Who this is for: Cambridge IGCSE Chemistry (0620) students who can draw a simple atom but lose marks on proton/neutron/electron numbers, nuclear symbols, or what group and period tell you about an element.
What query it owns: how to understand and revise atomic structure and the periodic table in Cambridge IGCSE Chemistry.
Why this is safe: this page owns the atomic-structure revision-guide angle, while Tutopiya’s Atomic Structure and the Periodic Table subtopic page owns the learning resource and the free Atomic Structure quiz owns the practice.

Atomic structure describes an atom as a nucleus (protons and neutrons) surrounded by electrons in shells. The periodic table arranges elements by atomic number so that group number reveals outer-shell electrons and period number reveals shell count. These ideas appear in almost every Cambridge IGCSE Chemistry (0620) paper. This guide covers particle charges, nuclear notation, and periodic trends examiners test.

Key takeaways

• Proton number (atomic number) = number of protons = number of electrons in a neutral atom.
• Mass number = protons + neutrons; neutrons = mass number − proton number.
• Group = columns — same outer-shell electrons; period = rows — same number of electron shells.
• Metals left, non-metals right; Group I = 1 outer electron; Group VII = 7 outer electrons.
• Confirm with the Atomic Structure quiz.

What is atomic structure in Cambridge IGCSE Chemistry?

Atomic structure is the model of the atom as a tiny, dense, positively charged nucleus containing protons and neutrons, with negatively charged electrons in energy levels (shells) around it. In Cambridge IGCSE Chemistry (0620) you interpret nuclear symbols, calculate subatomic particle numbers, and use the periodic table to predict electron arrangement.

Read the full notes on Tutopiya’s Atomic Structure and the Periodic Table subtopic page.

Subatomic particles — summary table

Particle	Relative charge	Relative mass	Location
Proton	+1	1	Nucleus
Neutron	0	1	Nucleus
Electron	−1	~1/1840	Shells around nucleus

Nuclear notation — how to read symbols

For ᵇₐX: a = proton number (bottom left conceptually in syllabus diagrams); b = mass number (top). Protons = a; electrons = a (if neutral); neutrons = b − a.

How to answer atomic structure questions — step by step

1. Write down proton number, mass number, and element symbol from the question.
2. Calculate protons (= atomic number), electrons (= protons if neutral), neutrons (= mass − protons).
3. For periodic table questions, use group for outer electrons and period for shell count.
4. State trends — metallic character decreases across a period; increases down a group.
5. Test with the free Atomic Structure quiz.

Past-paper wording: command words that matter

Command word / phrase	What the question wants	Typical stem
State	Short factual answer	”State the number of protons in sodium.”
Deduce	Work out from given data	”Deduce the number of neutrons in ²³₁₁Na.”
Describe	Structure or arrangement	”Describe the structure of an atom.”
Explain	Link periodic position to properties	”Explain why Group I elements are reactive.”

Worked exam-style stems (how to answer the wording)

1. “An atom of element X has mass number 24 and 12 protons. Deduce the number of neutrons.” Neutrons = 24 − 12 = 12. Deduce = show the subtraction.
2. “State the number of outer-shell electrons in an atom of chlorine (Group VII, Period 3).” 7 outer electrons (Group number for Groups I–VII). State = one value.
3. “Describe the structure of an atom of helium.” Target: nucleus containing protons (and neutrons if specified); electrons in shells; most of atom is empty space. Describe = arrangement of subatomic particles.

Practise more on the Atoms topical past paper questions.

How atomic structure connects to isotopes and bonding

Same proton number but different neutron counts leads to Isotopes. Outer-shell electron count drives Ions and Ionic Bonds and covalent bonding. The Cambridge IGCSE Chemistry hub links all Atoms subtopics.

Common mistakes students make

• Confusing mass number with relative atomic mass.
• Calculating electrons in ions as equal to protons — adjust for charge.
• Using group number for transition metals’ outer electrons (simplified model applies to Groups I–VII).
• Saying electrons orbit inside the nucleus.
• Skipping the Atomic Structure quiz.

When you need more support

If deduce questions on subatomic particles keep failing, drill the topical past paper questions and book a Cambridge IGCSE Chemistry tutor.

Frequently asked questions

What is the difference between atomic number and mass number? Atomic number is the number of protons; mass number is protons plus neutrons.

How many electrons does a neutral atom have? Equal to the number of protons (atomic number).

What does the group number tell you? For Groups I–VII, it equals the number of outer-shell electrons.

How do I revise atomic structure effectively? Read subtopic notes, practise deduce/state stems, take the Atomic Structure quiz, then study isotopes.

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