: 13 electrons.
3. Step 3
  Mass number A (the top number) = protons + neutrons (1 M). A = 27 = protons + neutrons.
  $n = A - Z = 27 - 13 = 14$
4. Step 4
  Explain method (1 B). Number of neutrons = mass number − atomic number = 27 − 13 = 14 neutrons.
Answer
Aluminium-27 has 13 protons, 14 neutrons, 13 electrons. Neutrons = A − Z = 27 − 13 = 14.
Examiner tip
Two of the four marks here are for METHOD — examiners want you to write out the relationship 'neutrons = mass number − atomic number'. Just stating '14 neutrons' without the working can lose the method mark. Always show the subtraction explicitly.
2Relative atomic mass from isotopic abundance (4 marks, Paper 1)
Core• Adapted from 4CH1/1C May/Jun 2024 Q4• isotopes, Ar, spec-1.20
Question
Chlorine exists as two isotopes: chlorine-35 (75.0% abundance) and chlorine-37 (25.0% abundance). Calculate the relative atomic mass of chlorine, giving your answer to 1 decimal place. (4 marks)
Step-by-step solution
1. Step 1
  State the relationship (1 M).
  $A_r = \dfrac{\sum (\text{isotope mass} \times \% \text{ abundance})}{100}$
2. Step 2
  Substitute isotope masses and abundances (1 M).
  $A_r = \dfrac{(35 \times 75.0) + (37 \times 25.0)}{100}$
3. Step 3
  Evaluate the numerator (1 A).
  $A_r = \dfrac{2625 + 925}{100} = \dfrac{3550}{100}$
4. Step 4
  Final answer (1 B). Quote to 1 dp as requested.
  $A_r = 35.5$
Answer
$A_r = 35.5$ (1 dp).
Examiner tip
Edexcel 4CH1 mark scheme awards: 1 for formula, 1 for substitution, 1 for the numerator, 1 for the final answer to correct sig figs. ECF applies — even if you transcribe an abundance wrongly, the division step still scores. Note: 35.5 is the standard value quoted on the 4CH1 periodic table.
3Electron configuration and the periodic table (4 marks, Paper 1)
Core• Adapted from 4CH1/1CR Jan 2024 Q5• spec-1.21, electron-configuration
Question
An atom of element X has 17 electrons. (a) Write its electron configuration. (b) State the group and period of element X in the periodic table, giving reasons in terms of its electron configuration. (4 marks)
Step-by-step solution
1. Step 1
  Fill shells using 2, 8, 8 rule (1 M). Shell 1 holds 2; shell 2 holds 8; shell 3 holds remaining 7.
2. Step 2
  Write the configuration (1 A).
  $X = 2, 8, 7$
3. Step 3
  Group (1 B). Group number = number of electrons in the OUTER shell = 7 → Group 7 (the halogens).
4. Step 4
  Period (1 B). Period number = number of OCCUPIED shells = 3 → Period 3. Element X is chlorine (Cl).
Answer
(a) 2, 8, 7. (b) Group 7, Period 3 (chlorine).
Examiner tip
Two-direction rule: outer-shell electrons → group; number of shells → period. Memorise this — it gets asked in some form on every Paper 1. Examiner reports note candidates confuse 'group' with 'total electrons' — don't.
4Identifying isotopes (3 marks, Paper 1)
Core• Adapted from 4CH1/1C May/Jun 2024 Q1• isotopes, spec-1.19
Question
Three atoms have the following structures: Atom 1 (8 protons, 8 neutrons), Atom 2 (8 protons, 10 neutrons), Atom 3 (7 protons, 8 neutrons). State which two atoms are isotopes of the same element and explain your reasoning. (3 marks)
Step-by-step solution
1. Step 1
  Identify the element (1 M). The element is determined by the number of protons. Atom 1 and Atom 2 both have 8 protons so they are both oxygen.
2. Step 2
  State the isotope definition (1 A). Isotopes are atoms of the same element (same proton number) with different numbers of neutrons (and so different mass numbers).
3. Step 3
  Identify the isotopes (1 B). Atom 1 (8p, 8n, A = 16) and Atom 2 (8p, 10n, A = 18) are both oxygen but with different mass numbers — they are isotopes of oxygen: ¹⁶O and ¹⁸O. Atom 3 has only 7 protons (nitrogen), so is a different element.
Answer
Atoms 1 and 2 are isotopes of oxygen (both have 8 protons but different neutron numbers — ¹⁶O and ¹⁸O). Atom 3 is nitrogen.
Examiner tip
Mark scheme requires (1) same number of protons, (2) different number of neutrons, (3) so different mass numbers. Don't just state 'they are isotopes' — define what isotopes ARE.

Key Formulae — Atomic Structure

The formulae you need to memorise for atomic structure on the Pearson Edexcel IGCSE 4CH1 paper, with every variable defined in plain English and a note on when to use it.

Relative atomic mass (weighted average) — spec 1.20
$A_r = \dfrac{\sum (\text{isotope mass} \times \% \text{ abundance})}{100}$
When to use
Use whenever a question gives you the percentage abundance of two or more isotopes and asks for the relative atomic mass. The denominator is 100 because the abundances total 100%. Ar has no units (it's a ratio).
Number of neutrons in an atom
$n = A - Z$
When to use
From the $^A_Z X$ notation: $A$ is the mass number (top), $Z$ is the atomic number (bottom). The difference is the number of neutrons. Number of protons = Z; number of electrons in a neutral atom = Z.
Shell-filling capacities (first 20 elements)
$2,\ 8,\ 8,\ 2$
When to use
Shell 1 holds up to 2 electrons; shell 2 up to 8; shell 3 up to 8; shell 4 starts to fill (with 2) for K and Ca. Group number = number of outer-shell electrons; period number = number of occupied shells.

Model Answers — Atomic Structure

High-scoring sample answers for atomic structure on the Pearson Edexcel IGCSE 4CH1 paper, with examiner-style notes mapping each response to the mark scheme and assessment objectives.

Question
4CH1/1C-style — atomic structure overview6 marks
Describe the structure of an atom, naming the three subatomic particles and giving the relative mass, relative charge and approximate location of each. Include a comment on the size of the nucleus relative to the atom. (6 marks)
Model answer
An atom consists of a central nucleus containing protons and neutrons, surrounded by electrons moving in shells (energy levels) around the nucleus.

Particle Relative mass Relative charge Location
Proton 1 +1 Inside the nucleus
Neutron 1 0 (neutral) Inside the nucleus
Electron 1/1840 (≈ 0, negligible) -1 Shells around the nucleus

Charge balance. In a neutral atom, the number of protons equals the number of electrons, so the equal and opposite charges cancel and the atom is overall neutral.

Mass distribution. Because electrons have negligible mass, essentially all the mass of an atom is concentrated in the tiny nucleus. The nucleus is very small compared to the atom — about 1/10,000 of the atomic diameter — so the atom is mostly empty space. This was first demonstrated by the Geiger–Marsden (Rutherford) alpha-scattering experiment: most alpha particles passed straight through a thin gold foil, but a small fraction were deflected backwards, showing that the positive charge and mass are concentrated in a tiny central nucleus.
Why this scores
Edexcel 6-marker. Mark scheme: 1 mark each for proton, neutron, electron mass/charge (≈ 3 marks); 1 for location of each (≈ 1-2 marks); 1 for nucleus tiny vs atom big; 1 for 'most of the atom is empty space' OR 'mass concentrated in nucleus'. The relative electron mass 1/1840 is officially on the spec but examiners also credit 'negligible'.
Question
4CH1/2C-style — isotopes6 marks
Hydrogen has three isotopes: protium (¹H), deuterium (²H) and tritium (³H). Explain what is meant by the term 'isotope' and predict, with reasons, whether (a) protium and deuterium have the same chemical properties, and (b) protium and deuterium have the same physical properties. (6 marks)
Model answer
Isotope. Atoms of the same element (same number of protons) but with different numbers of neutrons — so they have the same atomic number but different mass numbers.

For hydrogen: ¹H has 1 proton + 0 neutrons; ²H has 1 proton + 1 neutron; ³H has 1 proton + 2 neutrons.

(a) Chemical properties — SAME. Chemical reactions are governed by the outer-shell electrons of the atom. Because all three isotopes have the same number of protons (1) and therefore the same number of electrons (1) in the same electron configuration, they have identical chemical properties. Protium and deuterium both react with oxygen to form water (H₂O and D₂O), with sodium to release hydrogen gas, etc.

(b) Physical properties — DIFFERENT. Physical properties that depend on mass differ between isotopes. Deuterium has roughly TWICE the mass of protium. As a result:

Density of D₂O ("heavy water") is about 1.11 g/cm³, compared with 1.00 g/cm³ for ordinary H₂O.

Boiling point of D₂O (101.4 °C) is higher than that of H₂O (100 °C).

Rate of diffusion of D₂ gas is slower than that of H₂ gas at the same temperature.

Radioactivity differs — tritium (³H) is radioactive whereas protium and deuterium are not.

The key principle to remember: chemistry depends on electrons (same for all isotopes); physics depends on mass (different for isotopes).
Why this scores
Edexcel 6-marker. Mark scheme: 2 for the definition (same protons + different neutrons + same atomic number + different mass numbers); 2 for chemistry same with REASON (same outer electrons); 2 for physics different with at least ONE specific example (density / bp / diffusion rate). Note the standard phrase 'chemistry depends on electrons' is what mark schemes credit.
Question
4CH1/2C-style — reverse Ar calculation4 marks
An element X has two stable isotopes: ⁶³X and ⁶⁵X. The relative atomic mass of X is 63.5. Calculate the percentage abundance of each isotope. (4 marks)
Model answer
Let the percentage abundance of ⁶³X be $x$ %. Then the abundance of ⁶⁵X is $(100 - x)$ %.

Using the weighted-average relationship:

$A_r = \dfrac{(63 \times x) + (65 \times (100 - x))}{100} = 63.5$

Multiply both sides by 100:

$63x + 65(100 - x) = 6350$

$63x + 6500 - 65x = 6350$

$-2x = -150$

$x = 75$

So the percentage abundances are:

⁶³X = 75 %

⁶⁵X = 25 %

(Element X is copper — Cu, Ar 63.5 is the actual periodic-table value.)
Why this scores
Edexcel 4-mark mark scheme: 1 for setting up x and 100-x correctly; 1 for the weighted-average equation; 1 for algebraic manipulation; 1 for the final answer. ECF applies. Tip: always check the answer by substituting back: (63×75 + 65×25)/100 = (4725 + 1625)/100 = 6350/100 = 63.5 ✓.
Question
4CH1/1C-style — electron configuration5 marks
Explain how electrons are arranged in the shells of an atom. Use sodium (Na, Z = 11) and argon (Ar, Z = 18) as examples. Comment on how the electron configuration of argon relates to its chemical behaviour. (5 marks)
Model answer
Shell-filling rules. Electrons occupy energy levels (shells) around the nucleus, filling the lowest shell first. The maximum capacity of each shell at IGCSE level is:

1st shell: 2 electrons.

2nd shell: 8 electrons.

3rd shell: 8 electrons (for the first 20 elements).

Once a shell is full, electrons begin filling the next shell.

Sodium (Z = 11). Total electrons = 11. Filling sequence: 2 → fills shell 1; 8 → fills shell 2; remaining 1 → goes into shell 3. Configuration: 2, 8, 1. The single outer-shell electron makes Na very reactive — it is easily lost to form Na⁺.

Argon (Z = 18). Total electrons = 18. Filling: 2 → shell 1; 8 → shell 2; 8 → shell 3. Configuration: 2, 8, 8. All three shells are full.

Chemical behaviour of argon. Argon has a full outer shell (8 electrons in shell 3, the noble-gas configuration). This is a stable arrangement — argon has very little tendency to gain, lose or share electrons. As a result, argon is chemically inert / unreactive and does not normally form chemical compounds, which is why it (along with the other noble gases of Group 0) sits as a separate group on the periodic table.
Why this scores
Edexcel 5-mark mark scheme: 1 for shell capacities (2, 8, 8); 1 each for Na and Ar configurations; 1 for 'full outer shell' = stable; 1 for 'argon is inert / unreactive'. The phrase 'noble gas configuration' or 'full outer shell' is the keyword that examiners credit — 'happy electrons' or similar informal language scores 0.

Particle	Relative mass	Relative charge	Location
Proton	1	+1	Inside the nucleus
Neutron	1	0 (neutral)	Inside the nucleus
Electron	1/1840 (≈ 0, negligible)	-1	Shells around the nucleus

Key Definitions and Keywords — Atomic Structure

Definitions to memorise and the exact keywords mark schemes credit for atomic structure answers — sharpened from recent examiner reports for the 2026 Pearson Edexcel IGCSE 4CH1 sitting.

Atom
Examiner keyword
The smallest particle of an element that retains the chemical identity of that element. Consists of a tiny dense nucleus (protons + neutrons) surrounded by electrons in shells.
Proton
Examiner keyword
Subatomic particle found in the nucleus. Relative mass = 1; relative charge = +1. The number of protons defines the element (its atomic number, Z).
Neutron
Examiner keyword
Subatomic particle found in the nucleus. Relative mass = 1; relative charge = 0 (neutral). Adds mass without adding charge — different numbers of neutrons give different isotopes.
Electron
Examiner keyword
Subatomic particle that occupies shells (energy levels) around the nucleus. Relative mass = 1/1840 (negligible); relative charge = -1. Outer-shell electrons determine the chemistry of the atom.
Atomic number (Z)
Examiner keyword
The number of protons in the nucleus of an atom. Defines the element. Also equal to the number of electrons in a neutral atom.
Mass number (A)
Examiner keyword
The total number of protons + neutrons in the nucleus. Sometimes called the nucleon number.
Isotope
Examiner keyword
Atoms of the SAME element (same number of protons) with DIFFERENT numbers of neutrons (and so different mass numbers). Isotopes have the same chemical properties but different physical properties.
Relative atomic mass (Ar)
Examiner keyword
The weighted average mass of the atoms of an element compared with 1/12 of the mass of a carbon-12 atom. No units (it is a ratio).
Electron configuration
Examiner keyword
The arrangement of electrons in the shells (energy levels) of an atom, written as a comma-separated list, e.g. sodium = 2, 8, 1 and argon = 2, 8, 8.

Common Mistakes and Misconceptions — Atomic Structure

The traps other students keep falling into on atomic structure questions — taken from recent Pearson Edexcel IGCSE 4CH1 examiner reports and mark schemes — and how to avoid them.

✕Saying isotopes have different numbers of protons or electrons
4CH1 Examiner Reports 2022-2024
Why it happens
Confusing isotopes with ions.
How to avoid it
Isotopes differ ONLY in the number of NEUTRONS. Same protons (same element), same electrons (same chemistry), different neutrons (different mass). Ions are formed by gaining or losing electrons — a different concept.
✕Putting 18 electrons in the 3rd shell at IGCSE
Why it happens
Misremembering 2n² shell capacity rule from A-level.
How to avoid it
At IGCSE for the FIRST 20 ELEMENTS, the 3rd shell holds only 8 electrons before the 4th shell starts to fill (with K and Ca). Use 2, 8, 8, 2.
✕Calculating Ar as a simple average (e.g. (35 + 37)/2 = 36 for chlorine)
4CH1 Examiner Reports 2022-2024
Why it happens
Treating the two isotopes as if they had equal abundance.
How to avoid it
Use the WEIGHTED average: Ar = (isotope mass × % abundance) summed, divided by 100. For chlorine: (35 × 75 + 37 × 25)/100 = 35.5, not 36.
✕Treating the electron mass as 1
Why it happens
Treating all three subatomic particles symmetrically.
How to avoid it
Electron relative mass is 1/1840 (effectively zero / negligible). Only protons and neutrons contribute to the mass number. Electrons contribute to charge but not to mass.
✕Saying noble gases react readily because they have full outer shells
4CH1 Examiner Reports 2022-2024
Why it happens
Confusing 'full' with 'available to share'.
How to avoid it
A full outer shell is a STABLE arrangement — noble gases (He, Ne, Ar, Kr, Xe) are UNREACTIVE/INERT for exactly this reason. Other atoms gain, lose or share electrons to ACHIEVE the noble-gas configuration.

Practice questions

Exam-style questions with step-by-step worked solutions. Try one before checking the method.

Past paper style quiz

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

Video lesson

Short walkthrough of the concepts students most often get stuck on.

Atomic Structure — frequently asked questions

The things students keep getting wrong in this sub-topic, answered.

Atomic Structure

Short Study Notes in the form of Slides

Detailed Study Notes

What you’ll learn

How it’s examined

1Particles in an atom from ZAX^A_Z XZA​X notation (4 marks, Paper 1)

2Relative atomic mass from isotopic abundance (4 marks, Paper 1)

3Electron configuration and the periodic table (4 marks, Paper 1)

4Identifying isotopes (3 marks, Paper 1)

Relative atomic mass (weighted average) — spec 1.20

Number of neutrons in an atom

Shell-filling capacities (first 20 elements)

Atom

Proton

Neutron

Electron

Atomic number (Z)

Mass number (A)

Isotope

Relative atomic mass (Ar)

Electron configuration

✕Saying isotopes have different numbers of protons or electrons

✕Putting 18 electrons in the 3rd shell at IGCSE

✕Calculating Ar as a simple average (e.g. (35 + 37)/2 = 36 for chlorine)

✕Treating the electron mass as 1

✕Saying noble gases react readily because they have full outer shells

Practice questions

Past paper style quiz

Assess your understanding

Video lesson

Atomic Structure — frequently asked questions

1Particles in an atom from $^A_Z X$ notation (4 marks, Paper 1)