What is the basic structure of an atom according to the Cambridge IGCSE Coordinated Science syllabus?

In the Cambridge IGCSE Coordinated Science syllabus, an atom is described as consisting of a central nucleus containing protons and neutrons, surrounded by electrons in shells. Protons are positively charged, electrons are negatively charged, and neutrons have no charge. The number of protons in the nucleus determines the element's atomic number.

How does the Periodic Table organize elements in the context of atomic structure?

The Periodic Table organizes elements based on their atomic number, which is the number of protons in an atom's nucleus. Elements are arranged in rows called periods and columns known as groups. Elements in the same group have similar chemical properties and the same number of electrons in their outer shell, which influences their reactivity and bonding behavior.

What is the significance of electron configuration in understanding atomic structure?

Electron configuration refers to the distribution of electrons in an atom's shells. It is significant because it determines how an atom interacts with other atoms, influencing chemical bonding and reactivity. For example, elements with a full outer electron shell are typically inert, while those with one or two electrons in their outer shell are more reactive.

How do isotopes differ from each other in terms of atomic structure?

Isotopes are variants of the same element that have the same number of protons but different numbers of neutrons in their nuclei. This difference in neutron number results in different atomic masses for the isotopes. Despite these differences, isotopes of an element exhibit similar chemical properties because they have the same electron configuration.

Why is the Periodic Table important for understanding chemical reactions in Coordinated Science?

The Periodic Table is crucial for understanding chemical reactions because it provides a framework for predicting how elements will interact. By organizing elements according to their atomic structure and properties, the table allows scientists and students to anticipate the types of bonds that will form and the reactivity of different elements, which is essential for studying chemical reactions in Coordinated Science.

Why is "Calculating neutrons as mass number − number of electrons instead of mass number − atomic number" a common mistake in Atomic Structure and the Periodic Table?

Why it happens: Students confuse atomic number (protons) with electrons, especially in ions. How to avoid it: Neutrons = mass number − atomic number (not electrons). Atomic number = protons always.

Why is "Saying isotopes have different chemical properties" a common mistake in Atomic Structure and the Periodic Table?

Why it happens: Students confuse physical differences (mass, radioactivity) with chemical behaviour. How to avoid it: Isotopes have IDENTICAL chemical properties (same electron configuration, same outer electrons) but different physical properties (different mass, density, rate of diffusion). Source: 0654 Examiner Report 2023

Why is "Writing units (g or g/mol) after a relative atomic mass value" a common mistake in Atomic Structure and the Periodic Table?

Why it happens: Students confuse Ar (a dimensionless ratio) with molar mass (g/mol). How to avoid it: Relative atomic mass has no units. Molar mass has units of g/mol.

Atoms, Elements and CompoundsCambridge IGCSE Coordinated Sciences 0654

Atomic Structure and the Periodic Table

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Atomic Structure — Cambridge IGCSE Coordinated Science 0654

Atoms consist of a nucleus (protons + neutrons) surrounded by electrons in shells. Cambridge tests proton number, nucleon number, isotopes, electron configuration, and how atomic structure determines chemical behaviour.

What you’ll learn

Mapped to the Cambridge IGCSE 0654 syllabus (2025-2027).

C3.4 — Describe the structure of an atom (protons, neutrons, electrons).
C3.5 — Use proton number and nucleon number to determine composition of atoms and ions.
C3.6 — Define isotopes and explain why they have the same chemical properties.
C3.7 — Write electron configurations for elements 1–20.

Inside the Atom

Atoms have a tiny dense nucleus (protons and neutrons) surrounded by electrons in shells — most of the atom is empty space.

Particle	Relative mass	Relative charge	Location
Proton	1	+1	Nucleus
Neutron	1	0	Nucleus
Electron	1/1836 (negligible)	-1	Shells (orbitals) outside nucleus

In a neutral atom: number of protons = number of electrons (charges cancel).

Proton number (Z) = number of protons = atomic number. Unique to each element.

Nucleon number (A) = protons + neutrons = mass number.

Number of neutrons = A − Z

Example — Carbon-12 (¹²C):

Proton number = 6 → 6 protons, 6 electrons
Nucleon number = 12 → 6 neutrons

Ions: Atoms gain or lose electrons to become ions.

Losing electrons → fewer electrons than protons → positive charge → cation (e.g. Na⁺ has 11 protons, 10 electrons)
Gaining electrons → more electrons than protons → negative charge → anion (e.g. Cl⁻ has 17 protons, 18 electrons)

Protons: +1, in nucleus. Neutrons: 0, in nucleus. Electrons: -1, in shells.
Proton number = number of protons = number of electrons (neutral atom).
Neutrons = nucleon number − proton number.

Isotopes

Isotopes are atoms of the same element with different numbers of neutrons — same chemical properties, different masses.

Isotopes: atoms of the same element that have the same number of protons but different numbers of neutrons.

Examples:

¹H (protium): 1 proton, 0 neutrons
²H (deuterium): 1 proton, 1 neutron
³H (tritium): 1 proton, 2 neutrons
³⁵Cl: 17 protons, 18 neutrons
³⁷Cl: 17 protons, 20 neutrons (chlorine exists as ~75% ³⁵Cl and ~25% ³⁷Cl → average atomic mass ≈ 35.5)

Why same chemical properties? Chemical properties depend on the number of ELECTRONS (and hence electron configuration). Isotopes have the same number of protons → same number of electrons → same electron configuration → same chemical reactions.

Why different physical properties? Different masses → different densities, melting points (slightly), boiling points. Radioactive isotopes (radioisotopes) have unstable nuclei that decay.

Relative atomic mass (Ar): weighted average of the masses of all isotopes of an element, relative to ¹²C = 12.

Isotopes: same proton number, different neutron number → same element, different mass.
Same chemical properties (same electrons). Different physical properties (different mass).
Ar = weighted average mass of all isotopes relative to ¹²C.

Electron Configuration

Electrons occupy shells around the nucleus. The first holds up to 2, the second and third up to 8 each. Outer shell electrons determine chemical behaviour.

Electron shells: electrons occupy energy levels (shells) around the nucleus.

Shell 1: maximum 2 electrons
Shell 2: maximum 8 electrons
Shell 3: maximum 8 electrons (at IGCSE level — simplified)

Writing electron configurations (for elements 1–20):

H: 1 | He: 2 | Li: 2,1 | Na: 2,8,1 | Cl: 2,8,7 | Ca: 2,8,8,2

Why electron configuration matters:

Noble gas configuration (full outer shell): very stable → noble gases are unreactive
Elements react to achieve a full outer shell (noble gas configuration)
Metals lose outer electrons → form positive ions
Non-metals gain electrons → form negative ions

Element	Configuration	Outer electrons	Expected behaviour
Na	2,8,1	1	Loses 1e⁻ → Na⁺
Cl	2,8,7	7	Gains 1e⁻ → Cl⁻
Mg	2,8,2	2	Loses 2e⁻ → Mg²⁺
O	2,6	6	Gains 2e⁻ → O²⁻

Sodium has a tiny nucleus (11p + 12n) with 11 electrons in shells of 2, 8 and 1; the lone outer electron is easily lost to form Na⁺.

Shell 1: max 2e. Shell 2: max 8e. Shell 3: max 8e (simplified at IGCSE).
Full outer shell = stable (noble gas configuration).
Metals lose electrons (cations). Non-metals gain electrons (anions).

How it’s examined

Paper 4: 'An atom of phosphorus has nucleon number 31 and proton number 15. State the number of neutrons' (1 mark — 16). 'Explain why ³⁵Cl and ³⁷Cl have the same chemical properties' (2 marks — same number of protons/electrons, same electron configuration). 'Write the electron configuration of a calcium atom' (1 mark — 2,8,8,2). MCQ frequently tests isotopes and ion formation from atomic structure data.

Sources: Cambridge IGCSE Coordinated Sciences 0654 syllabus 2025-2027 (C3); 0654 Examiner Reports 2022-2024. Last reviewed 2026-05-14.

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Step-by-step worked examples — Atomic Structure and the Periodic Table

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

1Sub-atomic particles in phosphorus
Core
Question
An atom of phosphorus has atomic number 15 and mass number 31. State the number of protons, neutrons, and electrons in this atom.
Step-by-step solution
1. Step 1
  Protons = atomic number = 15.
  $p = Z = 15$
2. Step 2
  Neutrons = mass number − atomic number.
  $n = A - Z = 31 - 15 = 16$
3. Step 3
  Electrons = protons (neutral atom).
  $e = 15$
Answer
15 protons, 16 neutrons, 15 electrons.
2Electronic configuration of chlorine
Core
Question
Write the electronic configuration of a chlorine atom (atomic number 17).
Step-by-step solution
1. Step 1
  Shell 1 holds a maximum of 2 electrons: fill with 2.
2. Step 2
  Shell 2 holds a maximum of 8 electrons: fill with 8.
3. Step 3
  Remaining electrons: 17 − 2 − 8 = 7 go in shell 3.
Answer
2, 8, 7
Examiner tip
Write shell occupancies separated by commas. Always fill inner shells before outer shells.
3Isotopes of carbon
Extended
Question
Carbon-12 and carbon-14 are isotopes of carbon. Explain what this means and state ONE difference between them.
Step-by-step solution
1. Step 1
  Isotopes are atoms of the same element (same atomic number) with different numbers of neutrons (different mass numbers).
2. Step 2
  Carbon-12: 6 protons, 6 neutrons. Carbon-14: 6 protons, 8 neutrons.
3. Step 3
  One difference: carbon-14 has a greater mass than carbon-12 (different mass numbers: 12 vs 14). Both have the same chemical properties (same electron configuration: 2, 4).
Answer
Isotopes: same number of protons (6), different neutron numbers (6 vs 8). Difference: different mass numbers (12 vs 14); same chemical properties.
4Calculating relative atomic mass
Extended
Question
Chlorine has two isotopes: $^{35}\text{Cl}$ (75%) and $^{37}\text{Cl}$ (25%). Calculate the relative atomic mass of chlorine.
Step-by-step solution
1. Step 1
  Apply the weighted average formula.
  $A_r = \frac{(35 \times 75) + (37 \times 25)}{100}$
2. Step 2
  Calculate the numerator.
  $= \frac{2625 + 925}{100} = \frac{3550}{100}$
3. Step 3
  Divide to get the final answer.
  $A_r = 35.5$
Answer
$A_r(\text{Cl}) = 35.5$
Examiner tip
The answer must be expressed to 1 decimal place. Note that Ar is a ratio with no units.

Key Formulae — Atomic Structure and the Periodic Table

The formulae you need to memorise for atomic structure and the periodic table on the Cambridge IGCSE 0654 paper, with every variable defined in plain English and a note on when to use it.

Relative atomic mass
$A_r = \frac{\sum(\text{isotope mass} \times \% \text{abundance})}{100}$
$isotope mass$
mass number of each isotope
$% abundance$
percentage of each isotope in the natural element
When to use
Calculating the relative atomic mass from isotope data.
Example
Cl: (35 × 75 + 37 × 25) / 100 = 35.5

Key Definitions and Keywords — Atomic Structure and the Periodic Table

Definitions to memorise and the exact keywords mark schemes credit for atomic structure and the periodic table answers — sharpened from recent examiner reports for the 2026 0654 sitting.

Atomic number (Z)
Examiner keyword
The number of protons in the nucleus of an atom; defines the element and determines the atom's position in the periodic table.
Mass number (A)
Examiner keyword
The total number of protons plus neutrons in the nucleus of an atom.
Isotopes
Examiner keyword
Atoms of the same element (same atomic number) with different mass numbers due to different numbers of neutrons. Isotopes have identical chemical properties but different physical properties.
Relative atomic mass (Ar)
Examiner keyword
The weighted average mass of atoms of an element relative to 1/12 the mass of a carbon-12 atom. It takes into account the natural abundance of all isotopes.
Electron shell
An energy level at a fixed distance from the nucleus that can hold a maximum number of electrons (shell 1: 2; shell 2: 8; shell 3: 8).
Nucleon
A particle found in the nucleus of an atom — either a proton or a neutron.

Common Mistakes and Misconceptions — Atomic Structure and the Periodic Table

The traps other students keep falling into on atomic structure and the periodic table questions — taken from recent Cambridge IGCSE 0654 examiner reports and mark schemes — and how to avoid them.

✕Calculating neutrons as mass number − number of electrons instead of mass number − atomic number
Why it happens
Students confuse atomic number (protons) with electrons, especially in ions.
How to avoid it
Neutrons = mass number − atomic number (not electrons). Atomic number = protons always.
✕Saying isotopes have different chemical properties
0654 Examiner Report 2023
Why it happens
Students confuse physical differences (mass, radioactivity) with chemical behaviour.
How to avoid it
Isotopes have IDENTICAL chemical properties (same electron configuration, same outer electrons) but different physical properties (different mass, density, rate of diffusion).
✕Writing units (g or g/mol) after a relative atomic mass value
Why it happens
Students confuse Ar (a dimensionless ratio) with molar mass (g/mol).
How to avoid it
Relative atomic mass has no units. Molar mass has units of g/mol.

Practice questions

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Past paper style quiz

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