This module covers what atoms are made of, how ions and isotopes differ from a neutral atom, how electrons are arranged, and how to tell an element from a compound. It matters because later chemistry topics build on it: bonding, formulas, equations and mole calculations all assume you can count particles reliably. Check the current Cambridge IGCSE Chemistry 0620 syllabus for your exam year to see exactly how each point is worded.
What should I know before starting?
You need the idea that matter is made of tiny particles, which is covered in particle models and changes of state. You also need comfortable subtraction and addition with small whole numbers. A periodic table is used throughout and is provided in the exam.
One orienting example
Consider the ion 27/13 Al³⁺. One symbol answers four questions.
Protons: the lower number, 13. Neutrons: 27 − 13 = 14. Electrons: 13 protons minus 3 for the 3+ charge gives 10. Arrangement: 10 electrons fill 2,8, the same as neon.
If another aluminium atom had 15 neutrons, it would still be aluminium because it still has 13 protons. That would be an isotope. Every lesson in this module is a piece of that example.
In what order should I study the lessons?
- Read proton and nucleon numbers: the counting rules everything else uses.
- Determine particles in a simple ion: the same counting when electrons are lost or gained.
- Compare isotopes without changing element identity: what stays the same and what differs when neutrons change, plus the weighted average.
- Use an electron arrangement to explain a pattern: why electrons, not neutrons, decide chemical behaviour.
- Distinguish an element from a compound sample: turning atom types into a classification.
- Mixed practice: original questions with full explanations.
What are the common traps?
- Treating the nucleon number as the number of protons or neutrons.
- Changing the protons when an ion forms, when only electrons move.
- Saying isotopes are different elements because their masses differ.
- Writing 2,8,9 instead of 2,8,8,1 for potassium.
- Calling O₂ a compound because it has two atoms.
Most of these come from reaching for the wrong number under time pressure, not from missing the idea. Saying aloud what each number counts is a simple fix.
How do I use the practice set?
Do the mixed practice set after the lessons, with full working on paper, and open each answer only after you have a written attempt. Use the error table at the end to decide which lesson to reread.
The equation balance reasoning trainer and mole and equation-ratio tutor are the natural follow-ups once counting is secure.
Next in the subject, bonding and structure uses electron arrangements directly. If you want a teacher to check how you reason through these chains, online one-to-one Chemistry tuition is available across Malaysia.